Make it possible to debug failed hook imports via use of --traceback

Prior to this change, if a Python hook module failed to load (e.g. due
to an import error or path problem), it was impossible to figure out
why the error occurred, because the ImportErrors that got raised were
caught but never displayed.

If run with --traceback or ui.traceback=True, hg now prints tracebacks
of both of the ImportError instances that get raised before it bails.

mercurial/hook.py

@@ -37,10 +37,18 b' def _pythonhook(ui, repo, name, hname, f'
         try:
             obj = __import__(modname)
         except ImportError:
+            e1 = sys.exc_type, sys.exc_value, sys.exc_traceback
             try:
                 # extensions are loaded with hgext_ prefix
                 obj = __import__("hgext_%s" % modname)
             except ImportError:
+                e2 = sys.exc_type, sys.exc_value, sys.exc_traceback
+                if ui.tracebackflag:
+                    ui.warn(_('exception from first failed import attempt:\n'))
+                ui.traceback(e1)
+                if ui.tracebackflag:
+                    ui.warn(_('exception from second failed import attempt:\n'))
+                ui.traceback(e2)
                 raise util.Abort(_('%s hook is invalid '
                                    '(import of "%s" failed)') %
                                  (hname, modname))

mercurial/ui.py

@@ -15,7 +15,7 b' import config, util, error'
 class ui(object):
     def __init__(self, src=None):
         self._buffers = []
-        self.quiet = self.verbose = self.debugflag = self._traceback = False
+        self.quiet = self.verbose = self.debugflag = self.tracebackflag = False
         self._reportuntrusted = True
         self._ocfg = config.config() # overlay
         self._tcfg = config.config() # trusted
@@ -101,7 +101,7 b' class ui(object):'
         if self.verbose and self.quiet:
             self.quiet = self.verbose = False
         self._reportuntrusted = self.configbool("ui", "report_untrusted", True)
-        self._traceback = self.configbool('ui', 'traceback', False)
+        self.tracebackflag = self.configbool('ui', 'traceback', False)
 
         # update trust information
         self._trustusers.update(self.configlist('trusted', 'users'))
@@ -337,13 +337,16 b' class ui(object):'
 
         return t
 
-    def traceback(self):
+    def traceback(self, exc=None):
         '''print exception traceback if traceback printing enabled.
         only to call in exception handler. returns true if traceback
         printed.'''
-        if self._traceback:
+        if self.tracebackflag:
+            if exc:
+                traceback.print_exception(exc[0], exc[1], exc[2])
+            else:
             traceback.print_exc()
-        return self._traceback
+        return self.tracebackflag
 
     def geteditor(self):
         '''return editor to use'''

tests/test-hook

@@ -248,4 +248,18 b' echo "pre-commit.test = python:`pwd`/tes'
 cd ../repo
 hg commit
 
+cd ../../b
+echo '# make sure --traceback works on hook import failure'
+cat > importfail.py <<EOF
+import somebogusmodule
+# dereference something in the module to force demandimport to load it
+somebogusmodule.whatever
+EOF
+
+echo '[hooks]' > .hg/hgrc
+echo 'precommit.importfail = python:importfail.whatever' >> .hg/hgrc
+
+echo a >> a
+hg --traceback commit -Ama 2>&1 | grep '^\(exception\|Traceback\|ImportError\)'
+
 exit 0

tests/test-hook.out

@@ -163,3 +163,11 b' hooks.commit.auto=<function autohook>'
 # test python hook configured with python:[file]:[hook] syntax
 hook works
 nothing changed
+# make sure --traceback works on hook import failure
+exception from first failed import attempt:
+Traceback (most recent call last):
+ImportError: No module named somebogusmodule
+exception from second failed import attempt:
+Traceback (most recent call last):
+ImportError: No module named hgext_importfail
+Traceback (most recent call last):