add docs

lib/feed_reader/transport.ex

@@ -2,9 +2,44 @@ defmodule FeedReader.Transport do
   @moduledoc """
   Injectable HTTP transport boundary used by remote feed operations.
 
-  Implementations return raw response chunks as an enumerable. The response is
-  intentionally not decompressed here; the download layer owns decoded-body
-  checksums and decompression.
+  Implementations return raw response chunks as an enumerable. They must not
+  automatically decompress the response: the downloader owns decompression,
+  decoded-byte accounting, and checksums. `final_url` must include redirects;
+  a 304 response has an empty body and is returned without downloading content.
+
+  `request/2` receives the full option list passed to the remote downloader.
+  The default `FeedReader.ReqTransport` recognizes these transport options:
+
+  - `:http_client` — module implementing this behaviour, selected by
+    `Transport.request/2`; defaults to `FeedReader.ReqTransport`. The selected
+    module receives the full options list, including this option.
+  - `:http_options` — Req request options forwarded to `Req.run/2`. The transport
+    enforces streamed raw bodies, disables automatic decompression, and sets the
+    retry and redirect options described below; redirects are limited to five.
+    A custom `:headers` list replaces the defaults. Otherwise, headers default to
+    `user-agent: feedex/1` and `accept-encoding: gzip, deflate, identity`.
+  - `:previous_etag` — if binary, sent verbatim as `If-None-Match` on the
+    GET request. It replaces any existing `If-None-Match` header. A 304 response
+    returns `{:ok, response}` with `status: 304` and `body: []`; no HEAD request
+    is made.
+  - `:http_retry_attempts` — non-negative number of retries after the initial
+    request; defaults to `5`. The Req transport retries status 400 and 500 and
+    most transport errors. TLS alerts and redirect failures are terminal.
+  - `:retry_delay` — non-negative delay in milliseconds or an arity-one Req
+    retry-delay function; defaults to `0`. Invalid values use the default.
+
+  The downloader also passes these recognized options through to custom
+  transports, but consumes them itself rather than using them to configure HTTP:
+
+  - `:previous_checksum` — decoded-body checksum used for the unchanged result.
+  - `:max_compressed_bytes` — maximum total raw response bytes, default 50 MiB.
+  - `:max_decoded_bytes` — maximum total decompressed bytes, default 50 MiB.
+  - `:max_temp_bytes` — maximum bytes written to the owned temporary file,
+    default 50 MiB.
+  - `:max_expansion_ratio` — maximum decoded-to-compressed byte ratio, default
+    `100`.
+
+  Other options remain available to injected transport implementations.
   """
 
   @type response :: %{
@@ -14,10 +49,11 @@ defmodule FeedReader.Transport do
           required(:body) => Enumerable.t()
         }
 
+  @doc "Performs a GET and returns raw response chunks and response metadata."
   @callback request(url :: String.t(), opts :: keyword()) ::
               {:ok, response()} | {:error, term()}
 
-  @doc "Dispatches a request to the configured or default HTTP client."
+  @doc "Dispatches the request to `:http_client`, or `FeedReader.ReqTransport` by default."
   @spec request(String.t(), keyword()) :: {:ok, response()} | {:error, term()}
   def request(url, opts \\ []) do
     client = Keyword.get(opts, :http_client, FeedReader.ReqTransport)