zf

conn.ex (67803B)

---

 alias Plug.Conn.Unfetched
 
 defmodule Plug.Conn do
   @moduledoc """
   The Plug connection.
 
   This module defines a struct and the main functions for working with
   requests and responses in an HTTP connection.
 
   Note request headers are normalized to lowercase and response
   headers are expected to have lowercase keys.
 
   ## Request fields
 
   These fields contain request information:
 
     * `host` - the requested host as a binary, example: `"www.example.com"`
     * `method` - the request method as a binary, example: `"GET"`
     * `path_info` - the path split into segments, example: `["hello", "world"]`
     * `script_name` - the initial portion of the URL's path that corresponds to
       the application routing, as segments, example: `["sub","app"]`
     * `request_path` - the requested path, example: `/trailing/and//double//slashes/`
     * `port` - the requested port as an integer, example: `80`
     * `remote_ip` - the IP of the client, example: `{151, 236, 219, 228}`. This field
       is meant to be overwritten by plugs that understand e.g. the `X-Forwarded-For`
       header or HAProxy's PROXY protocol. It defaults to peer's IP
     * `req_headers` - the request headers as a list, example: `[{"content-type", "text/plain"}]`.
       Note all headers will be downcased
     * `scheme` - the request scheme as an atom, example: `:http`
     * `query_string` - the request query string as a binary, example: `"foo=bar"`
 
   ## Fetchable fields
 
   The request information in these fields is not populated until it is fetched
   using the associated `fetch_` function. For example, the `cookies` field uses
   `fetch_cookies/2`.
 
   If you access these fields before fetching them, they will be returned as
   `Plug.Conn.Unfetched` structs.
 
     * `cookies`- the request cookies with the response cookies
     * `body_params` - the request body params, populated through a `Plug.Parsers` parser.
     * `query_params` - the request query params, populated through `fetch_query_params/2`
     * `path_params` - the request path params, populated by routers such as `Plug.Router`
     * `params` - the request params, the result of merging the `:path_params` on top of
        `:body_params` on top of `:query_params`
     * `req_cookies` - the request cookies (without the response ones)
 
   ## Session vs Assigns
 
   HTTP is stateless.
   This means that a server begins each request cycle with no knowledge about
   the client except the request itself.
   Its response may include one or more `"Set-Cookie"` headers, asking the client
   to send that value back in a `"Cookie"` header on subsequent requests.
   This is the basis for stateful interactions with a client, so that the server
   can remember the client's name, the contents of their shopping cart, and so on.
 
   In Plug, a "session" is a place to store data that persists from one request
   to the next.
   Typically, this data is stored in a cookie using `Plug.Session.COOKIE`.
   A minimal approach would be to store only a user's id in the session, then
   use that during the request cycle to look up other information (in a database
   or elsewhere).
   More can be stored in a session cookie, but be careful: this makes requests
   and responses heavier, and clients may reject cookies beyond a certain size.
   Also, whatever is stored in a session cookie is not shared between a user's
   different browsers or devices.
 
   If the session is stored elsewhere, such as with `Plug.Session.ETS`,
   something like a user id would still be needed to look it up on each request.
 
   Unlike data in a session, data in the `assigns` field lasts only for a single
   request.
   A typical use case would be for an authentication plug to look up a
   user by id and store the user's details in the assigns for later plugs to
   access during the same request.
   When the next request happens, this data will be gone.
 
   To summarize: `assigns` is for storing data to be accessed during the current
   request, and the session is for storing data to be accessed in subsequent
   requests.
 
   ## Response fields
 
   These fields contain response information:
 
     * `resp_body` - the response body, by default is an empty string. It is set
       to nil after the response is sent, except for test connections. The response
       charset used defaults to "utf-8".
     * `resp_cookies` - the response cookies with their name and options
     * `resp_headers` - the response headers as a list of tuples, by default `cache-control`
       is set to `"max-age=0, private, must-revalidate"`. Note, response headers
       are expected to have lowercase keys.
     * `status` - the response status
 
   ## Connection fields
 
     * `assigns` - shared user data as a map
     * `owner` - the Elixir process that owns the connection
     * `halted` - the boolean status on whether the pipeline was halted
     * `secret_key_base` - a secret key used to verify and encrypt cookies.
       the field must be set manually whenever one of those features are used.
       This data must be kept in the connection and never used directly, always
       use `Plug.Crypto.KeyGenerator.generate/3` to derive keys from it
     * `state` - the connection state
 
   The connection state is used to track the connection lifecycle. It starts as
   `:unset` but is changed to `:set` (via `resp/3`) or `:set_chunked`
   (used only for `before_send` callbacks by `send_chunked/2`) or `:file`
   (when invoked via `send_file/3`). Its final result is `:sent`, `:file`, `:chunked` 
   or `:upgraded` depending on the response model.
 
   ## Private fields
 
   These fields are reserved for libraries/framework usage.
 
     * `adapter` - holds the adapter information in a tuple
     * `private` - shared library data as a map
 
   ## Custom status codes
 
   Plug allows status codes to be overridden or added in order to allow new codes
   not directly specified by Plug or its adapters. Adding or overriding a status
   code is done through the Mix configuration of the `:plug` application. For
   example, to override the existing 404 reason phrase for the 404 status code
   ("Not Found" by default) and add a new 998 status code, the following config
   can be specified:
 
       config :plug, :statuses, %{
         404 => "Actually This Was Found",
         998 => "Not An RFC Status Code"
       }
 
   As this configuration is Plug specific, Plug will need to be recompiled for
   the changes to take place: this will not happen automatically as dependencies
   are not automatically recompiled when their configuration changes. To recompile
   Plug:
 
       mix deps.clean --build plug
 
   The atoms that can be used in place of the status code in many functions are
   inflected from the reason phrase of the status code. With the above
   configuration, the following will all work:
 
       put_status(conn, :not_found)                     # 404
       put_status(conn, :actually_this_was_found)       # 404
       put_status(conn, :not_an_rfc_status_code)        # 998
 
   Even though 404 has been overridden, the `:not_found` atom can still be used
   to set the status to 404 as well as the new atom `:actually_this_was_found`
   inflected from the reason phrase "Actually This Was Found".
 
   ## Protocol Upgrades
 
   Plug provides basic support for protocol upgrades via the `upgrade_adapter/3`
   function to facilitate connection upgrades to protocols such as WebSockets.
   As the name suggests, this functionality is adapter-dependent and the 
   functionality & requirements of a given upgrade require explicit coordination
   between a Plug application & the underlying adapter. Plug provides upgrade 
   related functionality only to the extent necessary to allow a Plug application 
   to request protocol upgrades from the underlying adapter. See the documentation
   for `upgrade_adapter/3` for details.
   """
 
   @type adapter :: {module, term}
   @type assigns :: %{optional(atom) => any}
   @type body :: iodata
   @type req_cookies :: %{optional(binary) => binary}
   @type cookies :: %{optional(binary) => term}
   @type halted :: boolean
   @type headers :: [{binary, binary}]
   @type host :: binary
   @type int_status :: non_neg_integer | nil
   @type owner :: pid
   @type method :: binary
   @type query_param :: binary | %{optional(binary) => query_param} | [query_param]
   @type query_params :: %{optional(binary) => query_param}
   @type params :: %{optional(binary) => term}
   @type port_number :: :inet.port_number()
   @type query_string :: String.t()
   @type resp_cookies :: %{optional(binary) => map()}
   @type scheme :: :http | :https
   @type secret_key_base :: binary | nil
   @type segments :: [binary]
   @type state :: :unset | :set | :set_chunked | :set_file | :file | :chunked | :sent | :upgraded
   @type status :: atom | int_status
 
   @type t :: %__MODULE__{
           adapter: adapter,
           assigns: assigns,
           body_params: params | Unfetched.t(),
           cookies: cookies | Unfetched.t(),
           halted: halted,
           host: host,
           method: method,
           owner: owner,
           params: params | Unfetched.t(),
           path_info: segments,
           path_params: query_params,
           port: :inet.port_number(),
           private: assigns,
           query_params: query_params | Unfetched.t(),
           query_string: query_string,
           remote_ip: :inet.ip_address(),
           req_cookies: req_cookies | Unfetched.t(),
           req_headers: headers,
           request_path: binary,
           resp_body: body | nil,
           resp_cookies: resp_cookies,
           resp_headers: headers,
           scheme: scheme,
           script_name: segments,
           secret_key_base: secret_key_base,
           state: state,
           status: int_status
         }
 
   defstruct adapter: {Plug.MissingAdapter, nil},
             assigns: %{},
             body_params: %Unfetched{aspect: :body_params},
             cookies: %Unfetched{aspect: :cookies},
             halted: false,
             host: "www.example.com",
             method: "GET",
             owner: nil,
             params: %Unfetched{aspect: :params},
             path_info: [],
             path_params: %{},
             port: 0,
             private: %{},
             query_params: %Unfetched{aspect: :query_params},
             query_string: "",
             remote_ip: nil,
             req_cookies: %Unfetched{aspect: :cookies},
             req_headers: [],
             request_path: "",
             resp_body: nil,
             resp_cookies: %{},
             resp_headers: [{"cache-control", "max-age=0, private, must-revalidate"}],
             scheme: :http,
             script_name: [],
             secret_key_base: nil,
             state: :unset,
             status: nil
 
   defmodule NotSentError do
     defexception message: "a response was neither set nor sent from the connection"
 
