worker: rewrite error handling so os._exit covers all cases

Previously the worker error handling is like:

    pid = os.fork()   --+
    if pid == 0:        |
        ....            | problematic
        ....          --+
        try:          --+
            ....        | worker error handling
                      --+

If a signal arrives when Python is executing the "problematic" lines, an
external error handling (dispatch.py) will take over the control flow and
it's no longer guaranteed "os._exit" is called (see 86cd09bc13ba for why it
is necessary).

This patch rewrites the error handling so it covers all possible code paths
for a worker even during fork.

Note: "os.getpid() == parentpid" is used to test if the process is parent or
not intentionally, instead of checking "pid", because "pid = os.fork()" may
be not atomic - it's possible that that a signal hits the worker before the
assignment completes [1].  The newly added test replaces "os.fork" to
exercise that extreme case.

[1]: CPython compiles "pid = os.fork()" to 2 byte codes: "CALL_FUNCTION" and
"STORE_FAST", so it's probably not atomic:

    def f():
        pid = os.fork()

    dis.dis(f)
      2           0 LOAD_GLOBAL              0 (os)
                  3 LOAD_ATTR                1 (fork)
                  6 CALL_FUNCTION            0
                  9 STORE_FAST               0 (pid)
                 12 LOAD_CONST               0 (None)
                 15 RETURN_VALUE

mercurial/worker.py

@@ -1,234 +1,240
 # worker.py - master-slave parallelism support
 #
 # Copyright 2013 Facebook, Inc.
 #
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 
 from __future__ import absolute_import
 
 import errno
 import os
 import signal
 import sys
 
 from .i18n import _
 from . import (
     encoding,
     error,
     pycompat,
     scmutil,
     util,
 )
 
 def countcpus():
     '''try to count the number of CPUs on the system'''
 
     # posix
     try:
         n = int(os.sysconf('SC_NPROCESSORS_ONLN'))
         if n > 0:
             return n
     except (AttributeError, ValueError):
         pass
 
     # windows
     try:
         n = int(encoding.environ['NUMBER_OF_PROCESSORS'])
         if n > 0:
             return n
     except (KeyError, ValueError):
         pass
 
     return 1
 
 def _numworkers(ui):
     s = ui.config('worker', 'numcpus')
     if s:
         try:
             n = int(s)
             if n >= 1:
                 return n
         except ValueError:
             raise error.Abort(_('number of cpus must be an integer'))
     return min(max(countcpus(), 4), 32)
 
 if pycompat.osname == 'posix':
     _startupcost = 0.01
 else:
     _startupcost = 1e30
 
 def worthwhile(ui, costperop, nops):
     '''try to determine whether the benefit of multiple processes can
     outweigh the cost of starting them'''
     linear = costperop * nops
     workers = _numworkers(ui)
     benefit = linear - (_startupcost * workers + linear / workers)
     return benefit >= 0.15
 
 def worker(ui, costperarg, func, staticargs, args):
     '''run a function, possibly in parallel in multiple worker
     processes.
 
     returns a progress iterator
 
     costperarg - cost of a single task
 
     func - function to run
 
     staticargs - arguments to pass to every invocation of the function
 
     args - arguments to split into chunks, to pass to individual
     workers
     '''
     if worthwhile(ui, costperarg, len(args)):
         return _platformworker(ui, func, staticargs, args)
     return func(*staticargs + (args,))
 
