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Single lookups into concurrent dictionary #2231

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Jun 12, 2023
Merged

Single lookups into concurrent dictionary #2231

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14 changes: 2 additions & 12 deletions lib/PuppeteerSharp/ChromeTargetManager.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -89,18 +89,8 @@ public async Task InitializeAsync()

public void AddTargetInterceptor(CDPSession session, TargetInterceptor interceptor)
{
lock (_targetInterceptors)
{
_targetInterceptors.TryGetValue(session, out var interceptors);

if (interceptors == null)
{
interceptors = new List<TargetInterceptor>();
_targetInterceptors.TryAdd(session, interceptors);
}

interceptors.Add(interceptor);
}
var interceptors = _targetInterceptors.GetOrAdd(session, static _ => new());
interceptors.Add(interceptor);
Comment on lines -92 to +93
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@kblok Do you recall if the lock in AddTargetInterceptor intended to lock both the usage of _targetInterceptors and adding an element to interceptors?

A colleague of mine pointed out that I might have introduced a race-condition here
If two threads get the same interceptors out of _targetInterceptors and concurrently calls the thread-unsafe List<T>.Add.

On the other hand the other usages of interceptors are also not locked

  • RemoveTargetInterceptor removes an item from the list
  • OnAttachedToTarget enumerates the list

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A colleague of mine pointed out that I might have introduced a race condition here

Is that a theory or a suspicion? We are talking about multiple threads, but I don't see how those events coming from the browser could be racy.

But yeah. The goal of the lock was to make those two actions thread-safe.
We could restore the lock. We could also turn the list into a ConcurrentSet.

On the other hand the other usages of interceptors are also not locked

  • RemoveTargetInterceptor removes an item from the list

Again, I don't see how we can get a race in RemoveTargetInterceptor

OnAttachedToTarget enumerates the list

I can see the need for a lock there.

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A colleague of mine pointed out that I might have introduced a race condition here

Is that a theory or a suspicion? We are talking about multiple threads, but I don't see how those events coming from the browser could be racy.

Before this PR the non-thread safe interceptors.Add(interceptor); was inside a lock and now it isn't.
I didn't notice that slight change of semantics when I created the PR, that's why I'm bringing it to your attention.
We haven't experienced that any problems that I suspect stem from this PR, so for now it's a theoretical race-condition.
I assume that's what you meant by "theory or a suspicion"?

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@jnyrup yes. I was curious whether you did got a race condition or you found it could be a problem.
I think we could make it a ConcurrentSet just in case.

}

public void RemoveTargetInterceptor(CDPSession session, TargetInterceptor interceptor)
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14 changes: 2 additions & 12 deletions lib/PuppeteerSharp/FirefoxTargetManager.cs
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Expand Up @@ -46,18 +46,8 @@ public FirefoxTargetManager(

public void AddTargetInterceptor(CDPSession session, TargetInterceptor interceptor)
{
lock (_targetInterceptors)
{
_targetInterceptors.TryGetValue(session, out var interceptors);

if (interceptors == null)
{
interceptors = new List<TargetInterceptor>();
_targetInterceptors.TryAdd(session, interceptors);
}

interceptors.Add(interceptor);
}
var interceptors = _targetInterceptors.GetOrAdd(session, static _ => new());
interceptors.Add(interceptor);
}

public void RemoveTargetInterceptor(CDPSession session, TargetInterceptor interceptor)
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6 changes: 2 additions & 4 deletions lib/PuppeteerSharp/FrameTree.cs
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Expand Up @@ -44,8 +44,7 @@ internal Task<Frame> WaitForFrameAsync(string frameId)
}

var deferred = new TaskCompletionSource<Frame>(TaskCreationOptions.RunContinuationsAsynchronously);
_waitRequests.TryAdd(frameId, new List<TaskCompletionSource<Frame>>());
_waitRequests.TryGetValue(frameId, out var callbacks);
var callbacks = _waitRequests.GetOrAdd(frameId, static _ => new());
callbacks.Add(deferred);

return deferred.Task;
Expand All @@ -58,8 +57,7 @@ internal void AddFrame(Frame frame)
{
_parentIds.TryAdd(frame.Id, frame.ParentId);

_childIds.TryAdd(frame.ParentId, new List<string>());
_childIds.TryGetValue(frame.ParentId, out var childIds);
var childIds = _childIds.GetOrAdd(frame.ParentId, static _ => new());
childIds.Add(frame.Id);
}
else
Expand Down