     @moduledoc """
     Error raised when no response is sent in a request
     """
   end
 
   defmodule AlreadySentError do
     defexception message: "the response was already sent"
 
     @moduledoc """
     Error raised when trying to modify or send an already sent response
     """
   end
 
   defmodule CookieOverflowError do
     defexception message: "cookie exceeds maximum size of 4096 bytes"
 
     @moduledoc """
     Error raised when the cookie exceeds the maximum size of 4096 bytes.
     """
   end
 
   defmodule InvalidHeaderError do
     defexception message: "header is invalid"
 
     @moduledoc ~S"""
     Error raised when trying to send a header that has errors, for example:
 
       * the header key contains uppercase chars
       * the header value contains newlines \n
     """
   end
 
   defmodule InvalidQueryError do
     @moduledoc """
     Raised when the request string is malformed, for example:
 
       * the query has bad utf-8 encoding
       * the query fails to www-form decode
     """
 
     defexception message: "query string is invalid", plug_status: 400
   end
 
   alias Plug.Conn
   @epoch {{1970, 1, 1}, {0, 0, 0}}
   @already_sent {:plug_conn, :sent}
   @unsent [:unset, :set, :set_chunked, :set_file]
 
   @doc """
   Assigns a value to a key in the connection.
 
   The "assigns" storage is meant to be used to store values in the connection
   so that other plugs in your plug pipeline can access them. The assigns storage
   is a map.
 
   ## Examples
 
       iex> conn.assigns[:hello]
       nil
       iex> conn = assign(conn, :hello, :world)
       iex> conn.assigns[:hello]
       :world
 
   """
   @spec assign(t, atom, term) :: t
   def assign(%Conn{assigns: assigns} = conn, key, value) when is_atom(key) do
     %{conn | assigns: Map.put(assigns, key, value)}
   end
 
   @doc """
   Assigns multiple values to keys in the connection.
 
   Equivalent to multiple calls to `assign/3`.
 
   ## Examples
 
       iex> conn.assigns[:hello]
       nil
       iex> conn = merge_assigns(conn, hello: :world)
       iex> conn.assigns[:hello]
       :world
 
   """
   @spec merge_assigns(t, Keyword.t()) :: t
   def merge_assigns(%Conn{assigns: assigns} = conn, keyword) when is_list(keyword) do
     %{conn | assigns: Enum.into(keyword, assigns)}
   end
 
   @doc false
   @spec async_assign(t, atom, (() -> term)) :: t
   def async_assign(%Conn{} = conn, key, fun) when is_atom(key) and is_function(fun, 0) do
     IO.warn("Plug.Conn.async_assign/3 is deprecated, please call assign + Task.async instead")
     assign(conn, key, Task.async(fun))
   end
 
   @doc false
   @spec await_assign(t, atom, timeout) :: t
   def await_assign(%Conn{} = conn, key, timeout \\ 5000) when is_atom(key) do
     IO.warn(
       "Plug.Conn.await_assign/3 is deprecated, please fetch the assign and call Task.await instead"
     )
 
     task = Map.fetch!(conn.assigns, key)
     assign(conn, key, Task.await(task, timeout))
   end
 
   @doc """
   Assigns a new **private** key and value in the connection.
 
   This storage is meant to be used by libraries and frameworks to avoid writing
   to the user storage (the `:assigns` field). It is recommended for
   libraries/frameworks to prefix the keys with the library name.
 
   For example, if a plug called `my_plug` needs to store a `:hello`
   key, it would store it as `:my_plug_hello`:
 
       iex> conn.private[:my_plug_hello]
       nil
       iex> conn = put_private(conn, :my_plug_hello, :world)
       iex> conn.private[:my_plug_hello]
       :world
 
   """
   @spec put_private(t, atom, term) :: t
   def put_private(%Conn{private: private} = conn, key, value) when is_atom(key) do
     %{conn | private: Map.put(private, key, value)}
   end
 
   @doc """
   Assigns multiple **private** keys and values in the connection.
 
   Equivalent to multiple `put_private/3` calls.
 
   ## Examples
 
       iex> conn.private[:my_plug_hello]
       nil
       iex> conn = merge_private(conn, my_plug_hello: :world)
       iex> conn.private[:my_plug_hello]
       :world
 
   """
   @spec merge_private(t, Keyword.t()) :: t
   def merge_private(%Conn{private: private} = conn, keyword) when is_list(keyword) do
     %{conn | private: Enum.into(keyword, private)}
   end
 
   @doc """
   Stores the given status code in the connection.
 
   The status code can be `nil`, an integer, or an atom. The list of allowed
   atoms is available in `Plug.Conn.Status`.
 
   Raises a `Plug.Conn.AlreadySentError` if the connection has already been
   `:sent`, `:chunked` or `:upgraded`.
 
   ## Examples
 
       Plug.Conn.put_status(conn, :not_found)
       Plug.Conn.put_status(conn, 200)
 
   """
   @spec put_status(t, status) :: t
   def put_status(%Conn{state: state}, _status) when state not in @unsent do
     raise AlreadySentError
   end
 
   def put_status(%Conn{} = conn, nil), do: %{conn | status: nil}
   def put_status(%Conn{} = conn, status), do: %{conn | status: Plug.Conn.Status.code(status)}
 
   @doc """
   Sends a response to the client.
 
   It expects the connection state to be `:set`, otherwise raises an
   `ArgumentError` for `:unset` connections or a `Plug.Conn.AlreadySentError` for
   already `:sent`, `:chunked` or `:upgraded` connections.
 
   At the end sets the connection state to `:sent`.
 
   Note that this function does not halt the connection, so if
   subsequent plugs try to send another response, it will error out.
   Use `halt/1` after this function if you want to halt the plug pipeline.
 
   ## Examples
 
       conn
       |> Plug.Conn.resp(404, "Not found")
       |> Plug.Conn.send_resp()
 
   """
   @spec send_resp(t) :: t | no_return
   def send_resp(conn)
 
   def send_resp(%Conn{state: :unset}) do
     raise ArgumentError, "cannot send a response that was not set"
   end
 
   def send_resp(%Conn{adapter: {adapter, payload}, state: :set, owner: owner} = conn) do
     conn = run_before_send(conn, :set)
 
     {:ok, body, payload} =
       adapter.send_resp(payload, conn.status, conn.resp_headers, conn.resp_body)
 
     send(owner, @already_sent)
     %{conn | adapter: {adapter, payload}, resp_body: body, state: :sent}
   end
 
   def send_resp(%Conn{}) do
     raise AlreadySentError
   end
 
   @doc """
   Sends a file as the response body with the given `status`
   and optionally starting at the given offset until the given length.
 
   If available, the file is sent directly over the socket using
   the operating system `sendfile` operation.
 
   It expects a connection that has not been `:sent`, `:chunked` or `:upgraded` yet and sets its
   state to `:file` afterwards. Otherwise raises `Plug.Conn.AlreadySentError`.
 
   ## Examples
 
       Plug.Conn.send_file(conn, 200, "README.md")
 
   """
   @spec send_file(t, status, filename :: binary, offset :: integer, length :: integer | :all) ::
           t | no_return
   def send_file(conn, status, file, offset \\ 0, length \\ :all)
 
   def send_file(%Conn{state: state}, status, _file, _offset, _length)
       when state not in @unsent do
     _ = Plug.Conn.Status.code(status)
     raise AlreadySentError
   end
 
   def send_file(
         %Conn{adapter: {adapter, payload}, owner: owner} = conn,
         status,
         file,
         offset,
         length
       )
       when is_binary(file) do
     if file =~ "\0" do
       raise ArgumentError, "cannot send_file/5 with null byte"
     end
 
     conn =
       run_before_send(%{conn | status: Plug.Conn.Status.code(status), resp_body: nil}, :set_file)
 
     {:ok, body, payload} =
       adapter.send_file(payload, conn.status, conn.resp_headers, file, offset, length)
 
     send(owner, @already_sent)
     %{conn | adapter: {adapter, payload}, state: :file, resp_body: body}
   end
 
   @doc """
   Sends the response headers as a chunked response.
 