 def _posixworker(ui, func, staticargs, args):
     rfd, wfd = os.pipe()
     workers = _numworkers(ui)
     oldhandler = signal.getsignal(signal.SIGINT)
     signal.signal(signal.SIGINT, signal.SIG_IGN)
     pids, problem = set(), [0]
     def killworkers():
         # unregister SIGCHLD handler as all children will be killed. This
         # function shouldn't be interrupted by another SIGCHLD; otherwise pids
         # could be updated while iterating, which would cause inconsistency.
         signal.signal(signal.SIGCHLD, oldchldhandler)
         # if one worker bails, there's no good reason to wait for the rest
         for p in pids:
             try:
                 os.kill(p, signal.SIGTERM)
             except OSError as err:
                 if err.errno != errno.ESRCH:
                     raise
     def waitforworkers(blocking=True):
         for pid in pids.copy():
             p = st = 0
             while True:
                 try:
                     p, st = os.waitpid(pid, (0 if blocking else os.WNOHANG))
                     break
                 except OSError as e:
                     if e.errno == errno.EINTR:
                         continue
                     elif e.errno == errno.ECHILD:
                         # child would already be reaped, but pids yet been
                         # updated (maybe interrupted just after waitpid)
                         pids.discard(pid)
                         break
                     else:
                         raise
             if not p:
                 # skip subsequent steps, because child process should
                 # be still running in this case
                 continue
             pids.discard(p)
             st = _exitstatus(st)
             if st and not problem[0]:
                 problem[0] = st
     def sigchldhandler(signum, frame):
         waitforworkers(blocking=False)
         if problem[0]:
             killworkers()
     oldchldhandler = signal.signal(signal.SIGCHLD, sigchldhandler)
     ui.flush()
+    parentpid = os.getpid()
     for pargs in partition(args, workers):
-        pid = os.fork()
-        if pid == 0:
-            signal.signal(signal.SIGINT, oldhandler)
-            signal.signal(signal.SIGCHLD, oldchldhandler)
-
-            def workerfunc():
-                os.close(rfd)
-                for i, item in func(*(staticargs + (pargs,))):
-                    os.write(wfd, '%d %s\n' % (i, item))
-                return 0
+        # make sure we use os._exit in all worker code paths. otherwise the
+        # worker may do some clean-ups which could cause surprises like
+        # deadlock. see sshpeer.cleanup for example.
+        # override error handling *before* fork. this is necessary because
+        # exception (signal) may arrive after fork, before "pid =" assignment
+        # completes, and other exception handler (dispatch.py) can lead to
+        # unexpected code path without os._exit.
+        ret = -1
+        try:
+            pid = os.fork()
+            if pid == 0:
+                signal.signal(signal.SIGINT, oldhandler)
+                signal.signal(signal.SIGCHLD, oldchldhandler)
 
-            # make sure we use os._exit in all code paths. otherwise the worker
-            # may do some clean-ups which could cause surprises like deadlock.
-            # see sshpeer.cleanup for example.
-            ret = 0
-            try:
+                def workerfunc():
+                    os.close(rfd)
+                    for i, item in func(*(staticargs + (pargs,))):
+                        os.write(wfd, '%d %s\n' % (i, item))
+                    return 0
+
+                ret = scmutil.callcatch(ui, workerfunc)
+        except: # parent re-raises, child never returns
+            if os.getpid() == parentpid:
+                raise
+            exctype = sys.exc_info()[0]
+            force = not issubclass(exctype, KeyboardInterrupt)
+            ui.traceback(force=force)
+        finally:
+            if os.getpid() != parentpid:
                 try:
-                    ret = scmutil.callcatch(ui, workerfunc)
-                finally:
                     ui.flush()
-            except KeyboardInterrupt:
-                os._exit(255)
-            except: # never return, therefore no re-raises
-                try:
-                    ui.traceback(force=True)
-                    ui.flush()
+                except: # never returns, no re-raises
+                    pass
                 finally:
-                    os._exit(255)
-            else:
-                os._exit(ret & 255)
+                    os._exit(ret & 255)
         pids.add(pid)
     os.close(wfd)
     fp = os.fdopen(rfd, pycompat.sysstr('rb'), 0)
     def cleanup():
         signal.signal(signal.SIGINT, oldhandler)
         waitforworkers()
         signal.signal(signal.SIGCHLD, oldchldhandler)
         status = problem[0]
         if status:
             if status < 0:
                 os.kill(os.getpid(), -status)
             sys.exit(status)
     try:
         for line in util.iterfile(fp):
             l = line.split(' ', 1)
             yield int(l[0]), l[1][:-1]
     except: # re-raises
         killworkers()
         cleanup()
         raise
     cleanup()
 
 def _posixexitstatus(code):
     '''convert a posix exit status into the same form returned by
     os.spawnv
 
     returns None if the process was stopped instead of exiting'''
     if os.WIFEXITED(code):
         return os.WEXITSTATUS(code)
     elif os.WIFSIGNALED(code):
         return -os.WTERMSIG(code)
 
 if pycompat.osname != 'nt':
     _platformworker = _posixworker
     _exitstatus = _posixexitstatus
 
 def partition(lst, nslices):
     '''partition a list into N slices of roughly equal size
 
