Thrift
  1. Thrift
  2. THRIFT-747

C++ TSocket->close calls shutdown breaking forked parent process

    Details

    • Type: Bug Bug
    • Status: Open
    • Priority: Minor Minor
    • Resolution: Unresolved
    • Affects Version/s: 0.2, 0.3
    • Fix Version/s: None
    • Component/s: C++ - Library
    • Labels:
      None
    • Environment:

      Cygwin 1.7.1 on Windows XP SP3, Thrift 0.2.0 & r760184 & Trunk

      Description

      If a Thrift C++ Client opens a TSocket, then calls fork(), the child process can terminate the parent processes' connection by deleting its copy of the parent TSocket.

      In particular,
      TSocket->close() calls shutdown(socket_,SHUT_RDWR) before close(socket_)

      Discussion:

      This behaviour is inconsistent, as it is:

      • unlike the unix socket close() semantics - close() only affects the process that calls it, and the socket is shut down when all copies of it are closed
      • unlike the python and java code, which (appears) to only use close()

      This design choice makes it really difficult to program a Thrift client that forks other clients in C++, as the first process to call TSocket->close() terminates all copies of the connection. The child process is unable to cleanup its copy of the parent's connection - this is a particular issue when using shared_ptr because the child process can not even exit().

      However, the design choice also prevents deadlock/slowdown issues where a forked process holds open a copy of the parent's Thrift connections.

      The design choice may also be an attempt to implement the 'block to send then close' behaviour described in http://blog.netherlabs.nl/articles/2009/01/18/the-ultimate-so_linger-page-or-why-is-my-tcp-not-reliable
      However, the shutdown call, and close with the default linger interval of 0 are both hard resets.
      In the absence of linger/shutdown, the kernel can usually send small thrift messages by itself.

      Options:

      • The most functional resolution would be to implement TSocket->setShutdownOnClose() that allows Thrift users to set their preference for shutdown on socket close or delete. However, this change may also need to be made to other language libraries.
      • Removing shutdown() from TSocket->close() could break programs that expect TSockets not to stay open if children are still running.

      TODO:

      • Confirm issue on Linux - see attached test code
      • Decide how to resolve issue
      • Create Patch - see attached TSocket.h & TSocket.cpp from Thrift 0.2.0 (I don't know how to generate patches but I'm happy to try and work it out)
      1. TSocket-020-fix.cpp
        16 kB
        Tim Wilson-Brown
      2. TSocket-020-fix.h
        5 kB
        Tim Wilson-Brown
      3. thrift_shutdown_example.cpp
        4 kB
        Tim Wilson-Brown

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            • Assignee:
              Unassigned
              Reporter:
              Tim Wilson-Brown
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