   It expects a connection that has not been `:sent` or `:upgraded` yet and sets its
   state to `:chunked` afterwards. Otherwise, raises `Plug.Conn.AlreadySentError`.
   After `send_chunked/2` is called, chunks can be sent to the client via
   the `chunk/2` function.
 
   HTTP/2 does not support chunking and will instead stream the response without a
   transfer encoding. When using HTTP/1.1, the Cowboy adapter will stream the response
   instead of emitting chunks if the `content-length` header has been set before calling
   `send_chunked/2`.
   """
   @spec send_chunked(t, status) :: t | no_return
   def send_chunked(%Conn{state: state}, status)
       when state not in @unsent do
     _ = Plug.Conn.Status.code(status)
     raise AlreadySentError
   end
 
   def send_chunked(%Conn{adapter: {adapter, payload}, owner: owner} = conn, status) do
     conn = %{conn | status: Plug.Conn.Status.code(status), resp_body: nil}
     conn = run_before_send(conn, :set_chunked)
     {:ok, body, payload} = adapter.send_chunked(payload, conn.status, conn.resp_headers)
     send(owner, @already_sent)
     %{conn | adapter: {adapter, payload}, state: :chunked, resp_body: body}
   end
 
   @doc """
   Sends a chunk as part of a chunked response.
 
   It expects a connection with state `:chunked` as set by
   `send_chunked/2`. It returns `{:ok, conn}` in case of success,
   otherwise `{:error, reason}`.
 
   To stream data use `Enum.reduce_while/3` instead of `Enum.into/2`.
   `Enum.reduce_while/3` allows aborting the execution if `chunk/2` fails to
   deliver the chunk of data.
 
   ## Example
 
       Enum.reduce_while(~w(each chunk as a word), conn, fn (chunk, conn) ->
         case Plug.Conn.chunk(conn, chunk) do
           {:ok, conn} ->
             {:cont, conn}
           {:error, :closed} ->
             {:halt, conn}
         end
       end)
 
   """
   @spec chunk(t, body) :: {:ok, t} | {:error, term} | no_return
   def chunk(%Conn{adapter: {adapter, payload}, state: :chunked} = conn, chunk) do
     if iodata_empty?(chunk) do
       {:ok, conn}
     else
       case adapter.chunk(payload, chunk) do
         :ok -> {:ok, conn}
         {:ok, body, payload} -> {:ok, %{conn | resp_body: body, adapter: {adapter, payload}}}
         {:error, _} = error -> error
       end
     end
   end
 
   def chunk(%Conn{}, chunk) when is_binary(chunk) or is_list(chunk) do
     raise ArgumentError,
           "chunk/2 expects a chunked response. Please ensure " <>
             "you have called send_chunked/2 before you send a chunk"
   end
 
   defp iodata_empty?(""), do: true
   defp iodata_empty?([]), do: true
   defp iodata_empty?([head | tail]), do: iodata_empty?(head) and iodata_empty?(tail)
   defp iodata_empty?(_), do: false
 
   @doc """
   Sends a response with the given status and body.
 
   This is equivalent to setting the status and the body and then
   calling `send_resp/1`.
 
   Note that this function does not halt the connection, so if
   subsequent plugs try to send another response, it will error out.
   Use `halt/1` after this function if you want to halt the plug pipeline.
 
   ## Examples
 
       Plug.Conn.send_resp(conn, 404, "Not found")
 
   """
   @spec send_resp(t, status, body) :: t | no_return
   def send_resp(%Conn{} = conn, status, body) do
     conn |> resp(status, body) |> send_resp()
   end
 
   @doc """
   Sets the response to the given `status` and `body`.
 
   It sets the connection state to `:set` (if not already `:set`)
   and raises `Plug.Conn.AlreadySentError` if it was already `:sent`, `:chunked` or `:upgraded`.
 
   If you also want to send the response, use `send_resp/1` after this
   or use `send_resp/3`.
 
   The status can be an integer, an atom, or `nil`. See `Plug.Conn.Status`
   for more information.
 
   ## Examples
 
       Plug.Conn.resp(conn, 404, "Not found")
 
   """
   @spec resp(t, status, body) :: t
   def resp(%Conn{state: state}, status, _body)
       when state not in @unsent do
     _ = Plug.Conn.Status.code(status)
     raise AlreadySentError
   end
 
   def resp(%Conn{}, _status, nil) do
     raise ArgumentError, "response body cannot be set to nil"
   end
 
   def resp(%Conn{} = conn, status, body)
       when is_binary(body) or is_list(body) do
     %{conn | status: Plug.Conn.Status.code(status), resp_body: body, state: :set}
   end
 
   @doc """
   Returns the request peer data if one is present.
   """
   @spec get_peer_data(t) :: Plug.Conn.Adapter.peer_data()
   def get_peer_data(%Conn{adapter: {adapter, payload}}) do
     adapter.get_peer_data(payload)
   end
 
   @doc """
   Returns the HTTP protocol and version.
 
   ## Examples
 
       iex> get_http_protocol(conn)
       :"HTTP/1.1"
 
   """
   @spec get_http_protocol(t) :: Plug.Conn.Adapter.http_protocol()
   def get_http_protocol(%Conn{adapter: {adapter, payload}}) do
     adapter.get_http_protocol(payload)
   end
 
   @doc """
   Returns the values of the request header specified by `key`.
 
   ## Examples
 
       iex> get_req_header(conn, "accept")
       ["application/json"]
 
   """
   @spec get_req_header(t, binary) :: [binary]
   def get_req_header(%Conn{req_headers: headers}, key) when is_binary(key) do
     for {^key, value} <- headers, do: value
   end
 
   @doc ~S"""
   Prepends the list of headers to the connection request headers.
 
   Similar to `put_req_header` this functions adds a new request header
   (`key`) but rather than replacing the existing one it prepends another
   header with the same `key`.
 
   The "host" header will be overridden by `conn.host` and should not be set
   with this method. Instead, do `%Plug.Conn{conn | host: value}`.
 
   Because header keys are case-insensitive in both HTTP/1.1 and HTTP/2,
   it is recommended for header keys to be in lowercase, to avoid sending
   duplicate keys in a request.
   Additionally, requests with mixed-case headers served over HTTP/2 are not
   considered valid by common clients, resulting in dropped requests.
   As a convenience, when using the `Plug.Adapters.Conn.Test` adapter, any
   headers that aren't lowercase will raise a `Plug.Conn.InvalidHeaderError`.
 
   Raises a `Plug.Conn.AlreadySentError` if the connection has already been
   `:sent`, `:chunked` or `:upgraded`.
 
   ## Examples
 
       Plug.Conn.prepend_req_headers(conn, [{"accept", "application/json"}])
 
   """
   @spec prepend_req_headers(t, headers) :: t
   def prepend_req_headers(conn, headers)
 
   def prepend_req_headers(%Conn{state: state}, _headers) when state not in @unsent do
     raise AlreadySentError
   end
 
   def prepend_req_headers(%Conn{adapter: adapter, req_headers: req_headers} = conn, headers)
       when is_list(headers) do
     for {key, _value} <- headers do
       validate_req_header!(adapter, key)
     end
 
     %{conn | req_headers: headers ++ req_headers}
   end
 
   @doc """
   Merges a series of request headers into the connection.
 
   The "host" header will be overridden by `conn.host` and should not be set
   with this method. Instead, do `%Plug.Conn{conn | host: value}`.
 
   Because header keys are case-insensitive in both HTTP/1.1 and HTTP/2,
   it is recommended for header keys to be in lowercase, to avoid sending
   duplicate keys in a request.
   Additionally, requests with mixed-case headers served over HTTP/2 are not
   considered valid by common clients, resulting in dropped requests.
   As a convenience, when using the `Plug.Adapters.Conn.Test` adapter, any
   headers that aren't lowercase will raise a `Plug.Conn.InvalidHeaderError`.
 
   ## Example
 
       Plug.Conn.merge_req_headers(conn, [{"accept", "text/plain"}, {"X-1337", "5P34K"}])
 
   """
   @spec merge_req_headers(t, Enum.t()) :: t
   def merge_req_headers(conn, headers)
 
   def merge_req_headers(%Conn{state: state}, _headers) when state not in @unsent do
     raise AlreadySentError
   end
 
   def merge_req_headers(conn, headers) when headers == %{} do
     conn
   end
 
   def merge_req_headers(%Conn{req_headers: current, adapter: adapter} = conn, headers) do
     headers =
       Enum.reduce(headers, current, fn {key, value}, acc
                                        when is_binary(key) and is_binary(value) ->
         validate_req_header!(adapter, key)
         List.keystore(acc, key, 0, {key, value})
       end)
 
     %{conn | req_headers: headers}
   end
 
   @doc """
   Adds a new request header (`key`) if not present, otherwise replaces the
   previous value of that header with `value`.
 
