[THRIFT-747] C++ TSocket->close calls shutdown breaking forked parent process

• Type: Bug
• Status: Closed
• Priority: Minor
• Resolution: Won't Fix
• Affects Version/s: 0.2, 0.3
• Fix Version/s: 0.10.0
• Component/s: C++ - Library

Environment

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

• Assignee: Unassigned
• Reporter: Tim Wilson-Brown
• Created: 01/Apr/10 09:11
• Updated: 18/Mar/21 23:25
• Resolved: 13/Dec/15 20:05

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)

Issue Links

• blocks: THRIFT-748 C++ TSocket default linger setting breaks forked parent process
• relates to: THRIFT-5374 SO_LINGER should not be set on AF_UNIX socket on WSL (portability issue)