bundle2: fail faster when interrupted

Before this patch, bundle2 application attempted to consume remaining
bundle2 part data when the process is interrupted (SIGINT) or when
sys.exit is called (translated into a SystemExit exception). This
meant that if one of these occurred when applying a say 1 GB
changegroup bundle2 part being downloaded over a network, it may take
Mercurial *several minutes* to terminate after a SIGINT because the
process is waiting on the network to stream megabytes of data. This is
not a great user experience and a regression from bundle1. Furthermore,
many process supervisors tend to only give processes a finite amount of
time to exit after delivering SIGINT: if processes take too long to
self-terminate, a SIGKILL is issued and Mercurial has no opportunity to
clean up. This would mean orphaned locks and transactions. Not good.

This patch changes the bundle2 application behavior to fail faster
when an interrupt or system exit is requested. It does so by not
catching BaseException (which includes KeyboardInterrupt and
SystemExit) and by explicitly checking for these conditions in
yet another handler which would also seek to the end of the current
bundle2 part on failure.

The end result of this patch is that SIGINT is now reacted to
significantly faster: the active transaction is rolled back
immediately without waiting for incoming bundle2 data to be consumed.
This restores the pre-bundle2 behavior and makes Mercurial treat
signals with the urgency they deserve.

mercurial/bundle2.py

@@ -353,7 +353,7 def processbundle(repo, unbundler, trans
     try:
         for nbpart, part in iterparts:
             _processpart(op, part)
-    except BaseException as exc:
+    except Exception as exc:
         for nbpart, part in iterparts:
             # consume the bundle content
             part.seek(0, 2)
@@ -382,6 +382,7 def _processpart(op, part):
     The part is guaranteed to have been fully consumed when the function exits
     (even if an exception is raised)."""
     status = 'unknown' # used by debug output
+    hardabort = False
     try:
         try:
             handler = parthandlermapping.get(part.type)
@@ -436,8 +437,14 def _processpart(op, part):
                 outpart = op.reply.newpart('output', data=output,
                                            mandatory=False)
                 outpart.addparam('in-reply-to', str(part.id), mandatory=False)
+    # If exiting or interrupted, do not attempt to seek the stream in the
+    # finally block below. This makes abort faster.
+    except (SystemExit, KeyboardInterrupt):
+        hardabort = True
+        raise
     finally:
         # consume the part content to not corrupt the stream.
+        if not hardabort:
         part.seek(0, 2)