{"action":"opened","number":2952,"pull_request":{"url":"https://api.github.com/repos/dotnet/corefx/pulls/2952","id":43213329,"html_url":"https://github.com/dotnet/corefx/pull/2952","diff_url":"https://github.com/dotnet/corefx/pull/2952.diff","patch_url":"https://github.com/dotnet/corefx/pull/2952.patch","issue_url":"https://api.github.com/repos/dotnet/corefx/issues/2952","number":2952,"state":"open","locked":false,"title":"Make PipeStream.ReadAsync on an async stream cancelable on Unix","user":{"login":"stephentoub","id":2642209,"avatar_url":"https://avatars.githubusercontent.com/u/2642209?v=3","gravatar_id":"","url":"https://api.github.com/users/stephentoub","html_url":"https://github.com/stephentoub","followers_url":"https://api.github.com/users/stephentoub/followers","following_url":"https://api.github.com/users/stephentoub/following{/other_user}","gists_url":"https://api.github.com/users/stephentoub/gists{/gist_id}","starred_url":"https://api.github.com/users/stephentoub/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/stephentoub/subscriptions","organizations_url":"https://api.github.com/users/stephentoub/orgs","repos_url":"https://api.github.com/users/stephentoub/repos","events_url":"https://api.github.com/users/stephentoub/events{/privacy}","received_events_url":"https://api.github.com/users/stephentoub/received_events","type":"User","site_admin":false},"body":"Contributes to #812.\r\ncc: @ianhays, @ericstj, @ellismg, @bpschoch, @Maxwe11 \r\nI suggest reviewing a commit at a time; I had to make some changes along the way to how things were structured, and that'll make it easier to follow along.\r\n\r\nToday on Unix PipeStream.Read/WriteAsync just delegate to the base Stream's ReadAsync/WriteAsync methods, which in turn queue work items to call Read/Write.  This is how Window's implementation of PipeStream works, too, but only when the stream has isAsync false.  When the stream is created to be asynchronous, on Windows ReadAsync/WriteAsync use overlapped I/O.  In addition to avoiding blocked threads, this has a big advantage: the operations are cancelable.  With our Unix implementation, once we do the initial check for whether cancellation has been requested, we then call the blocking read/write libc methods, and ignore cancellation from that point on. This PR addresses that, at least for ReadAsync.\r\n\r\nFirst, I refactored the implementation to avoid relying on the base Stream's ReadAsync/WriteAsync for the isAsync==true case.  This doesn't in and of itself enable cancellation, but it does give us a place to plug it in.  It also fixes a difference we had between Windows and Unix: on Windows when isAsync==true, Read/Write calls delegated to ReadAsync/WriteAsync, such that they were all appropriately synchronized.  That wasn't the case on Unix, which meant the same usage patterns could result in non-thread-safe accesses to the type.  That's now fixed.\r\n\r\nWith that in place, I added a cancellation mechanism to ReadAsync.  Instead of blocking in a call to read, we instead block in a call to poll, passing to it both the target pipe's descriptor along with a special descriptor we create and signal when the cancellation token is signaled.  I did not enable this for WriteAsync because the same approach either requires a very expensive change (writing one byte at a time) or is still potentially blocking (poll doesn't tell us whether there's enough room to write all of the data, just whether we can write something).\r\n\r\n(I considered other approaches here, e.g. aio_read/aio_write/aio_cancel, io_submit, etc., but from everything I could find about these approaches, they were either too platform-specific, or didn't work with pipes, or would yield no better efficiency than what this approach is doing.  If folks have other suggestions, I'd be happy to consider alternative approaches.)","created_at":"2015-08-24T19:42:43Z","updated_at":"2015-08-24T19:42:44Z","closed_at":null,"merged_at":null,"merge_commit_sha":null,"assignee":null,"milestone":null,"commits_url":"https://api.github.com/repos/dotnet/corefx/pulls/2952/commits","review_comments_url":"https://api.github.com/repos/dotnet/corefx/pulls/2952/comments","review_comment_url":"https://api.github.com/repos/dotnet/corefx/pulls/comments{/number}","comments_url":"https://api.github.com/repos/dotnet/corefx/issues/2952/comments","statuses_url":"https://api.github.com/repos/dotnet/corefx/statuses/83bc24dbe8113737300c57a96b030ac85d680ace","head":{"label":"stephentoub:cancelable_pipes_readwrite","ref":"cancelable_pipes_readwrite","sha":"83bc24dbe8113737300c57a96b030ac85d680ace","user":{"login":"stephentoub","id":2642209,"avatar_url":"https://avatars.githubusercontent.com/u/2642209?v=3","gravatar_id":"","url":"https://api.github.com/users/stephentoub","html_url":"https://github.com/stephentoub","followers_url":"https://api.github.com/users/stephentoub/followers","following_url":"https://api.github.com/users/stephentoub/following{/other_user}","gists_url":"https://api.github.com/users/stephentoub/gists{/gist_id}","starred_url":"https://api.github.com/users/stephentoub/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/stephentoub/subscriptions","organizations_url":"https://api.github.com/users/stephentoub/orgs","repos_url":"https://api.github.com/users/stephentoub/repos","events_url":"https://api.github.com/users/stephentoub/events{/privacy}","received_events_url":"https://api.github.com/users/stephentoub/received_events","type":"User","site_admin":false},"repo":{"id":26551529,"name":"corefx","full_name":"stephentoub/corefx","owner":{"login":"stephentoub","id":2642209,"avatar_url":"https://avatars.githubusercontent.com/u/2642209?v=3","gravatar_id":"","url":"https://api.github.com/users/stephentoub","html_url":"https://github.com/stephentoub","followers_url":"https://api.github.com/users/stephentoub/followers","following_url":"https://api.github.com/users/stephentoub/following{/other_user}","gists_url":"https://api.github.com/users/stephentoub/gists{/gist_id}","starred_url":"https://api.github.com/users/stephentoub/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/stephentoub/subscriptions","organizations_url":"https://api.github.com/users/stephentoub/orgs","repos_url":"https://api.github.com/users/stephentoub/repos","events_url":"https://api.github.com/users/stephentoub/events{/privacy}","received_events_url":"https://api.github.com/users/stephentoub/received_events","type":"User","site_admin":false},"private":false,"html_url":"https://github.com/stephentoub/corefx","description":"This 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