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Re: Clearing O_NONBLOCK from a pipe may lose data
- From: Andrey Repin <anrdaemon at yandex dot ru>
- To: Thomas Wolff <towo at towo dot net>, cygwin at cygwin dot com
- Date: Fri, 20 Feb 2015 02:47:00 +0300
- Subject: Re: Clearing O_NONBLOCK from a pipe may lose data
- Authentication-results: sourceware.org; auth=none
- References: <20150218220859 dot 1e8f8b19 at tukaani dot org> <20150219095147 dot GC26084 at calimero dot vinschen dot de> <54E660F1 dot 3040509 at towo dot net>
- Reply-to: cygwin at cygwin dot com
Greetings, Thomas Wolff!
> Am 19.02.2015 um 10:51 schrieb Corinna Vinschen:
>> On Feb 18 22:08, Lasse Collin wrote:
>>> (Please Cc me when replying, I'm not subscribed to the list.)
>>> I suspect that there is a bug in Cygwin:
>>> 1. Create a pipe with both ends in blocking mode (O_NONBLOCK
>>> is not set).
>>> 2. The writer sets its end to non-blocking mode.
>>> 3. The writer writes to the pipe.
>>> 4. The writer restores its end of the pipe to blocking mode
>>> before the reader has read anything from the pipe.
>>> 5. The writer closes its end of the pipe.
>>> 6. The reader reads from the pipe in blocking mode. The last
>>> bytes written by the writer never appear at the reader,
>>> thus data is silently lost.
>>> Omitting the step 4 above makes the problem go away.
>> I can imagine. A few years back, when changing the pipe code to
>> using overlapped IO, we stumbled over a problem in Windows. When
>> closing an overlapped pipe while I/O is still ongoing, Windows
>> simply destroys the pipe buffers without flushing the data to the
>> reader. This is not much of a problem for blocking IO, but it
>> obviously is for non-blocking.
>> The workaround for this behaviour is this: If the pipe is closed, and
>> this is the writing side of a nonblocking pipe, a background thread gets
>> started which keeps the overlapped structure open and continues to wait
>> for IO completion (i.e. the data has been sent to the reader).
>> However, if you switch back to blocking before closing the pipe, the
>> aforementioned mechanism does not kick in.
> Could not "switching back to blocking" simply be handled like closing as
> far as the waiting is concerned,
> thus effectively flushing the pipe buffer?
You can't "just flush" it, if the receiving end isn't reading from it.
Andrey Repin (firstname.lastname@example.org) 20.02.2015, <02:46>
Sorry for my terrible english...
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