   The "host" header will be overridden by `conn.host` and should not be set
   with this method. Instead, do `%Plug.Conn{conn | host: value}`.
 
   Because header keys are case-insensitive in both HTTP/1.1 and HTTP/2,
   it is recommended for header keys to be in lowercase, to avoid sending
   duplicate keys in a request.
   Additionally, requests with mixed-case headers served over HTTP/2 are not
   considered valid by common clients, resulting in dropped requests.
   As a convenience, when using the `Plug.Adapters.Conn.Test` adapter, any
   headers that aren't lowercase will raise a `Plug.Conn.InvalidHeaderError`.
 
   Raises a `Plug.Conn.AlreadySentError` if the connection has already been
   `:sent`, `:chunked` or `:upgraded`.
 
   ## Examples
 
       Plug.Conn.put_req_header(conn, "accept", "application/json")
 
   """
   @spec put_req_header(t, binary, binary) :: t
   def put_req_header(conn, key, value)
 
   def put_req_header(%Conn{state: state}, _key, _value) when state not in @unsent do
     raise AlreadySentError
   end
 
   def put_req_header(%Conn{adapter: adapter, req_headers: headers} = conn, key, value)
       when is_binary(key) and is_binary(value) do
     validate_req_header!(adapter, key)
     %{conn | req_headers: List.keystore(headers, key, 0, {key, value})}
   end
 
   @doc """
   Deletes a request header if present.
 
   Raises a `Plug.Conn.AlreadySentError` if the connection has already been
   `:sent`, `:chunked` or `:upgraded`.
 
   ## Examples
 
       Plug.Conn.delete_req_header(conn, "content-type")
 
   """
   @spec delete_req_header(t, binary) :: t
   def delete_req_header(conn, key)
 
   def delete_req_header(%Conn{state: state}, _key) when state not in @unsent do
     raise AlreadySentError
   end
 
   def delete_req_header(%Conn{req_headers: headers} = conn, key)
       when is_binary(key) do
     %{conn | req_headers: List.keydelete(headers, key, 0)}
   end
 
   @doc """
   Updates a request header if present, otherwise it sets it to an initial
   value.
 
   Raises a `Plug.Conn.AlreadySentError` if the connection has already been
   `:sent`, `:chunked` or `:upgraded`.
 
   Only the first value of the header `key` is updated if present.
 
   ## Examples
 
       Plug.Conn.update_req_header(
         conn,
         "accept",
         "application/json; charset=utf-8",
         &(&1 <> "; charset=utf-8")
       )
 
   """
   @spec update_req_header(t, binary, binary, (binary -> binary)) :: t
   def update_req_header(conn, key, initial, fun)
 
   def update_req_header(%Conn{state: state}, _key, _initial, _fun) when state not in @unsent do
     raise AlreadySentError
   end
 
   def update_req_header(%Conn{} = conn, key, initial, fun)
       when is_binary(key) and is_binary(initial) and is_function(fun, 1) do
     case get_req_header(conn, key) do
       [] -> put_req_header(conn, key, initial)
       [current | _] -> put_req_header(conn, key, fun.(current))
     end
   end
 
   @doc """
   Returns the values of the response header specified by `key`.
 
   ## Examples
 
       iex> conn = %{conn | resp_headers: [{"content-type", "text/plain"}]}
       iex> get_resp_header(conn, "content-type")
       ["text/plain"]
 
   """
   @spec get_resp_header(t, binary) :: [binary]
   def get_resp_header(%Conn{resp_headers: headers}, key) when is_binary(key) do
     for {^key, value} <- headers, do: value
   end
 
   @doc ~S"""
   Adds a new response header (`key`) if not present, otherwise replaces the
   previous value of that header with `value`.
 
   Because header keys are case-insensitive in both HTTP/1.1 and HTTP/2,
   it is recommended for header keys to be in lowercase, to avoid sending
   duplicate keys in a request.
   Additionally, responses with mixed-case headers served over HTTP/2 are not
   considered valid by common clients, resulting in dropped responses.
   As a convenience, when using the `Plug.Adapters.Conn.Test` adapter, any
   headers that aren't lowercase will raise a `Plug.Conn.InvalidHeaderError`.
 
   Raises a `Plug.Conn.AlreadySentError` if the connection has already been
   `:sent`, `:chunked` or `:upgraded`.
 
   Raises a `Plug.Conn.InvalidHeaderError` if the header value contains control
   feed (`\r`) or newline (`\n`) characters.
 
   ## Examples
 
       Plug.Conn.put_resp_header(conn, "content-type", "application/json")
 
   """
   @spec put_resp_header(t, binary, binary) :: t
   def put_resp_header(%Conn{state: state}, _key, _value) when state not in @unsent do
     raise AlreadySentError
   end
 
   def put_resp_header(%Conn{adapter: adapter, resp_headers: headers} = conn, key, value)
       when is_binary(key) and is_binary(value) do
     validate_header_key_if_test!(adapter, key)
     validate_header_value!(key, value)
     %{conn | resp_headers: List.keystore(headers, key, 0, {key, value})}
   end
 
   @doc ~S"""
   Prepends the list of headers to the connection response headers.
 
   Similar to `put_resp_header` this functions adds a new response header
   (`key`) but rather than replacing the existing one it prepends another header
   with the same `key`.
 
   It is recommended for header keys to be in lowercase, to avoid sending
   duplicate keys in a request.
   Additionally, responses with mixed-case headers served over HTTP/2 are not
   considered valid by common clients, resulting in dropped responses.
   As a convenience, when using the `Plug.Adapters.Conn.Test` adapter, any
   headers that aren't lowercase will raise a `Plug.Conn.InvalidHeaderError`.
 
   Raises a `Plug.Conn.AlreadySentError` if the connection has already been
   `:sent`, `:chunked` or `:upgraded`.
 
   Raises a `Plug.Conn.InvalidHeaderError` if the header value contains control
   feed (`\r`) or newline (`\n`) characters.
 
   ## Examples
 
       Plug.Conn.prepend_resp_headers(conn, [{"content-type", "application/json"}])
 
   """
   @spec prepend_resp_headers(t, headers) :: t
   def prepend_resp_headers(conn, headers)
 
   def prepend_resp_headers(%Conn{state: state}, _headers) when state not in @unsent do
     raise AlreadySentError
   end
 
   def prepend_resp_headers(%Conn{adapter: adapter, resp_headers: resp_headers} = conn, headers)
       when is_list(headers) do
     for {key, value} <- headers do
       validate_header_key_if_test!(adapter, key)
       validate_header_value!(key, value)
     end
 
     %{conn | resp_headers: headers ++ resp_headers}
   end
 
   @doc """
   Merges a series of response headers into the connection.
 
   It is recommended for header keys to be in lowercase, to avoid sending
   duplicate keys in a request.
   Additionally, responses with mixed-case headers served over HTTP/2 are not
   considered valid by common clients, resulting in dropped responses.
   As a convenience, when using the `Plug.Adapters.Conn.Test` adapter, any
   headers that aren't lowercase will raise a `Plug.Conn.InvalidHeaderError`.
 
   ## Example
 
       Plug.Conn.merge_resp_headers(conn, [{"content-type", "text/plain"}, {"X-1337", "5P34K"}])
 
   """
   @spec merge_resp_headers(t, Enum.t()) :: t
   def merge_resp_headers(conn, headers)
 
   def merge_resp_headers(%Conn{state: state}, _headers) when state not in @unsent do
     raise AlreadySentError
   end
 
   def merge_resp_headers(conn, headers) when headers == %{} do
     conn
   end
 
   def merge_resp_headers(%Conn{resp_headers: current, adapter: adapter} = conn, headers) do
     headers =
       Enum.reduce(headers, current, fn {key, value}, acc
                                        when is_binary(key) and is_binary(value) ->
         validate_header_key_if_test!(adapter, key)
         validate_header_value!(key, value)
         List.keystore(acc, key, 0, {key, value})
       end)
 
     %{conn | resp_headers: headers}
   end
 
   @doc """
   Deletes a response header if present.
 
   Raises a `Plug.Conn.AlreadySentError` if the connection has already been
   `:sent`, `:chunked` or `:upgraded`.
 
