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<dt>Revision</dt> <dd><a href="http://trac.webkit.org/projects/webkit/changeset/285330">285330</a></dd>
<dt>Author</dt> <dd>heycam@apple.com</dd>
<dt>Date</dt> <dd>2021-11-04 20:27:54 -0700 (Thu, 04 Nov 2021)</dd>
</dl>

<h3>Log Message</h3>
<pre>Nested run loops under MediaPlayerPrivateAVFoundationObjC::waitForVideoOutputMediaDataWillChange can cause hang when timeout fires
https://bugs.webkit.org/show_bug.cgi?id=232695
<rdar://problem/85004449>

Reviewed by Jer Noble.

It's possible for MediaPlayerPrivateAVFoundationObjC::waitForVideoOutputMediaDataWillChange
to be called re-entrantly, if the RunLoop::run call ends up processing
an event that also wants to synchronously update the media image. This
can cause a hang:

1. Enter the outer waitForVideoOutputMediaDataWillChange call.
2. Set up the outer timeout timer.
3. Call RunLoop::run.
    3.1. Enter the inner waitForVideoOutputMediaDataWillChange call.
    3.2. Set up the inner timeout timer.
    3.3. Call RunLoop::run.
        3.3.1. Wait for new RunLoop events, and none arrive.
        3.3.2. The outer timeout timer fires, calling RunLoop::stop.
    3.4. Return from waitForVideoOutputMediaDataWillChange, cancelling
         the inner timeout timer.
    3.5. Wait for more events on the run loop, forever.

To avoid this, we can set up a single timeout timer, and track the
nesting level of our RunLoop::run calls. The innermost RunLoop::run call
will finish either by the timer firing (which calls RunLoop::stop) or by
the video data updating (which also calls RunLoop::stop, under
outputMediaDataWillChange). Either way, once the innermost
RunLoop::run call is finished, we know we can stop processing all of
the ancestor RunLoop:run calls.

* platform/graphics/avfoundation/objc/MediaPlayerPrivateAVFoundationObjC.h:
* platform/graphics/avfoundation/objc/MediaPlayerPrivateAVFoundationObjC.mm:
(WebCore::MediaPlayerPrivateAVFoundationObjC::waitForVideoOutputMediaDataWillChange):
(WebCore::MediaPlayerPrivateAVFoundationObjC::outputMediaDataWillChange):</pre>

<h3>Modified Paths</h3>
<ul>
<li><a href="#trunkSourceWebCoreChangeLog">trunk/Source/WebCore/ChangeLog</a></li>
<li><a href="#trunkSourceWebCoreplatformgraphicsavfoundationobjcMediaPlayerPrivateAVFoundationObjCh">trunk/Source/WebCore/platform/graphics/avfoundation/objc/MediaPlayerPrivateAVFoundationObjC.h</a></li>
<li><a href="#trunkSourceWebCoreplatformgraphicsavfoundationobjcMediaPlayerPrivateAVFoundationObjCmm">trunk/Source/WebCore/platform/graphics/avfoundation/objc/MediaPlayerPrivateAVFoundationObjC.mm</a></li>
</ul>

