Fix concurrency issues around writing files to global locations in kpm

[davidfowl]

We need to do this for the kpm cache and when writing the nupkg files on disk.

[ChengTian]

@davidfowl , I wrote a program which starts two kpm pack simultaneously and found the following race condition:

When we want to write a nukpg to global cache, we first delete the old one in cache and copy the new nupkg from a temp file to the final destination. See https://github.com/aspnet/KRuntime/blob/dev/src/Microsoft.Framework.PackageManager/Restore/NuGet/HttpSource.cs#L119.

After we write the nupkg to global cache, we also want to extract its content into current project's local package dir, so we need to read it. See https://github.com/aspnet/KRuntime/blob/dev/src/Microsoft.Framework.PackageManager/Restore/NuGet/PackageFeed.cs#L204

So part of the logic in kpm restore can be simplified to:

1. File.Delete("global_cache\A.1.0.0.0.nukpg");
2. File.Copy("tmp.nukpg", "global_cache\A.1.0.0.0.nukpg");
3. ZipArchive.Extract("global_cache\A.1.0.0.0.nukpg", "MyProject\packages\A.1.0.0.0");

When we execute two kpm restore concurrently (p1 and p2), let's assume the two processes interleave in the following way:

p1: 1    2      3
p2:          1

That is, after p2 deletes the nupkg in global cache, p1 tries to read the nukpg and throws an exception because the nukpg is not there.

We can fix this problem by keeping the global nukpg "locked" before it's extracted. I'll send my proposed solution as a PR later.

[ChengTian]

@davidfowl , BTW, you mentioned that you have some further instructions for this issue. Please put them here at your earliest convenience. Thanks!

[davidfowl]

http://nuget.codeplex.com/SourceControl/latest#src/CommandLine/Common/PackageExtractor.cs

Take a look at ExecuteLocked

[ChengTian]

@davidfowl , thanks for the useful hints. However, because we are using a lot of async methods, I have trouble using Mutex to resolve race conditions. In the PR below I use Semaphore to eliminate contentions successfully. To learn about the problem caused by mixing Mutex and async methods, please see http://www.dzhang.com/blog/2012/08/29/synchronization-in-async-csharp-methods