From f5bf9642d80ad20bdff2d79130346f510bc35da4 Mon Sep 17 00:00:00 2001
From: Adrian Chaves <adrian@zyte.com>
Date: Mon, 10 Aug 2026 11:31:03 +0200
Subject: [PATCH] Fail HTTP/2 requests when the connection cannot be
 established

---
 scrapy/core/http2/agent.py                    | 37 ++++++++++++++-----
 .../test_downloader_handler_twisted_http2.py  | 16 --------
 2 files changed, 28 insertions(+), 25 deletions(-)

diff --git a/scrapy/core/http2/agent.py b/scrapy/core/http2/agent.py
index 0f04c1862..8850a512d 100644
--- a/scrapy/core/http2/agent.py
+++ b/scrapy/core/http2/agent.py
@@ -48,11 +48,7 @@ class H2ConnectionPool:
     ) -> Deferred[H2ClientProtocol]:
         if key in self._pending_requests:
             # Received a request while connecting to remote
-            # Create a deferred which will fire with the H2ClientProtocol
-            # instance
-            d: Deferred[H2ClientProtocol] = Deferred()
-            self._pending_requests[key].append(d)
-            return d
+            return self._pending_request(key)
 
         # Check if we already have a usable connection to the remote, moving
         # it to the end so that the pool stays ordered from least to most
@@ -71,8 +67,7 @@ class H2ConnectionPool:
         self, key: ConnectionKeyT, uri: URI, endpoint: HostnameEndpoint
     ) -> Deferred[H2ClientProtocol]:
         self._enforce_limit()
-        pending_requests: deque[Deferred[H2ClientProtocol]] = deque()
-        self._pending_requests[key] = pending_requests
+        self._pending_requests[key] = deque()
 
         conn_lost_deferred: Deferred[list[BaseException]] = Deferred()
 
@@ -83,9 +78,25 @@ class H2ConnectionPool:
             tls_verbose_logging=self._tls_verbose_logging,
         )
         conn_d = endpoint.connect(factory)
-        conn_d.addCallback(self.put_connection, key, conn_lost_deferred)
+        d = self._pending_request(key)
+        conn_d.addCallbacks(
+            self.put_connection,
+            self._connection_failed,
+            callbackArgs=(key, conn_lost_deferred),
+            errbackArgs=(key,),
+        )
+        return d
 
-        d: Deferred[H2ClientProtocol] = Deferred()
+    def _pending_request(self, key: ConnectionKeyT) -> Deferred[H2ClientProtocol]:
+        """Return a deferred that fires with the connection being established
+        for *key*.
+
+        Cancelling it takes it out of the queue, so that the connection, once
+        established or found unreachable, only fires the deferreds of the
+        requests that are still waiting for it.
+        """
+        pending_requests = self._pending_requests[key]
+        d: Deferred[H2ClientProtocol] = Deferred(pending_requests.remove)
         pending_requests.append(d)
         return d
 
@@ -127,6 +138,14 @@ class H2ConnectionPool:
 
         return conn
 
+    def _connection_failed(self, failure: Failure, key: ConnectionKeyT) -> None:
+        """Fail the requests waiting for a connection that could not be
+        established, and let a later request try to connect again."""
+        pending_requests = self._pending_requests.pop(key)
+        while pending_requests:
+            d = pending_requests.popleft()
+            d.errback(failure)
+
     def _remove_connection(
         self, errors: list[BaseException], key: ConnectionKeyT, conn: H2ClientProtocol
     ) -> None:
diff --git a/tests/test_downloader_handler_twisted_http2.py b/tests/test_downloader_handler_twisted_http2.py
index 3f6d1adb1..f14545b7e 100644
--- a/tests/test_downloader_handler_twisted_http2.py
+++ b/tests/test_downloader_handler_twisted_http2.py
@@ -82,22 +82,6 @@ class TestHttp2(H2DownloadHandlerMixin, TestHttpsBase):
             response = await download_handler.download_request(request)
         assert response.protocol == "h2"
 
-    def test_download_conn_failed(self) -> None:  # type: ignore[override]
-        # Unlike HTTP11DownloadHandler which raises it from download_request()
-        # (without any special handling), here ConnectionRefusedError (raised in
-        # twisted.internet.endpoints.startConnectionAttempts()) bubbles up as
-        # an unhandled exception in a Deferred and the handler waits until
-        # DOWNLOAD_TIMEOUT.
-        pytest.skip("The handler doesn't properly reraise ConnectionRefusedError")
-
-    def test_download_dns_error(self) -> None:  # type: ignore[override]
-        # Unlike HTTP11DownloadHandler which raises it from download_request()
-        # (without any special handling), here DNSLookupError (raised in
-        # twisted.internet.endpoints.startConnectionAttempts()) bubbles up as
-        # an unhandled exception in a Deferred and the handler waits until
-        # DOWNLOAD_TIMEOUT.
-        pytest.skip("The handler doesn't properly reraise DNSLookupError")
-
     @coroutine_test
     async def test_concurrent_requests_same_domain(
         self, mockserver: MockServer