   ## Examples
 
       Plug.Conn.delete_resp_header(conn, "content-type")
 
   """
   @spec delete_resp_header(t, binary) :: t
   def delete_resp_header(%Conn{state: state}, _key) when state not in @unsent do
     raise AlreadySentError
   end
 
   def delete_resp_header(%Conn{resp_headers: headers} = conn, key)
       when is_binary(key) do
     %{conn | resp_headers: List.keydelete(headers, key, 0)}
   end
 
   @doc """
   Updates a response header if present, otherwise it sets it to an initial
   value.
 
   Raises a `Plug.Conn.AlreadySentError` if the connection has already been
   `:sent`, `:chunked` or `:upgraded`.
 
   Only the first value of the header `key` is updated if present.
 
   ## Examples
 
       Plug.Conn.update_resp_header(
         conn,
         "content-type",
         "application/json; charset=utf-8",
         &(&1 <> "; charset=utf-8")
       )
 
   """
   @spec update_resp_header(t, binary, binary, (binary -> binary)) :: t
   def update_resp_header(conn, key, initial, fun)
 
   def update_resp_header(%Conn{state: state}, _key, _initial, _fun) when state not in @unsent do
     raise AlreadySentError
   end
 
   def update_resp_header(%Conn{} = conn, key, initial, fun)
       when is_binary(key) and is_binary(initial) and is_function(fun, 1) do
     case get_resp_header(conn, key) do
       [] -> put_resp_header(conn, key, initial)
       [current | _] -> put_resp_header(conn, key, fun.(current))
     end
   end
 
   @doc """
   Sets the value of the `"content-type"` response header taking into account the
   `charset`.
 
   If `charset` is `nil`, the value of the `"content-type"` response header won't
   specify a charset.
 
   ## Examples
 
       iex> conn = put_resp_content_type(conn, "application/json")
       iex> get_resp_header(conn, "content-type")
       ["application/json; charset=utf-8"]
 
   """
   @spec put_resp_content_type(t, binary, binary | nil) :: t
   def put_resp_content_type(conn, content_type, charset \\ "utf-8")
 
   def put_resp_content_type(conn, content_type, nil) when is_binary(content_type) do
     put_resp_header(conn, "content-type", content_type)
   end
 
   def put_resp_content_type(conn, content_type, charset)
       when is_binary(content_type) and is_binary(charset) do
     put_resp_header(conn, "content-type", "#{content_type}; charset=#{charset}")
   end
 
   @doc """
   Fetches query parameters from the query string.
 
   Params are decoded as `"x-www-form-urlencoded"` in which key/value pairs
   are separated by `&` and keys are separated from values by `=`.
 
   This function does not fetch parameters from the body. To fetch
   parameters from the body, use the `Plug.Parsers` plug.
 
   ## Options
 
     * `:length` - the maximum query string length. Defaults to `1_000_000` bytes.
       Keep in mind the webserver you are using may have a more strict limit. For
       example, for the Cowboy webserver, [please read](https://hexdocs.pm/plug_cowboy/Plug.Cowboy.html#module-safety-limits).
 
     * `:validate_utf8` - boolean that tells whether or not to validate the keys and
       values of the decoded query string are UTF-8 encoded. Defaults to `true`.
 
   """
   @spec fetch_query_params(t, Keyword.t()) :: t
   def fetch_query_params(conn, opts \\ [])
 
   def fetch_query_params(%Conn{query_params: %Unfetched{}} = conn, opts) do
     %{params: params, query_string: query_string} = conn
     length = Keyword.get(opts, :length, 1_000_000)
 
     if byte_size(query_string) > length do
       raise InvalidQueryError,
         message:
           "maximum query string length is #{length}, got a query with #{byte_size(query_string)} bytes",
         plug_status: 414
     end
 
     query_params =
       Plug.Conn.Query.decode(
         query_string,
         %{},
         Plug.Conn.InvalidQueryError,
         Keyword.get(opts, :validate_utf8, true)
       )
 
     case params do
       %Unfetched{} -> %{conn | query_params: query_params, params: query_params}
       %{} -> %{conn | query_params: query_params, params: Map.merge(query_params, params)}
     end
   end
 
   def fetch_query_params(%Conn{} = conn, _opts) do
     conn
   end
 
   @doc """
   Reads the request body.
 
   This function reads a chunk of the request body up to a given length (specified
   by the `:length` option). If there is more data to be read, then
   `{:more, partial_body, conn}` is returned. Otherwise `{:ok, body, conn}` is
   returned. In case of an error reading the socket, `{:error, reason}` is
   returned as per `:gen_tcp.recv/2`.
 
   Like all functions in this module, the `conn` returned by `read_body` must
   be passed to the next stage of your pipeline and should not be ignored.
 
   In order to, for instance, support slower clients you can tune the
   `:read_length` and `:read_timeout` options. These specify how much time should
   be allowed to pass for each read from the underlying socket.
 
   Because the request body can be of any size, reading the body will only
   work once, as Plug will not cache the result of these operations. If you
   need to access the body multiple times, it is your responsibility to store
   it. Finally keep in mind some plugs like `Plug.Parsers` may read the body,
   so the body may be unavailable after being accessed by such plugs.
 
   This function is able to handle both chunked and identity transfer-encoding
   by default.
 
   ## Options
 
     * `:length` - sets the maximum number of bytes to read from the body on
       every call, defaults to `8_000_000` bytes
     * `:read_length` - sets the amount of bytes to read at one time from the
       underlying socket to fill the chunk, defaults to `1_000_000` bytes
     * `:read_timeout` - sets the timeout for each socket read, defaults to
       `15_000` milliseconds
 
   The values above are not meant to be exact. For example, setting the
   length to `8_000_000` may end up reading some hundred bytes more from
   the socket until we halt.
 
   ## Examples
 
       {:ok, body, conn} = Plug.Conn.read_body(conn, length: 1_000_000)
 
   """
   @spec read_body(t, Keyword.t()) ::
           {:ok, binary, t}
           | {:more, binary, t}
           | {:error, term}
   def read_body(%Conn{adapter: {adapter, state}} = conn, opts \\ []) do
     case adapter.read_req_body(state, opts) do
       {:ok, data, state} ->
         {:ok, data, %{conn | adapter: {adapter, state}}}
 
       {:more, data, state} ->
         {:more, data, %{conn | adapter: {adapter, state}}}
 
       {:error, reason} ->
         {:error, reason}
     end
   end
 
   @doc """
   Reads the headers of a multipart request.
 
   It returns `{:ok, headers, conn}` with the headers or
   `{:done, conn}` if there are no more parts.
 
   Once `read_part_headers/2` is invoked, you may call
   `read_part_body/2` to read the body associated to the headers.
   If `read_part_headers/2` is called instead, the body is automatically
   skipped until the next part headers.
 
   ## Options
 
     * `:length` - sets the maximum number of bytes to read from the body for
       each chunk, defaults to `64_000` bytes
     * `:read_length` - sets the amount of bytes to read at one time from the
       underlying socket to fill the chunk, defaults to `64_000` bytes
     * `:read_timeout` - sets the timeout for each socket read, defaults to
       `5_000` milliseconds
 
   """
   @spec read_part_headers(t, Keyword.t()) :: {:ok, headers, t} | {:done, t}
   def read_part_headers(%Conn{adapter: {adapter, state}} = conn, opts \\ []) do
     opts = opts ++ [length: 64_000, read_length: 64_000, read_timeout: 5000]
 
     case init_multipart(conn) do
       {boundary, buffer} ->
         {data, state} = read_multipart_from_buffer_or_adapter(buffer, adapter, state, opts)
         read_part_headers(conn, data, boundary, adapter, state, opts)
 
       :done ->
         {:done, conn}
     end
   end
 
   defp read_part_headers(conn, data, boundary, adapter, state, opts) do
     case :plug_multipart.parse_headers(data, boundary) do
       {:ok, headers, rest} ->
         {:ok, headers, store_multipart(conn, {boundary, rest}, adapter, state)}
 
       :more ->
         {_, next, state} = next_multipart(adapter, state, opts)
         read_part_headers(conn, data <> next, boundary, adapter, state, opts)
 
       {:more, rest} ->
         {_, next, state} = next_multipart(adapter, state, opts)
         read_part_headers(conn, rest <> next, boundary, adapter, state, opts)
 
       {:done, _} ->
         {:done, store_multipart(conn, :done, adapter, state)}
     end
   end
 
   @doc """
   Reads the body of a multipart request.
 
   Returns `{:ok, body, conn}` if all body has been read,
   `{:more, binary, conn}` otherwise, and `{:done, conn}`
   if there is no more body.
 
