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https://issues.apache.org/jira/browse/HADOOP-12487?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14976634#comment-14976634
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Alan Burlison commented on HADOOP-12487:
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If Al Viro thought the suggestions were without merit then I'd have expected
him to have simply terminated the discussion, which he hasn't. The current
Linux shutdown() and close() behaviour are not POSIX compliant. Whether or not
the Linux developers think it's worth complying with POSIX in this area is an
interesting discussion, but not really pertinent here as even if it does
change, Hadoop has to deal with the current Linux behaviour.
I understand your *theoretical* race scenario however after looking at the
Hadoop code carefully I can't see how it will ever occur *in practice*.
DomainSocket uses a CloseableReferenceCount to make sure that once the FD
encapsulated by the DomainSocket is closed then it isn't used any more. It
therefore doesn't matter if the FD is recycled elsewhere, because the copy of
it inside the DomainSocket is 'dead' and therefore irrelevant.
All the read/write uses of the FD are made from inside DomainSocket as far as I
can tell, and are therefore protected by the CloseableReferenceCount. However
the fd field itself is not private. As far as I can tell the only place the FD
is used externally to DomainSocket is from within DomainSocketWatcher and
again, as far as I can tell it's only used for poll(), DomainSocketWatcher
doesn't read or write to it, doesn't open any FDs itself and re-validates the
DomainSocket FDs it uses by calling sock.refCount.unreferenceCheckClosed() etc,
so I think that is safe as well.
Unless I've missed something, I believe the close routine in DomainSocket()
could be changed to call shutdown() immediately followed by close(), then wait
for the CloseableReferenceCount to reach zero and return. If we ignore any
failures from shutdown() I think that would then work on both Linux and Solaris.
If I'm missing a pitfall here somewhere here I'd be grateful if you could point
it out, thanks.
> DomainSocket.close() assumes incorrect Linux behaviour
> ------------------------------------------------------
>
> Key: HADOOP-12487
> URL: https://issues.apache.org/jira/browse/HADOOP-12487
> Project: Hadoop Common
> Issue Type: Sub-task
> Components: net
> Affects Versions: 2.7.1
> Environment: Linux Solaris
> Reporter: Alan Burlison
> Assignee: Alan Burlison
> Attachments: shutdown.c
>
>
> I'm getting a test failure in TestDomainSocket.java, in the
> testSocketAcceptAndClose test. That test creates a socket which one thread
> waits on in DomainSocket.accept() whilst a second thread sleeps for a short
> time before closing the same socket with DomainSocket.close().
> DomainSocket.close() first calls shutdown0() on the socket before closing
> close0() - both those are thin wrappers around the corresponding libc socket
> calls. DomainSocket.close() contains the following comment, explaining the
> logic involved:
> {code}
> // Calling shutdown on the socket will interrupt blocking system
> // calls like accept, write, and read that are going on in a
> // different thread.
> {code}
> Unfortunately that relies on non-standards-compliant Linux behaviour. I've
> written a simple C test case that replicates the scenario above:
> # ThreadA opens, binds, listens and accepts on a socket, waiting for
> connections.
> # Some time later ThreadB calls shutdown on the socket ThreadA is waiting in
> accept on.
> Here is what happens:
> On Linux, the shutdown call in ThreadB succeeds and the accept call in
> ThreadA returns with EINVAL.
> On Solaris, the shutdown call in ThreadB fails and returns ENOTCONN. ThreadA
> continues to wait in accept.
> Relevant POSIX manpages:
> http://pubs.opengroup.org/onlinepubs/9699919799/functions/accept.html
> http://pubs.opengroup.org/onlinepubs/9699919799/functions/shutdown.html
> The POSIX shutdown manpage says:
> "The shutdown() function shall cause all or part of a full-duplex connection
> on the socket associated with the file descriptor socket to be shut down."
> ...
> "\[ENOTCONN] The socket is not connected."
> Page 229 & 303 of "UNIX System V Network Programming" say:
> "shutdown can only be called on sockets that have been previously connected"
> "The socket \[passed to accept that] fd refers to does not participate in the
> connection. It remains available to receive further connect indications"
> That is pretty clear, sockets being waited on with accept are not connected
> by definition. Nor is it the accept socket connected when a client connects
> to it, it is the socket returned by accept that is connected to the client.
> Therefore the Solaris behaviour of failing the shutdown call is correct.
> In order to get the required behaviour of ThreadB causing ThreadA to exit the
> accept call with an error, the correct way is for ThreadB to call close on
> the socket that ThreadA is waiting on in accept.
> On Solaris, calling close in ThreadB succeeds, and the accept call in ThreadA
> fails and returns EBADF.
> On Linux, calling close in ThreadB succeeds but ThreadA continues to wait in
> accept until there is an incoming connection. That accept returns
> successfully. However subsequent accept calls on the same socket return EBADF.
> The Linux behaviour is fundamentally broken in three places:
> # Allowing shutdown to succeed on an unconnected socket is incorrect.
> # Returning a successful accept on a closed file descriptor is incorrect,
> especially as future accept calls on the same socket fail.
> # Once shutdown has been called on the socket, calling close on the socket
> fails with EBADF. That is incorrect, shutdown should just prevent further IO
> on the socket, it should not close it.
> The real issue though is that there's no single way of doing this that works
> on both Solaris and Linux, there will need to be platform-specific code in
> Hadoop to cater for the Linux brokenness.
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