</div>
<div id="patch">
<h3>Diff</h3>
<a id="trunkSourceWebCoreChangeLog"></a>
<div class="modfile"><h4>Modified: trunk/Source/WebCore/ChangeLog (285329 => 285330)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/Source/WebCore/ChangeLog   2021-11-05 03:25:23 UTC (rev 285329)
+++ trunk/Source/WebCore/ChangeLog      2021-11-05 03:27:54 UTC (rev 285330)
</span><span class="lines">@@ -1,3 +1,41 @@
</span><ins>+2021-11-04  Cameron McCormack  <heycam@apple.com>
+
+        Nested run loops under MediaPlayerPrivateAVFoundationObjC::waitForVideoOutputMediaDataWillChange can cause hang when timeout fires
+        https://bugs.webkit.org/show_bug.cgi?id=232695
+        <rdar://problem/85004449>
+
+        Reviewed by Jer Noble.
+
+        It's possible for MediaPlayerPrivateAVFoundationObjC::waitForVideoOutputMediaDataWillChange
+        to be called re-entrantly, if the RunLoop::run call ends up processing
+        an event that also wants to synchronously update the media image. This
+        can cause a hang:
+
+        1. Enter the outer waitForVideoOutputMediaDataWillChange call.
+        2. Set up the outer timeout timer.
+        3. Call RunLoop::run.
+            3.1. Enter the inner waitForVideoOutputMediaDataWillChange call.
+            3.2. Set up the inner timeout timer.
+            3.3. Call RunLoop::run.
+                3.3.1. Wait for new RunLoop events, and none arrive.
+                3.3.2. The outer timeout timer fires, calling RunLoop::stop.
+            3.4. Return from waitForVideoOutputMediaDataWillChange, cancelling
+                 the inner timeout timer.
+            3.5. Wait for more events on the run loop, forever.
+
+        To avoid this, we can set up a single timeout timer, and track the
+        nesting level of our RunLoop::run calls. The innermost RunLoop::run call
+        will finish either by the timer firing (which calls RunLoop::stop) or by
+        the video data updating (which also calls RunLoop::stop, under
+        outputMediaDataWillChange). Either way, once the innermost
+        RunLoop::run call is finished, we know we can stop processing all of
+        the ancestor RunLoop:run calls.
+
+        * platform/graphics/avfoundation/objc/MediaPlayerPrivateAVFoundationObjC.h:
+        * platform/graphics/avfoundation/objc/MediaPlayerPrivateAVFoundationObjC.mm:
+        (WebCore::MediaPlayerPrivateAVFoundationObjC::waitForVideoOutputMediaDataWillChange):
+        (WebCore::MediaPlayerPrivateAVFoundationObjC::outputMediaDataWillChange):
+
</ins><span class="cx"> 2021-11-04  Alan Bujtas  <zalan@apple.com>
</span><span class="cx"> 
</span><span class="cx">         [LFC][IFC] Use objectReplacementCharacter to mark atomic inline box position.
</span></span></pre></div>
<a id="trunkSourceWebCoreplatformgraphicsavfoundationobjcMediaPlayerPrivateAVFoundationObjCh"></a>
<div class="modfile"><h4>Modified: trunk/Source/WebCore/platform/graphics/avfoundation/objc/MediaPlayerPrivateAVFoundationObjC.h (285329 => 285330)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/Source/WebCore/platform/graphics/avfoundation/objc/MediaPlayerPrivateAVFoundationObjC.h    2021-11-05 03:25:23 UTC (rev 285329)
+++ trunk/Source/WebCore/platform/graphics/avfoundation/objc/MediaPlayerPrivateAVFoundationObjC.h       2021-11-05 03:27:54 UTC (rev 285330)
</span><span class="lines">@@ -428,6 +428,7 @@
</span><span class="cx">     mutable long long m_cachedTotalBytes { 0 };
</span><span class="cx">     unsigned m_pendingStatusChanges { 0 };
</span><span class="cx">     int m_cachedItemStatus;
</span><ins>+    int m_runLoopNestingLevel { 0 };
</ins><span class="cx">     MediaPlayer::BufferingPolicy m_bufferingPolicy { MediaPlayer::BufferingPolicy::Default };
</span><span class="cx">     bool m_cachedLikelyToKeepUp { false };
</span><span class="cx">     bool m_cachedBufferEmpty { false };
</span><span class="lines">@@ -447,7 +448,6 @@
</span><span class="cx">     mutable bool m_allowsWirelessVideoPlayback { true };
</span><span class="cx">     bool m_shouldPlayToPlaybackTarget { false };
</span><span class="cx"> #endif
</span><del>-    bool m_runningModalPaint { false };
</del><span class="cx">     bool m_haveProcessedChapterTracks { false };
</span><span class="cx">     bool m_waitForVideoOutputMediaDataWillChangeTimedOut { false };
</span><span class="cx">     bool m_haveBeenAskedToPaint { false };
</span></span></pre></div>
<a id="trunkSourceWebCoreplatformgraphicsavfoundationobjcMediaPlayerPrivateAVFoundationObjCmm"></a>
<div class="modfile"><h4>Modified: trunk/Source/WebCore/platform/graphics/avfoundation/objc/MediaPlayerPrivateAVFoundationObjC.mm (285329 => 285330)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/Source/WebCore/platform/graphics/avfoundation/objc/MediaPlayerPrivateAVFoundationObjC.mm   2021-11-05 03:25:23 UTC (rev 285329)
+++ trunk/Source/WebCore/platform/graphics/avfoundation/objc/MediaPlayerPrivateAVFoundationObjC.mm      2021-11-05 03:27:54 UTC (rev 285330)
</span><span class="lines">@@ -2609,21 +2609,31 @@
</span><span class="cx"> {
</span><span class="cx">     if (m_waitForVideoOutputMediaDataWillChangeTimedOut)
</span><span class="cx">         return;
</span><del>-    [m_videoOutput requestNotificationOfMediaDataChangeWithAdvanceInterval:0];
</del><span class="cx"> 
</span><span class="cx">     // Wait for 1 second.
</span><span class="cx">     MonotonicTime start = MonotonicTime::now();
</span><span class="cx"> 
</span><del>-    RunLoop::Timer<MediaPlayerPrivateAVFoundationObjC> timeoutTimer { RunLoop::main(), [] {
-        RunLoop::main().stop();
-    } };
-    timeoutTimer.startOneShot(1_s);
</del><ins>+    std::optional<RunLoop::Timer<MediaPlayerPrivateAVFoundationObjC>> timeoutTimer;
</ins><span class="cx"> 
</span><del>-    m_runningModalPaint = true;
</del><ins>+    if (!m_runLoopNestingLevel) {
+        [m_videoOutput requestNotificationOfMediaDataChangeWithAdvanceInterval:0];
+
+        timeoutTimer.emplace(RunLoop::main(), [&] {
+            RunLoop::main().stop();
+        });
+        timeoutTimer->startOneShot(1_s);
+    }
+
+    ++m_runLoopNestingLevel;
</ins><span class="cx">     RunLoop::run();
</span><del>-    m_runningModalPaint = false;
</del><ins>+    --m_runLoopNestingLevel;
</ins><span class="cx"> 
</span><del>-    bool satisfied = timeoutTimer.isActive();
</del><ins>+    if (m_runLoopNestingLevel) {
+        RunLoop::main().stop();
+        return;
+    }
+
+    bool satisfied = timeoutTimer->isActive();
</ins><span class="cx">     if (!satisfied) {
</span><span class="cx">         ERROR_LOG(LOGIDENTIFIER, "timed out");
</span><span class="cx">         m_waitForVideoOutputMediaDataWillChangeTimedOut = true;
</span><span class="lines">@@ -2633,7 +2643,7 @@
</span><span class="cx"> 
</span><span class="cx"> void MediaPlayerPrivateAVFoundationObjC::outputMediaDataWillChange()
</span><span class="cx"> {
</span><del>-    if (m_runningModalPaint) {
</del><ins>+    if (m_runLoopNestingLevel) {
</ins><span class="cx">         if (RunLoop::isMain())
</span><span class="cx">             RunLoop::main().stop();
</span><span class="cx">         else {
</span></span></pre>
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