bugdown: Spend at most 5 seconds rendering a message

(imported from commit bc092acc8b2b9f8a63af669de06c6f7512ccf8c9)

zephyr/lib/bugdown/__init__.py

@@ -7,6 +7,7 @@ import re
 from zephyr.lib.avatar  import gravatar_hash
 from zephyr.lib.bugdown import codehilite, fenced_code
 from zephyr.lib.bugdown.fenced_code import FENCE_RE
+from zephyr.lib.timeout import timeout
 
 class Gravatar(markdown.inlinepatterns.Pattern):
     def handleMatch(self, match):
@@ -179,7 +180,10 @@ def convert(md):
     _md_engine.reset()
 
     try:
-        html = _md_engine.convert(md)
+        # Spend at most 5 seconds rendering.
+        # Sometimes Python-Markdown is really slow; see
+        # https://trac.humbughq.com/ticket/345
+        html = timeout(5, _md_engine.convert, md)
     except:
         # FIXME: Do something more reasonable here!
         html = '<p>[Humbug note: Sorry, we could not understand the formatting of your message]</p>'

zephyr/lib/timeout.py

@@ -0,0 +1,45 @@
+import threading
+
+# Based on http://code.activestate.com/recipes/483752/
+
+class TimeoutExpired(Exception):
+    '''Exception raised when a function times out.'''
+    def __str__(self):
+        return 'Function call timed out.'
+
+def timeout(timeout, func, *args, **kwargs):
+    '''Call the function in a separate thread.
+       Return its return value, or raise an exception,
+       within 'timeout' seconds.
+
+       The function may still be running in the background!
+
+       This may also fail to interrupt functions which are
+       stuck in a long-running primitive interpreter
+       operation.'''
+
+    class TimeoutThread(threading.Thread):
+        def __init__(self):
+            threading.Thread.__init__(self)
+            self.result = None
+            self.exn    = None
+
+            # Don't block the whole program from exiting
+            # if this is the only thread left.
+            self.daemon = True
+
+        def run(self):
+            try:
+                self.result = func(*args, **kwargs)
+            except BaseException, e:
+                self.exn = e
+
+    thread = TimeoutThread()
+    thread.start()
+    thread.join(timeout)
+
+    if thread.isAlive():
+        raise TimeoutExpired
+    if thread.exn:
+        raise thread.exn
+    return thread.result