   It accepts the same options as `read_body/2`.
   """
   @spec read_part_body(t, Keyword.t()) :: {:ok, binary, t} | {:more, binary, t} | {:done, t}
   def read_part_body(%Conn{adapter: {adapter, state}} = conn, opts) do
     case init_multipart(conn) do
       {boundary, buffer} ->
         # Note we will read the whole length from the socket
         # and then break it apart as necessary.
         length = Keyword.get(opts, :length, 8_000_000)
         {data, state} = read_multipart_from_buffer_or_adapter(buffer, adapter, state, opts)
         read_part_body(conn, data, "", length, boundary, adapter, state, opts)
 
       :done ->
         {:done, conn}
     end
   end
 
   defp read_part_body(%Conn{} = conn, data, acc, length, boundary, adapter, state, _opts)
        when byte_size(acc) > length do
     {:more, acc, store_multipart(conn, {boundary, data}, adapter, state)}
   end
 
   defp read_part_body(%Conn{} = conn, data, acc, length, boundary, adapter, state, opts) do
     case :plug_multipart.parse_body(data, boundary) do
       {:ok, body} ->
         {_, next, state} = next_multipart(adapter, state, opts)
         acc = prepend_unless_empty(acc, body)
         read_part_body(conn, next, acc, length, boundary, adapter, state, opts)
 
       {:ok, body, rest} ->
         {_, next, state} = next_multipart(adapter, state, opts)
         next = prepend_unless_empty(rest, next)
         acc = prepend_unless_empty(acc, body)
         read_part_body(conn, next, acc, length, boundary, adapter, state, opts)
 
       :done ->
         {:ok, acc, store_multipart(conn, {boundary, ""}, adapter, state)}
 
       {:done, body} ->
         {:ok, acc <> body, store_multipart(conn, {boundary, ""}, adapter, state)}
 
       {:done, body, rest} ->
         {:ok, acc <> body, store_multipart(conn, {boundary, rest}, adapter, state)}
     end
   end
 
   @compile {:inline, prepend_unless_empty: 2}
   defp prepend_unless_empty("", body), do: body
   defp prepend_unless_empty(acc, body), do: acc <> body
 
   defp init_multipart(%{private: %{plug_multipart: plug_multipart}}) do
     plug_multipart
   end
 
   defp init_multipart(%{req_headers: req_headers}) do
     {_, content_type} = List.keyfind(req_headers, "content-type", 0)
     {:ok, "multipart", _, keys} = Plug.Conn.Utils.content_type(content_type)
 
     case keys do
       %{"boundary" => boundary} -> {boundary, ""}
       %{} -> :done
     end
   end
 
   defp next_multipart(adapter, state, opts) do
     case adapter.read_req_body(state, opts) do
       {:ok, "", _} -> raise "invalid multipart, body terminated too soon"
       valid -> valid
     end
   end
 
   defp store_multipart(conn, multipart, adapter, state) do
     %{put_in(conn.private[:plug_multipart], multipart) | adapter: {adapter, state}}
   end
 
   defp read_multipart_from_buffer_or_adapter("", adapter, state, opts) do
     {_, data, state} = adapter.read_req_body(state, opts)
     {data, state}
   end
 
   defp read_multipart_from_buffer_or_adapter(buffer, _adapter, state, _opts) do
     {buffer, state}
   end
 
   @doc """
   Sends an informational response to the client.
 
   An informational response, such as an early hint, must happen prior to a response
   being sent. If an informational request is attempted after a response is sent then
   a `Plug.Conn.AlreadySentError` will be raised. Only status codes from 100-199 are valid.
 
   To use inform for early hints send one or more informs with a status of 103.
 
   If the adapter does not support informational responses then this is a noop.
 
   Most HTTP/1.1 clients do not properly support informational responses but some
   proxies require it to support server push for HTTP/2. You can call
   `get_http_protocol/1` to retrieve the protocol and version.
   """
   @spec inform(t, status, Keyword.t()) :: t
   def inform(%Conn{} = conn, status, headers \\ []) do
     status_code = Plug.Conn.Status.code(status)
     adapter_inform(conn, status_code, headers)
     conn
   end
 
   @doc """
   Sends an information response to a client but raises if the adapter does not support inform.
 
   See `inform/3` for more information.
   """
   @spec inform!(t, status, Keyword.t()) :: t
   def inform!(%Conn{adapter: {adapter, _}} = conn, status, headers \\ []) do
     status_code = Plug.Conn.Status.code(status)
 
     case adapter_inform(conn, status_code, headers) do
       :ok ->
         conn
 
       _ ->
         raise "inform is not supported by #{inspect(adapter)}." <>
                 "You should either delete the call to `inform!/3` or switch to an " <>
                 "adapter that does support informational such as Plug.Cowboy"
     end
   end
 
   defp adapter_inform(_conn, status, _headers)
        when not (status >= 100 and status <= 199 and is_integer(status)) do
     raise ArgumentError, "inform expects a status code between 100 and 199, got: #{status}"
   end
 
   defp adapter_inform(%Conn{state: state}, _status, _headers)
        when state not in @unsent do
     raise AlreadySentError
   end
 
   defp adapter_inform(%Conn{adapter: {adapter, payload}}, status, headers),
     do: adapter.inform(payload, status, headers)
 
   @doc """
   Request a protocol upgrade from the underlying adapter.
 
   The precise semantics of an upgrade are deliberately left unspecified here in order to
   support arbitrary upgrades, even to protocols which may not exist today. The primary intent of
   this function is solely to allow an application to issue an upgrade request, not to manage how
   a given protocol upgrade takes place or what APIs the application must support in order to serve
   this updated protocol. For details in this regard, consult the documentation of the underlying
   adapter (such a Plug.Cowboy or Bandit).
 
   Takes an argument describing the requested upgrade (for example, `:websocket`), and an argument
   which contains arbitrary data which the underlying adapter is expected to interpret in the
   context of the requested upgrade.
 
   If the upgrade is accepted by the adapter, the returned `Plug.Conn` will have a `state` of
   `:upgraded`. This state is considered equivalently to a 'sent' state, and is subject to the same
   limitation on subsequent mutating operations. Note that there is no guarantee or expectation
   that the actual upgrade process is undertaken within this function; it is entirely possible that
   the server will only do the actual upgrade later in the connection lifecycle.
 
   If the adapter does not support the requested upgrade then this is a noop and the returned 
   `Plug.Conn` will be unchanged. The application can detect this and operate on the conn as it
   normally would in order to indicate an upgrade failure to the client.
   """
   @spec upgrade_adapter(t, atom, term) :: t
   def upgrade_adapter(%Conn{adapter: {adapter, payload}, state: state} = conn, protocol, args)
       when state in @unsent do
     case adapter.upgrade(payload, protocol, args) do
       {:ok, payload} ->
         %{conn | adapter: {adapter, payload}, state: :upgraded}
 
       {:error, :not_supported} ->
         raise ArgumentError, "upgrade to #{protocol} not supported by #{inspect(adapter)}"
     end
   end
 
   def upgrade_adapter(_conn, _protocol, _args) do
     raise AlreadySentError
   end
 
   @doc """
   Pushes a resource to the client.
 
   Server pushes must happen prior to a response being sent. If a server
   push is attempted after a response is sent then a `Plug.Conn.AlreadySentError`
   will be raised.
 
   If the adapter does not support server push then this is a noop.
 
   Note that certain browsers (such as Google Chrome) will not accept a pushed
   resource if your certificate is not trusted. In the case of Chrome this means
   a valid cert with a SAN. See https://www.chromestatus.com/feature/4981025180483584
   """
   @deprecated "Most browsers and clients have removed push support"
   @spec push(t, String.t(), Keyword.t()) :: t
   def push(%Conn{} = conn, path, headers \\ []) do
     adapter_push(conn, path, headers)
     conn
   end
 
   @doc """
   Pushes a resource to the client but raises if the adapter
   does not support server push.
   """
   @deprecated "Most browsers and clients have removed push support"
   @spec push!(t, String.t(), Keyword.t()) :: t
   def push!(%Conn{adapter: {adapter, _}} = conn, path, headers \\ []) do
     case adapter_push(conn, path, headers) do
       :ok ->
         conn
 
       _ ->
         raise "server push not supported by #{inspect(adapter)}." <>
                 "You should either delete the call to `push!/3` or switch to an " <>
                 "adapter that does support server push such as Plug.Cowboy."
     end
   end
 
   defp adapter_push(%Conn{state: state}, _path, _headers)
        when state not in @unsent do
     raise AlreadySentError
   end
 
   defp adapter_push(%Conn{adapter: {adapter, payload}}, path, headers) do
     headers =
       case List.keyfind(headers, "accept", 0) do
         nil -> [{"accept", MIME.from_path(path)} | headers]
         _ -> headers
       end
 
     adapter.push(payload, path, headers)
   end
 
   @doc """
   Fetches cookies from the request headers.
 
