websocket: pause writing and meta data fix

When writing a decoded chunk of websocket data, always flush the writer
chain so that buffered data gets delivered before the ws meta data gets
updated.

Add client writer flags CURL_CW_FLAG_BLOWUP for writer types that may
significantly enlarge write sizes. This flag causes the pause writer to
be added and shrinks the write chunk sizes. We do not want that for
content decoders like WS that do not change the size.

Add test_20_13 to check that large frames are paused/unpaused correctly
with the matching meta data.

Fixes #22413
Reported-by: Hendrik Hübner
Closes #22416

tests/http/test_20_websockets.py

@@ -325,12 +325,22 @@ class TestWebsockets:
         rss2 = r.profile.stats['rss'] / (1024 * 1024)
         assert (rss1 * 1.1) >= rss2, 'bad memory increase'
 
-    # test frame delivery when pausing
-    def test_20_12_pause_frames(self, env: Env, ws_4frames):
+    # test small frames delivery when pausing
+    def test_20_12_pause_frames_small(self, env: Env, ws_4frames):
         payload = 127 * "x"
         client = LocalClient(env=env, name='cli_ws_pause')
         if not client.exists():
             pytest.skip(f'example client not built: {client.name}')
-        url = f'ws://localhost:{ws_4frames.port}/'
+        url = f'ws://localhost:{ws_4frames.port}/small'
+        r = client.run(args=[url, payload])
+        r.check_exit_code(0)
+
+    # test small frames delivery when pausing
+    def test_20_13_pause_frames_large(self, env: Env, ws_4frames):
+        payload = 127 * "x"
+        client = LocalClient(env=env, name='cli_ws_pause')
+        if not client.exists():
+            pytest.skip(f'example client not built: {client.name}')
+        url = f'ws://localhost:{ws_4frames.port}/large'
         r = client.run(args=[url, payload])
         r.check_exit_code(0)

tests/http/testenv/ws_4frames_server.py

@@ -29,21 +29,30 @@ import logging
 
 import websockets
 
-MESSAGES = [
+MESSAGES_SMALL = [
     "Hello 1",
     "Hello 2",
     "Hello 3",
     "Hello 4",
 ]
 
+MESSAGES_LARGE = [
+    b"x" * 65536,
+    b"x" * 65536,
+    b"x" * 65536,
+    b"x" * 65536,
+]
+
 
 async def handler(websocket):
     peer = websocket.remote_address
     print(f"client from {peer[0]}:{peer[1]}", flush=True)
     print("handshake complete", flush=True)
 
+    msgs = MESSAGES_LARGE if websocket.request.path == '/large' else MESSAGES_SMALL
+
     await asyncio.sleep(0.1)
-    for index, payload in enumerate(MESSAGES, start=1):
+    for index, payload in enumerate(msgs, start=1):
         await websocket.send(payload)
         print(f"sent frame {index}: {payload!r}", flush=True)
         # await asyncio.sleep(0.2)