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CleanSignalHandling

Please Add comments at CleanSignalHandlingDiscussion^x.

Contact

Tres Seaver

Problem

The "stop" script we ship with Zope uses (in non-Win32^x platforms) a signal to down the Zope server processes:

   kill `cat /path/to/zope/var/Z2.pid`

The Zope process does nothing to arrange for handling this signal cleanly; hence, it gets taken down hard, without the opportunity to peform cleanup (e.g., the ZODB.FileStorage^x doesn't get a chance to construct its index).

This proposal is related to the broader one of easing Zope administration -- see BetterStartStopScripts^x for that problem.

Proposed Solutions

Add signal handling to ZServer^x, to permit more uniform control of the server from the command line. In particular:

• Trap the signal.SIGTERM signal, and call the moral equivalent of manage_shutdown.

• Trap the signal.SIGHUP signal, and call the moral equivalent of manage_restart.

Note

This has been figured out and done for the ZEO storage server.

Risk Factors

• Signal handling, especially in conjunction with threading, can be tricky:

  • Python, for instance, makes no guarantee that signal handlers will be called immediately (long running C-level extensions can induce lag between signal delivery and dispatch to the handler.)

  • Signals during I/O can cause the I/O operation to fail with errno == errno.EITNTR; this is one of the platform-dependent areas of Python.

  • Python signals are not well-supported on Win32^x; OTOH, this lack is mitigated by the fact that most "production" Zope servers on Win32^x will be controlled through the Win32^x Service Control Manager.

  • Do we have a way to test for specific flavors of Unix? If so, we might want only to enable this feature on the platforms we have tested it on.

Scope

• Add code to install signal handlers to the appropriate spot in the mainline, z2.py (they have to be installed by the main thread); bracket this installation with a test of os.name.

Deliverables

• Checkin to z2.py

• Add documentation of signal usage to the ZopeAdministratorsGuide^x (or its moral equivalent).