[THRIFT-748] C++ TSocket default linger setting breaks forked parent process

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

Environment

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

• Assignee: James E. King III
• Reporter: Tim Wilson-Brown
• Created: 01/Apr/10 10:40
• Updated: 18/Mar/21 23:25
• Resolved: 06/Mar/18 21:26

Description

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

In particular,
the default setting of lingerOn_ = 1 causes a RST to be sent in close(socket_) in TSocket->close()

Discussion:

This behaviour is identical to the behaviour of unix sockets when SO_LINGER is set (implementations vary).
However, the SO_LINGER default for sockets is off not on. This provides unexpected behaviour in TSocket.

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 processes all have to call TSocket->setLinger(0,0) or (1,timeout) before deleting the TSocket, closing the TSocket, or exiting. (This workaround only succeeds with the suggested fix in THRIFT-747 ).

However, the design choice also prevents deadlock/slowdown issues where a forked process holds open a copy of the parent's Thrift connections. It also makes close non-blocking, which is ideal in a destructor.

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 default linger interval of 0 turns the linger setting into a hard reset.
And in the absence of linger, the kernel can usually send small thrift messages by itself.

Options:

• Change the default lingerOn to 0 - rely on the kernel to resend a limited number of times
• Change the default lingerVal to > 0

• a large value like INT_MAX would match the default connection, send, and recv 'no timeout' behaviour

TODO:

• Confirm issue on Linux - see attached test code
• Decide if a change to the defaults is needed
• Document workaround after resolution of THRIFT-747 - call TSocket->setLinger(0,0) or (1,timeout) if forking

Issue Links

• is blocked by: THRIFT-747 C++ TSocket->close calls shutdown breaking forked parent process
• is related to: THRIFT-4479 Enhance Windows CI build with 32/64 cygwin/mingw builds like in Boost.Format
• relates to: THRIFT-5374 SO_LINGER should not be set on AF_UNIX socket on WSL (portability issue)