mod_http3: fix a session allocator race, a stuck graceful drain and a blocked-stream spin

changes-entries/h3_blocked_stream_spin.txt

@@ -0,0 +1,4 @@
+  *) mod_http3: Fixed an event-thread spin while a client holds its
+     flow-control window shut: a blocked stream is now blocked again each time
+     nghttp3 offers it.
+     [Alexander Gerasimov <codeguard gmail.com>]

changes-entries/h3_drain_pending_handshakes.txt

@@ -0,0 +1,4 @@
+  *) mod_http3: Fixed a graceful restart that never ended: a handshake still
+     pending when the child started to drain did not finish or time out, so
+     the old child kept its connection count and the port.
+     [Alexander Gerasimov <codeguard gmail.com>]

changes-entries/h3_session_allocator_race.txt

@@ -0,0 +1,3 @@
+  *) mod_http3: Fixed a heap race that crashed the child: worker threads and
+     the event thread allocated from one session allocator that had no mutex.
+     [Alexander Gerasimov <codeguard gmail.com>]

modules/http3/h3_io.c

@@ -269,6 +269,15 @@ static apr_status_t spawn_serviced_session(h3_io_t* io, h3q_conn* conn)
     }
     apr_allocator_owner_set(allocator, session_pool);
     apr_pool_tag(session_pool, "h3_session");
+    /* Workers allocate from stream subpools while this thread makes its own: share the allocator safely. */
+    apr_thread_mutex_t* alloc_mutex = NULL;
+    if (apr_thread_mutex_create(&alloc_mutex, APR_THREAD_MUTEX_DEFAULT, session_pool) != APR_SUCCESS)
+    {
+        apr_pool_destroy(session_pool);
+        h3q_conn_free(conn);
+        return APR_EGENERAL;
+    }
+    apr_allocator_mutex_set(allocator, alloc_mutex);
 
     h3_session* session = NULL;
     if (h3_session_create(&session, io->server, conn, session_pool) != APR_SUCCESS)

modules/http3/h3_stream.c

@@ -126,13 +126,14 @@ void flush_nghttp3(h3_session* session)
         nghttp3_conn_add_ack_offset(session->ngh3, sid, res.accepted);
         if (res.blocked)
         {
-            /* Send buffer full: skip this stream instead of busy-looping on the same vec. */
+            /* Send buffer full: skip this stream instead of busy-looping on the same vec.
+             * Block it again every time: resume_stream reschedules a blocked stream. */
             if (!h3s->write_blocked)
             {
                 h3s->write_blocked = 1;
                 session->blocked_streams++;
-                nghttp3_conn_block_stream(session->ngh3, sid);
             }
+            nghttp3_conn_block_stream(session->ngh3, sid);
             continue;
         }
     }

modules/http3/h3_threads.c

@@ -96,6 +96,10 @@ void* APR_THREAD_FUNC h3_event_thread(apr_thread_t* thread, void* data)
                     h3q_conn_free(conn);
                 }
             }
+        }
+        if (io->thread_running)
+        {
+            /* Also while draining: a pending handshake counts as an MPM connection, so it must finish or time out. */
             progress_pending_handshakes(io);
         }