refactor(http2): delegate HTTP/2 to erlang_h2 library

NEWS.md

@@ -5,6 +5,20 @@ UNRELEASED
 
 ### Refactor
 
+- HTTP/2 is now delegated to the `erlang_h2` library (hex `h2` 0.3.0).
+  Hackney no longer ships its own HTTP/2 framing, HPACK codec, or
+  connection/stream state machine:
+  - `hackney_http2.erl`, `hackney_http2_machine.erl`, `hackney_hpack.erl`
+    and the `hackney_hpack_huffman*` headers have been removed.
+  - `hackney_conn.erl` now starts an `h2_connection` gen_statem on the
+    post-ALPN socket, transfers socket ownership, and translates
+    `{h2, Conn, Event}` owner-messages into hackney's sync replies or
+    `{hackney_response, Ref, _}` async events.
+  - Server push handling (RFC 7540 §8.2, deprecated) is no longer exposed.
+    The `enable_push` option is a no-op.
+  - Public user-facing API is unchanged: `hackney:request/5`, streaming
+    async responses, pooled HTTP/2 connections, and `request_async` all
+    behave as before.
 - HTTP/3 is now delegated to the `erlang_quic` library's `quic_h3` module
   (hex `quic` 1.0.0). Hackney no longer ships its own HTTP/3 framing,
   QPACK codec, control-stream or unidirectional-stream handling:

guides/http2_guide.md

@@ -13,7 +13,8 @@ Hackney supports HTTP/2 with automatic protocol negotiation via ALPN (Applicatio
 - **Multiplexing** - Multiple requests share a single connection
 - **Header compression** - HPACK compression reduces overhead
 - **Flow control** - Automatic window management
-- **Server push** - Optional support for server-initiated streams
+- **Delegated to `erlang_h2`** - the underlying HTTP/2 stack is the `h2`
+  hex package; hackney exposes the same request API it always did
 
 ## Quick Start
 
@@ -243,20 +244,10 @@ end) || Path <- Paths].
 
 ## Server Push
 
-Server push allows the server to send resources before the client requests them. By default, hackney rejects server pushes. To handle them:
-
-```erlang
-%% Using low-level API with push handler
-{ok, Conn} = hackney_conn:start_link(#{
-    host => "example.com",
-    port => 443,
-    transport => hackney_ssl,
-    enable_push => self()  %% Receive push notifications
-}),
-
-%% Push notifications arrive as messages:
-%% {hackney_push, ConnPid, {PromisedStreamId, Method, Scheme, Authority, Path, Headers}}
-```
+Server push (RFC 7540 §8.2) is deprecated and no longer supported by the
+underlying `h2` library. The `enable_push` option is accepted for
+backwards-compatibility but is a no-op; pushes from the server are silently
+refused.
 
 ## Flow Control
 
@@ -275,11 +266,14 @@ HTTP/2 specific errors:
 case hackney:get(URL) of
     {ok, Status, Headers, Body} ->
         ok;
-    {error, {h2_connection_error, ErrorCode}} ->
-        %% HTTP/2 connection-level error
-        io:format("HTTP/2 error: ~p~n", [ErrorCode]);
-    {error, closed} ->
-        %% Connection closed (GOAWAY received)
+    {error, {goaway, ErrorCode}} ->
+        %% Peer sent GOAWAY
+        io:format("HTTP/2 GOAWAY: ~p~n", [ErrorCode]);
+    {error, {stream_error, ErrorCode}} ->
+        %% Peer sent RST_STREAM for this request
+        io:format("HTTP/2 stream reset: ~p~n", [ErrorCode]);
+    {error, {closed, _Reason}} ->
+        %% Connection closed
         ok;
     {error, Reason} ->
         io:format("Error: ~p~n", [Reason])

rebar.config

@@ -59,6 +59,8 @@
 {deps, [
         %% Pure Erlang QUIC + HTTP/3 stack
         {quic, "1.0.0"},
+        %% Pure Erlang HTTP/2 stack
+        {h2, "0.3.0"},
         {idna, "~>7.1.0"},
         {mimerl, "~>1.4"},
         {certifi, "~>2.16.0"},
@@ -115,7 +117,7 @@
   {exclude_mods, [hackney_metrics_prometheus]},
   {base_plt_apps, [erts, stdlib, kernel, crypto, runtime_tools]},
   {plt_apps, top_level_deps},
-  {plt_extra_apps, [quic]},
+  {plt_extra_apps, [quic, h2]},
   {plt_location, local},
   {plt_prefix, "hackney"},
   {base_plt_location, "."},

src/hackney.app.src

@@ -15,7 +15,8 @@
       idna,
       mimerl,
       certifi,
-      ssl_verify_fun]},
+      ssl_verify_fun,
+      h2]},
     {included_applications, []},
     {mod, { hackney_app, []}},
     {env, [{timeout, 150000},