   ## Options
 
     * `:signed` - a list of one or more cookies that are signed and must
       be verified accordingly
 
     * `:encrypted` - a list of one or more cookies that are encrypted and
       must be decrypted accordingly
 
   See `put_resp_cookie/4` for more information.
   """
   @spec fetch_cookies(t, Keyword.t()) :: t
   def fetch_cookies(conn, opts \\ [])
 
   def fetch_cookies(%Conn{req_cookies: %Unfetched{}} = conn, opts) do
     %{resp_cookies: resp_cookies, req_headers: req_headers} = conn
 
     req_cookies =
       for {"cookie", cookie} <- req_headers,
           kv <- Plug.Conn.Cookies.decode(cookie),
           into: %{},
           do: kv
 
     cookies =
       Enum.reduce(resp_cookies, req_cookies, fn {key, opts}, acc ->
         if value = Map.get(opts, :value) do
           Map.put(acc, key, value)
         else
           Map.delete(acc, key)
         end
       end)
 
     fetch_cookies(%{conn | req_cookies: req_cookies, cookies: cookies}, opts)
   end
 
   def fetch_cookies(%Conn{} = conn, []) do
     conn
   end
 
   def fetch_cookies(%Conn{} = conn, opts) do
     %{req_cookies: req_cookies, cookies: cookies, secret_key_base: secret_key_base} = conn
 
     cookies =
       verify_or_decrypt(
         opts[:signed],
         req_cookies,
         cookies,
         &Plug.Crypto.verify(secret_key_base, &1 <> "_cookie", &2, keys: Plug.Keys)
       )
 
     cookies =
       verify_or_decrypt(
         opts[:encrypted],
         req_cookies,
         cookies,
         &Plug.Crypto.decrypt(secret_key_base, &1 <> "_cookie", &2, keys: Plug.Keys)
       )
 
     %{conn | cookies: cookies}
   end
 
   defp verify_or_decrypt(names, req_cookies, cookies, fun) do
     names
     |> List.wrap()
     |> Enum.reduce(cookies, fn name, acc ->
       if value = req_cookies[name] do
         case fun.(name, value) do
           {:ok, verified_value} -> Map.put(acc, name, verified_value)
           {_, _} -> Map.delete(acc, name)
         end
       else
         acc
       end
     end)
   end
 
   @doc """
   Puts a response cookie in the connection.
 
   If the `:sign` or `:encrypt` flag are given, then the cookie
   value can be any term.
 
   If the cookie is not signed nor encrypted, then the value must be a binary.
   Note the value is not automatically escaped. Therefore if you want to store
   values with non-alphanumeric characters, you must either sign or encrypt
   the cookie or consider explicitly escaping the cookie value by using a
   function such as `Base.encode64(value, padding: false)` when writing and
   `Base.decode64(encoded, padding: false)` when reading the cookie.
   It is important for padding to be disabled since `=` is not a valid
   character in cookie values.
 
   ## Signing and encrypting cookies
 
   This function allows you to automatically sign and encrypt cookies.
   When signing or encryption is enabled, then any Elixir value can be
   stored in the cookie (except anonymous functions for security reasons).
   Once a value is signed or encrypted, you must also call `fetch_cookies/2`
   with the name of the cookies that are either signed or encrypted.
 
   To sign, you would do:
 
       put_resp_cookie(conn, "my-cookie", %{user_id: user.id}, sign: true)
 
   and then:
 
       fetch_cookies(conn, signed: ~w(my-cookie))
 
   To encrypt, you would do:
 
       put_resp_cookie(conn, "my-cookie", %{user_id: user.id}, encrypt: true)
 
   and then:
 
       fetch_cookies(conn, encrypted: ~w(my-cookie))
 
   By default a signed or encrypted cookie is only valid for a day, unless
   a `:max_age` is specified.
 
   The signing and encryption keys are derived from the connection's
   `secret_key_base` using a salt that is built by appending "_cookie" to
   the cookie name. Care should be taken not to derive other keys using
   this value as the salt. Similarly do not use the same cookie name to
   store different values with distinct purposes.
 
   ## Options
 
     * `:domain` - the domain the cookie applies to
     * `:max_age` - the cookie max-age, in seconds. Providing a value for this
       option will set both the _max-age_ and _expires_ cookie attributes.
     * `:path` - the path the cookie applies to
     * `:http_only` - when `false`, the cookie is accessible beyond HTTP
     * `:secure` - if the cookie must be sent only over https. Defaults
       to true when the connection is HTTPS
     * `:extra` - string to append to cookie. Use this to take advantage of
       non-standard cookie attributes.
     * `:sign` - when true, signs the cookie
     * `:encrypt` - when true, encrypts the cookie
     * `:same_site` - set the cookie SameSite attribute to a string value.
       If no string value is set, the attribute is omitted.
 
   """
   @spec put_resp_cookie(t, binary, any(), Keyword.t()) :: t
   def put_resp_cookie(%Conn{} = conn, key, value, opts \\ [])
       when is_binary(key) and is_list(opts) do
     %{resp_cookies: resp_cookies, scheme: scheme} = conn
     {to_send_value, opts} = maybe_sign_or_encrypt_cookie(conn, key, value, opts)
     cookie = [{:value, to_send_value} | opts] |> Map.new() |> maybe_secure_cookie(scheme)
     resp_cookies = Map.put(resp_cookies, key, cookie)
     update_cookies(%{conn | resp_cookies: resp_cookies}, &Map.put(&1, key, value))
   end
 
   defp maybe_sign_or_encrypt_cookie(conn, key, value, opts) do
     {sign?, opts} = Keyword.pop(opts, :sign, false)
     {encrypt?, opts} = Keyword.pop(opts, :encrypt, false)
 
     case {sign?, encrypt?} do
       {true, true} ->
         raise ArgumentError,
               ":encrypt automatically implies :sign. Please pass only one or the other"
 
       {true, false} ->
         {Plug.Crypto.sign(conn.secret_key_base, key <> "_cookie", value, max_age(opts)), opts}
 
       {false, true} ->
         {Plug.Crypto.encrypt(conn.secret_key_base, key <> "_cookie", value, max_age(opts)), opts}
 
       {false, false} when is_binary(value) ->
         {value, opts}
 
       {false, false} ->
         raise ArgumentError, "cookie value must be a binary unless the cookie is signed/encrypted"
     end
   end
 
   defp max_age(opts) do
     [keys: Plug.Keys, max_age: Keyword.get(opts, :max_age, 86400)]
   end
 
   defp maybe_secure_cookie(cookie, :https), do: Map.put_new(cookie, :secure, true)
   defp maybe_secure_cookie(cookie, _), do: cookie
 
   @doc """
   Deletes a response cookie.
 
   Deleting a cookie requires the same options as to when the cookie was put.
   Check `put_resp_cookie/4` for more information.
   """
   @spec delete_resp_cookie(t, binary, Keyword.t()) :: t
   def delete_resp_cookie(%Conn{} = conn, key, opts \\ [])
       when is_binary(key) and is_list(opts) do
     %{resp_cookies: resp_cookies, scheme: scheme} = conn
     opts = opts ++ [universal_time: @epoch, max_age: 0]
     cookie = opts |> Map.new() |> maybe_secure_cookie(scheme)
     resp_cookies = Map.put(resp_cookies, key, cookie)
     update_cookies(%{conn | resp_cookies: resp_cookies}, &Map.delete(&1, key))
   end
 
   @doc """
   Fetches the session from the session store. Will also fetch cookies.
   """
   @spec fetch_session(t, Keyword.t()) :: t
   def fetch_session(conn, opts \\ [])
 
   def fetch_session(%Conn{private: private} = conn, _opts) do
     case Map.fetch(private, :plug_session_fetch) do
       {:ok, :done} -> conn
       {:ok, fun} -> conn |> fetch_cookies |> fun.()
       :error -> raise ArgumentError, "cannot fetch session without a configured session plug"
     end
   end
 
   @doc """
   Puts the specified `value` in the session for the given `key`.
 
   The key can be a string or an atom, where atoms are
   automatically converted to strings. Can only be invoked
   on unsent `conn`s. Will raise otherwise.
   """
   @spec put_session(t, String.t() | atom, any) :: t
   def put_session(%Conn{state: state}, _key, _value) when state not in @unsent,
     do: raise(AlreadySentError)
 
   def put_session(conn, key, value) when is_atom(key) or is_binary(key) do
     put_session(conn, &Map.put(&1, session_key(key), value))
   end
 
   @doc """
   Returns session value for the given `key`. If `key`
   is not set, `nil` is returned.
 
