Implement stack trace dump for all running threads on SIGQUIT

This is handy when we're debugging the thread locks: we can try to
understand which thread does what and how it was called.

Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>

Changelog.rst

@@ -7,7 +7,8 @@ ChangeLog
 WIP (add new stuff for the next release)
 ========================================
 
-=======
+* Dump stacktrace for all threads on SIGQUIT: ease debugging
+  of threading and other issues
 * SIGHUP is now handled as the termination notification rather than
   the signal to reread the configuration (Dmitrijs Ledkovs)
 * Honor the timezone of emails (Tobias Thierer)

docs/MANUAL.rst

@@ -309,7 +309,7 @@ UNIX Signals
 ============
 
 OfflineImap listens to the unix signals SIGUSR1, SIGUSR2, SIGTERM,
-SIGINT, SIGHUP:
+SIGINT, SIGHUP, SIGQUIT:
 
 If sent a SIGUSR1 it will abort any current (or next future) sleep of all
 accounts that are configured to "autorefresh". In effect, this will trigger a
@@ -326,6 +326,9 @@ in each account, close keep alive connections, remove locks on the
 accounts and exit. It may take up to 10 seconds, if autorefresh option
 is used.
 
+SIGQUIT dumps stack traces for all threads and tries to dump process
+core.
+
 Folder filtering and nametrans
 ==============================
 

offlineimap/init.py

@@ -28,6 +28,7 @@ from offlineimap import accounts, threadutil, syncmaster
 from offlineimap.error import OfflineImapError
 from offlineimap.ui import UI_LIST, setglobalui, getglobalui
 from offlineimap.CustomConfig import CustomConfigParser
+from offlineimap.utils import stacktrace
 
 
 class OfflineImap:
@@ -341,12 +342,16 @@ class OfflineImap:
                     getglobalui().warn("Terminating NOW (this may "\
                                        "take a few seconds)...")
                     accounts.Account.set_abort_event(self.config, 3)
+                elif sig == signal.SIGQUIT:
+                    stacktrace.dump (sys.stderr)
+                    os.abort()
 
             signal.signal(signal.SIGHUP,sig_handler)
             signal.signal(signal.SIGUSR1,sig_handler)
             signal.signal(signal.SIGUSR2,sig_handler)
             signal.signal(signal.SIGTERM, sig_handler)
             signal.signal(signal.SIGINT, sig_handler)
+            signal.signal(signal.SIGQUIT, sig_handler)
 
             #various initializations that need to be performed:
             offlineimap.mbnames.init(self.config, syncaccounts)

offlineimap/utils/stacktrace.py

@@ -0,0 +1,25 @@
+# Copyright 2013 Eygene A. Ryabinkin
+# Functions to perform stack tracing (for multithreaded programs
+# as well as for single-threaded ones).
+
+import sys
+import threading
+import traceback
+
+
+def dump(out):
+	""" Dumps current stack trace into I/O object 'out' """
+	id2name = {}
+	for th in threading.enumerate():
+		id2name[th.ident] = th.name
+	n = 0
+	for i, stack in sys._current_frames().items():
+		out.write ("\n# Thread #%d (id=%d), %s\n" % \
+		  (n, i, id2name[i]))
+		n = n + 1
+		for f, lno, name, line in traceback.extract_stack (stack):
+			out.write ('File: "%s", line %d, in %s' % \
+			  (f, lno, name))
+			if line:
+				out.write (" %s" % (line.strip()))
+			out.write ("\n")