Re: [Xen-devel] [PATCH 2/2] libxl: Cope with pipes which signal POLLHUP|POLLIN on read eof

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Tue, Apr 07, 2015 at 02:05:28PM +0100, Ian Jackson wrote:
> Some operating systems (including Linux and FreeBSD[1]) signal not
> (only) POLLIN when a reading pipe reaches EOF, but POLLHUP (with or
> without POLLIN).  This is permitted[2].  The implications are that in
> the general case it is not possible to determine whether POLLHUP
> indicates an error or simply eof without attempting a read.
> 
> Datacopiers mishandle this, because they always treat POLLHUP
> exceptionally (either reporting it via callback_pollhup, or treating
> it as an error).  datacopiers reading from pipes on such OSs can fail
> (perhaps leaving some data unprocessed) rather than completing
> successfully.
> 
> [1] http://www.greenend.org.uk/rjk/tech/poll.html
> [2] http://pubs.opengroup.org/onlinepubs/9699919799/functions/poll.html
> 
> Distinguishing POLLHUP is needed for pty fds, but most callers in
> libxl do not care about POLLHUP except as an error or eof condition.
> 
> So change the datacopier semantics so that if callback_pollhup is not
> specified we treat POLLHUP almost like POLLIN.  The difference is that
> if we get HUP from poll, but EWOULDBLOCK from read, we must signal an
> error ratehr than attempting the read again.
> 
> This fixes the problem which 7e9ec50b0535 was aimed at.
> 
> Signed-off-by: Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
> CC: Ian Campbell <ian.campbell@xxxxxxxxxx>
> CC: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> CC: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
> CC: Wei Liu <wei.liu2@xxxxxxxxxx>

The code matches documents and commit log so:

Acked-by: Wei Liu <wei.liu2@xxxxxxxxxx>

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