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>

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")