util: introduce util.debugstacktrace for showing a stack trace without crashing

util: introduce util.debugstacktrace for showing a stack trace without crashing

This is often very handy when hacking/debugging.

Calling util.debugstacktrace('hey') from a place in hg will give something like:
  hey at:
   ./hg:38                                     in <module>
   /home/user/hgsrc/mercurial/dispatch.py:28   in run
   /home/user/hgsrc/mercurial/dispatch.py:65   in dispatch
   /home/user/hgsrc/mercurial/dispatch.py:88   in _runcatch
   /home/user/hgsrc/mercurial/dispatch.py:740  in _dispatch
   /home/user/hgsrc/mercurial/dispatch.py:514  in runcommand
   /home/user/hgsrc/mercurial/dispatch.py:830  in _runcommand
   /home/user/hgsrc/mercurial/dispatch.py:801  in checkargs
   /home/user/hgsrc/mercurial/dispatch.py:737  in <lambda>
   /home/user/hgsrc/mercurial/util.py:472      in check
...

--- a/contrib/lock-checker.py	Tue Jan 07 22:29:57 2014 +0100
+++ b/contrib/lock-checker.py	Sun Jan 12 23:28:21 2014 +0100
@@ -7,21 +7,12 @@
 This currently only checks store locks, not working copy locks.
 """
 import os
-import traceback
-
-def _warnstack(ui, msg, skip=1):
-    '''issue warning with the message and the current stack, skipping the
-    skip last entries'''
-    ui.warn('%s at:\n' % msg)
-    entries = traceback.extract_stack()[:-skip]
-    fnmax = max(len(entry[0]) for entry in entries)
-    for fn, ln, func, _text in entries:
-        ui.warn(' %*s:%-4s in %s\n' % (fnmax, fn, ln, func))
+from mercurial import util
 
 def _checklock(repo):
     l = repo._lockref and repo._lockref()
     if l is None or not l.held:
-        _warnstack(repo.ui, 'missing lock', skip=2)
+        util.debugstacktrace('missing lock', skip=1)
 
 def reposetup(ui, repo):
     orig = repo.__class__

--- a/mercurial/util.py	Tue Jan 07 22:29:57 2014 +0100
+++ b/mercurial/util.py	Sun Jan 12 23:28:21 2014 +0100
@@ -1983,3 +1983,20 @@
         self._hooks.sort(key=lambda x: x[0])
         for source, hook in self._hooks:
             hook(*args)
+
+def debugstacktrace(msg='stacktrace', skip=0, f=sys.stderr):
+    '''Writes a message to f (stderr) with a nicely formatted stacktrace.
+    Skips the 'skip' last entries.
+    It can be used everywhere and do intentionally not require an ui object.
+    Not be used in production code but very convenient while developing.
+    '''
+    f.write('%s at:\n' % msg)
+    entries = [('%s:%s' % (fn, ln), func)
+        for fn, ln, func, _text in traceback.extract_stack()[:-skip - 1]]
+    if entries:
+        fnmax = max(len(entry[0]) for entry in entries)
+        for fnln, func in entries:
+            f.write(' %-*s in %s\n' % (fnmax, fnln, func))
+
+# convenient shortcut
+dst = debugstacktrace

--- a/tests/test-debugcommands.t	Tue Jan 07 22:29:57 2014 +0100
+++ b/tests/test-debugcommands.t	Sun Jan 12 23:28:21 2014 +0100
@@ -23,3 +23,25 @@
   uncompressed data size (min/max/avg) : 43 / 43 / 43
   full revision size (min/max/avg)     : 44 / 44 / 44
   delta size (min/max/avg)             : 0 / 0 / 0
+
+
+Test internal debugstacktrace command
+
+  $ cat > debugstacktrace.py << EOF
+  > from mercurial.util import debugstacktrace, dst, sys
+  > def f():
+  >     dst('hello world')
+  > def g():
+  >     f()
+  >     debugstacktrace(skip=-5, f=sys.stdout)
+  > g()
+  > EOF
+  $ python debugstacktrace.py
+  hello world at:
+   debugstacktrace.py:7 in <module>
+   debugstacktrace.py:5 in g
+   debugstacktrace.py:3 in f
+  stacktrace at:
+   debugstacktrace.py:7 *in <module> (glob)
+   debugstacktrace.py:6 *in g (glob)
+   */util.py:* in debugstacktrace (glob)