@callback chunk(payload(), body :: Plug.Conn.body()) ::
  :ok | {:ok, sent_body :: binary() | nil, payload()} | {:error, term()}

Sends a chunk in the chunked response.

If the request has method "HEAD", the adapter should not send the response to the client.

Webservers are advised to return nil as the sent_body, since the complete sent body depends on the sum of all calls to this function. However, the test implementation tracks the overall body and payload so it can be used during testing.

@callback get_http_protocol(payload()) :: http_protocol()

Returns the HTTP protocol and its version.

@callback get_peer_data(payload()) :: peer_data()

Returns peer information such as the address, port and ssl cert.

@callback inform(payload(), status :: Plug.Conn.status(), headers :: Keyword.t()) ::
  :ok | {:ok, payload()} | {:error, term()}

Send an informational response to the client.

If the adapter does not support inform, then {:error, :not_supported} should be returned.

@callback push(payload(), path :: String.t(), headers :: Keyword.t()) ::
  :ok | {:error, term()}

Push a resource to the client.

If the adapter does not support server push then {:error, :not_supported} should be returned.

@callback read_req_body(payload(), options :: Keyword.t()) ::
  {:ok, data :: binary(), payload()}
  | {:more, data :: binary(), payload()}
  | {:error, term()}

Reads the request body.

Read the docs in Plug.Conn.read_body/2 for the supported options and expected behaviour.

@callback send_chunked(
  payload(),
  status :: Plug.Conn.status(),
  headers :: Plug.Conn.headers()
) ::
  {:ok, sent_body :: binary() | nil, payload()}

Sends the given status, headers as the beginning of a chunked response to the client.

Webservers are advised to return nil as the sent_body, since this function does not actually produce a body. However, the test implementation returns an empty binary as the body in order to be consistent with the built-up body returned by subsequent calls to the test implementation's chunk/2 function

@callback send_file(
  payload(),
  status :: Plug.Conn.status(),
  headers :: Plug.Conn.headers(),
  file :: binary(),
  offset :: integer(),
  length :: integer() | :all
) :: {:ok, sent_body :: binary() | nil, payload()}

Sends the given status, headers and file as a response back to the client.

If the request has method "HEAD", the adapter should not send the response to the client.

Webservers are advised to return nil as the sent_body, as the body can no longer be manipulated. However, the test implementation returns the actual body so it can be used during testing.

@callback send_resp(
  payload(),
  status :: Plug.Conn.status(),
  headers :: Plug.Conn.headers(),
  body :: Plug.Conn.body()
) :: {:ok, sent_body :: binary() | nil, payload()}

Sends the given status, headers and body as a response back to the client.

If the request has method "HEAD", the adapter should not send the response to the client.

Webservers are advised to return nil as the sent_body, as the body can no longer be manipulated. However, the test implementation returns the actual body so it can be used during testing.

@callback upgrade(payload(), protocol :: atom(), opts :: term()) ::
  {:ok, payload()} | {:error, term()}

Attempt to upgrade the connection with the client.

If the adapter does not support the indicated upgrade, then {:error, :not_supported} should be be returned.

If the adapter supports the indicated upgrade but is unable to proceed with it (due to a negotiation error, invalid opts being passed to this function, or some other reason), then an arbitrary error may be returned. Note that an adapter does not need to process the actual upgrade within this function; it is a wholly supported failure mode for an adapter to attempt the upgrade process later in the connection lifecycle and fail at that point.