WebKit export of https://bugs.webkit.org/show_bug.cgi?id=321107 (#62034)

Author: yulunwu2

css/selectors/media/support/stall-resume.py

@@ -1,11 +1,29 @@
 import os
 import time
 
+# How long a resume stays in effect. Every request that reaches the stall point
+# during this window is released, not just the first one to claim the key.
+RESUME_WINDOW = 5
+
+# Waiting is bounded so that a request the UA has already cancelled doesn't park
+# its server thread for the rest of the run.
+RESUME_TIMEOUT = 30
+
+
 def main(request, response):
     key = request.GET.first(b"key")
+    stash = request.server.stash
 
     if request.method == "POST":
-        request.server.stash.put(key, True)
+        stash.put(key, True)
+
+        # Hold the resume open long enough for every request already waiting --
+        # and any that arrive just afterwards -- to be released, then take it
+        # back out so the value doesn't outlive the test.
+        time.sleep(RESUME_WINDOW)
+        with stash.lock:
+            stash.take(key)
+
         return f"put {key} into stash"
 
     file_path = os.path.join(request.doc_root, "media", "movie_300.webm")
@@ -28,10 +46,21 @@ def main(request, response):
 
         response.writer.write(f.read(first_size))
 
-        # Wait for the key to appear in the stash.
+        # Wait for the key to appear in the stash. Several requests can be
+        # waiting on the same key at once, because the UA may try more than one
+        # media player before settling on one that can handle the response.
+        deadline = time.monotonic() + RESUME_TIMEOUT
         while True:
-            if request.server.stash.take(key) == True:
-                break
+            with stash.lock:
+                if stash.take(key) == True:
+                    # `take` is destructive, so put the value straight back --
+                    # under the lock, making the pair atomic -- so that every
+                    # request waiting on this key is released, not just whichever
+                    # one happened to get here first.
+                    stash.put(key, True)
+                    break
+            if time.monotonic() > deadline:
+                return
             time.sleep(0.1)
 
         # Send the rest of the data.