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From "Alan Burlison (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HADOOP-12487) DomainSocket.close() assumes incorrect Linux behaviour
Date Wed, 21 Oct 2015 15:06:27 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-12487?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14967280#comment-14967280
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Alan Burlison commented on HADOOP-12487:
----------------------------------------

The /dev/null & dup2 mechanism works on Solaris but it doesn't work on Linux unfortunately,
due to the brokenness in the handling of close() on a socket that's being used in an accept().

However having said that the /dev/null & dup2 mechanism works on Solaris I can't come
up with a race scenario where it's actually needed. DomainSocket encapsulates the underlying
FD and DomainSocket invalidates itself on close, so I can't see how the FD can actually be
used for anything, even if it is reused by an open operation in a different thread. If you
can come up with a scenario involving DomainSocket then I'll investigate, 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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