   The key can be a string or an atom, where atoms are
   automatically converted to strings.
   """
   @spec get_session(t, String.t() | atom) :: any
   def get_session(conn, key) when is_atom(key) or is_binary(key) do
     conn |> get_session |> Map.get(session_key(key))
   end
 
   @doc """
   Returns the whole session.
 
   Although `get_session/2` and `put_session/3` allow atom keys,
   they are always normalized to strings. So this function always
   returns a map with string keys.
 
   Raises if the session was not yet fetched.
   """
   @spec get_session(t) :: %{optional(String.t()) => any}
   def get_session(%Conn{private: private}) do
     if session = Map.get(private, :plug_session) do
       session
     else
       raise ArgumentError, "session not fetched, call fetch_session/2"
     end
   end
 
   @doc """
   Deletes `key` from session.
 
   The key can be a string or an atom, where atoms are
   automatically converted to strings.
   """
   @spec delete_session(t, String.t() | atom) :: t
   def delete_session(%Conn{state: state}, _key) when state not in @unsent,
     do: raise(AlreadySentError)
 
   def delete_session(conn, key) when is_atom(key) or is_binary(key) do
     put_session(conn, &Map.delete(&1, session_key(key)))
   end
 
   @doc """
   Clears the entire session.
 
   This function removes every key from the session, clearing the session.
 
   Note that, even if `clear_session/1` is used, the session is still sent to the
   client. If the session should be effectively *dropped*, `configure_session/2`
   should be used with the `:drop` option set to `true`.
   """
   @spec clear_session(t) :: t
   def clear_session(conn) do
     put_session(conn, fn _existing -> Map.new() end)
   end
 
   @doc """
   Configures the session.
 
   ## Options
 
     * `:renew` - When `true`, generates a new session id for the cookie
     * `:drop` - When `true`, drops the session, a session cookie will not be included in the
       response
     * `:ignore` - When `true`, ignores all changes made to the session in this request cycle
 
   ## Examples
 
       configure_session(conn, renew: true)
 
   """
   @spec configure_session(t, Keyword.t()) :: t
   def configure_session(conn, opts)
 
   def configure_session(%Conn{state: state}, _opts) when state not in @unsent,
     do: raise(AlreadySentError)
 
   def configure_session(conn, opts) do
     # Ensure the session is available.
     _ = get_session(conn)
 
     cond do
       opts[:renew] -> put_private(conn, :plug_session_info, :renew)
       opts[:drop] -> put_private(conn, :plug_session_info, :drop)
       opts[:ignore] -> put_private(conn, :plug_session_info, :ignore)
       true -> conn
     end
   end
 
   @doc ~S"""
   Registers a callback to be invoked before the response is sent.
 
   Callbacks are invoked in the reverse order they are defined (callbacks
   defined first are invoked last).
 
   ## Examples
 
   To log the status of response being sent:
 
       require Logger
 
       Plug.Conn.register_before_send(conn, fn conn ->
         Logger.info("Sent a #{conn.status} response")
         conn
       end)
 
   """
   @spec register_before_send(t, (t -> t)) :: t
   def register_before_send(conn, callback)
 
   def register_before_send(%Conn{state: state}, _callback)
       when state not in @unsent do
     raise AlreadySentError
   end
 
   def register_before_send(%Conn{} = conn, callback)
       when is_function(callback, 1) do
     update_in(conn.private[:before_send], &[callback | &1 || []])
   end
 
   @doc """
   Halts the Plug pipeline by preventing further plugs downstream from being
   invoked. See the docs for `Plug.Builder` for more information on halting a
   Plug pipeline.
   """
   @spec halt(t) :: t
   def halt(%Conn{} = conn) do
     %{conn | halted: true}
   end
 
   @doc """
   Returns the full request URL.
   """
   def request_url(%Conn{} = conn) do
     IO.iodata_to_binary([
       to_string(conn.scheme),
       "://",
       conn.host,
       request_url_port(conn.scheme, conn.port),
       conn.request_path,
       request_url_qs(conn.query_string)
     ])
   end
 
   ## Helpers
 
   defp run_before_send(%Conn{private: private} = conn, new) do
     conn = Enum.reduce(private[:before_send] || [], %{conn | state: new}, & &1.(&2))
 
     if conn.state != new do
       raise ArgumentError, "cannot send/change response from run_before_send callback"
     end
 
     %{conn | resp_headers: merge_headers(conn.resp_headers, conn.resp_cookies)}
   end
 
   defp merge_headers(headers, cookies) do
     Enum.reduce(cookies, headers, fn {key, opts}, acc ->
       value =
         key
         |> Plug.Conn.Cookies.encode(opts)
         |> verify_cookie!(key)
 
       [{"set-cookie", value} | acc]
     end)
   end
 
   defp verify_cookie!(cookie, key) when byte_size(cookie) > 4096 do
     raise Plug.Conn.CookieOverflowError,
           "cookie named #{inspect(key)} exceeds maximum size of 4096 bytes"
   end
 
   defp verify_cookie!(cookie, _key) do
     validate_header_value!("set-cookie", cookie)
   end
 
   defp update_cookies(%Conn{state: state}, _fun) when state not in @unsent do
     raise AlreadySentError
   end
 
   defp update_cookies(%Conn{cookies: %Unfetched{}} = conn, _fun), do: conn
   defp update_cookies(%Conn{cookies: cookies} = conn, fun), do: %{conn | cookies: fun.(cookies)}
 
   defp session_key(binary) when is_binary(binary), do: binary
   defp session_key(atom) when is_atom(atom), do: Atom.to_string(atom)
 
   defp put_session(conn, fun) do
     private =
       conn.private
       |> Map.put(:plug_session, fun.(get_session(conn)))
       |> Map.put_new(:plug_session_info, :write)
 
     %{conn | private: private}
   end
 
   # host is an HTTP header, but if you store it in the main list it will be
   # overridden by conn.host.
   defp validate_req_header!(_adapter, "host") do
     raise InvalidHeaderError,
           "set the host header with %Plug.Conn{conn | host: \"example.com\"}"
   end
 
   defp validate_req_header!(adapter, key), do: validate_header_key_if_test!(adapter, key)
 
   defp validate_header_key_if_test!({Plug.Adapters.Test.Conn, _}, key) do
     if Application.fetch_env!(:plug, :validate_header_keys_during_test) and
          not valid_header_key?(key) do
       raise InvalidHeaderError, "header key is not lowercase: " <> inspect(key)
     end
   end
 
   defp validate_header_key_if_test!(_adapter, _key) do
     :ok
   end
 
   # Any string containing an UPPERCASE char is not valid.
   defp valid_header_key?(<<h, _::binary>>) when h in ?A..?Z, do: false
   defp valid_header_key?(<<_, t::binary>>), do: valid_header_key?(t)
   defp valid_header_key?(<<>>), do: true
   defp valid_header_key?(_), do: false
 
   defp validate_header_value!(key, value) do
     case :binary.match(value, ["\n", "\r"]) do
       {_, _} ->
         raise InvalidHeaderError,
               "value for header #{inspect(key)} contains control feed (\\r) or newline " <>
                 "(\\n): #{inspect(value)}"
 
       :nomatch ->
         value
     end
   end
 
   defp request_url_port(:http, 80), do: ""
   defp request_url_port(:https, 443), do: ""
   defp request_url_port(_, port), do: [?:, Integer.to_string(port)]
 
   defp request_url_qs(""), do: ""
   defp request_url_qs(qs), do: [??, qs]
 end
 
 defimpl Inspect, for: Plug.Conn do
   def inspect(conn, opts) do
     conn
     |> no_secret_key_base()
     |> no_adapter_data(opts)
     |> Inspect.Any.inspect(opts)
   end
 
   defp no_secret_key_base(%{secret_key_base: nil} = conn), do: conn
   defp no_secret_key_base(conn), do: %{conn | secret_key_base: :...}
 
   defp no_adapter_data(conn, %{limit: :infinity}), do: conn
   defp no_adapter_data(%{adapter: {adapter, _}} = conn, _), do: %{conn | adapter: {adapter, :...}}
 end
 
 defimpl Collectable, for: Plug.Conn do
   def into(conn) do
     IO.warn(
       "using Enum.into/2 for conn is deprecated, use Plug.Conn.chunk/2 " <>
         "and Enum.reduce_while/3 instead (see the Plug.Conn.chunk/2 docs for an example)"
     )
 
     fun = fn
       conn, {:cont, x} ->
         {:ok, conn} = Plug.Conn.chunk(conn, x)
         conn
 
       conn, _ ->
         conn
     end
 
     {conn, fun}
   end
 end