Add ConcurrentQueue<T> Clear method

[AlexRadch]

Implement ConcurrentQueue.Clear API. See https://github.com/dotnet/corefx/issues/2338

[karelz]

Please add test: https://github.com/dotnet/corefx/blob/master/src/System.Collections.Concurrent/tests/ConcurrentQueueTests.cs

[stephentoub]

This is a lot of additional code. Is it measurably more efficient than just doing:

T item;
while (TryDequeue(out item));

? It looks like it's essentially a copy of the TryDequeue/TryRemove implementation, just with Clear/Clear naming, so I wouldn't expect it would be, unless I'm missing something. We shouldn't be adding all of this extra complicated code unless it actually makes a meaningful difference.

[AlexRadch]

TryDequeue remove only one element per call see if (Interlocked.CompareExchange(ref _low, lowLocal + 1, lowLocal) == lowLocal) code.

But head.Clear() remove all elements in Segment per call see if (Interlocked.CompareExchange(ref _low, highLocal + 1, lowLocal) == lowLocal) code.

[stephentoub]

I see. Ok. Please add a comment highlighting that fairly subtle difference between the two implementations.

[AlexRadch]

I added comment and created tests. But I have issues with config files to add API correctly.

[AlexRadch]

I added comment about Clear method.
I created tests for Clear method;
I do not clear understand Adding APIs Guidelines so test project can be compiled and tested locally in src\System.Collections.Concurrent\tests\ folder but global build return strange errors.
Can anybody help me with config files to add API correctly?

[Priya91]

@AlexRadch Can you cleanup the merge commits and have only your commits in history.. Also there's a compiler error: missing semicolon,

ConcurrentQueueTests.netstandard1.7.cs(58,43): error CS1002: ; expected [/mnt/resource/j/workspace/dotnet_corefx/master/centos7.1_debug_prtest/src/System.Collections.Concurrent/tests/System.Collections.Concurrent.Tests.csproj]

Please run tests locally, running build.cmd at root of repo, or for additional instructions here

[AlexRadch]

I tried to remove excess commits but I did not find how to do this so I think I have only one way - recreate my corefx fork. So I will close all my PRs, will recreate my corefx fork and will create new PRs.

[stephentoub]

As per #14084 (comment), it'll probably be easiest for you to wait for @justinvp to finish #13828 before proceeding with this change.

[AlexRadch]

@stephentoub Ok. Thanks.

[danmoseley]

@AlexRadch I see #13828 is now merged so you should be unblocked to finish this PR.

[karelz]

@AlexRadch do you plan to finish it?

[karelz]

@AlexRadch any update?
Or would you prefer to close the PR for now and make the issue "up for grabs" again for others to pick?

[stephentoub]

Thanks for your work on this, @AlexRadch. Since this has been sitting for a couple of months and needed to be significantly updated based on changes to the target types, I went ahead and took your commits and created a new PR at #15292.