     The current strategy takes every Nth element from the input. If
     we ever write workers that need to preserve grouping in input
     we should consider allowing callers to specify a partition strategy.
 
     mpm is not a fan of this partitioning strategy when files are involved.
     In his words:
 
         Single-threaded Mercurial makes a point of creating and visiting
         files in a fixed order (alphabetical). When creating files in order,
         a typical filesystem is likely to allocate them on nearby regions on
         disk. Thus, when revisiting in the same order, locality is maximized
         and various forms of OS and disk-level caching and read-ahead get a
         chance to work.
 
         This effect can be quite significant on spinning disks. I discovered it
         circa Mercurial v0.4 when revlogs were named by hashes of filenames.
         Tarring a repo and copying it to another disk effectively randomized
         the revlog ordering on disk by sorting the revlogs by hash and suddenly
         performance of my kernel checkout benchmark dropped by ~10x because the
         "working set" of sectors visited no longer fit in the drive's cache and
         the workload switched from streaming to random I/O.
 
         What we should really be doing is have workers read filenames from a
         ordered queue. This preserves locality and also keeps any worker from
         getting more than one file out of balance.
     '''
     for i in range(nslices):
         yield lst[i::nslices]

tests/test-worker.t

@@ -1,94 +1,126
 Test UI worker interaction
 
   $ cat > t.py <<EOF
   > from __future__ import absolute_import, print_function
   > from mercurial import (
   >     cmdutil,
   >     error,
   >     ui as uimod,
   >     worker,
   > )
   > def abort(ui, args):
   >     if args[0] == 0:
   >         # by first worker for test stability
   >         raise error.Abort('known exception')
   >     return runme(ui, [])
   > def exc(ui, args):
   >     if args[0] == 0:
   >         # by first worker for test stability
   >         raise Exception('unknown exception')
   >     return runme(ui, [])
   > def runme(ui, args):
   >     for arg in args:
   >         ui.status('run\n')
   >         yield 1, arg
   > functable = {
   >     'abort': abort,
   >     'exc': exc,
   >     'runme': runme,
   > }
   > cmdtable = {}
   > command = cmdutil.command(cmdtable)
   > @command('test', [], 'hg test [COST] [FUNC]')
   > def t(ui, repo, cost=1.0, func='runme'):
   >     cost = float(cost)
   >     func = functable[func]
   >     ui.status('start\n')
   >     runs = worker.worker(ui, cost, func, (ui,), range(8))
   >     for n, i in runs:
   >         pass
   >     ui.status('done\n')
   > EOF
   $ abspath=`pwd`/t.py
   $ hg init
 
 Run tests with worker enable by forcing a heigh cost
 
   $ hg --config "extensions.t=$abspath" test 100000.0
   start
   run
   run
   run
   run
   run
   run
   run
   run
   done
 
 Run tests without worker by forcing a low cost
 
   $ hg --config "extensions.t=$abspath" test 0.0000001
   start
   run
   run
   run
   run
   run
   run
   run
   run
   done
 
 #if no-windows
 
 Known exception should be caught, but printed if --traceback is enabled
 
   $ hg --config "extensions.t=$abspath" --config 'worker.numcpus=2' \
   > test 100000.0 abort
   start
   abort: known exception
   [255]
 
   $ hg --config "extensions.t=$abspath" --config 'worker.numcpus=2' \
   > test 100000.0 abort --traceback 2>&1 | grep '^Traceback'
   Traceback (most recent call last):
   Traceback (most recent call last):
 
 Traceback must be printed for unknown exceptions
 
   $ hg --config "extensions.t=$abspath" --config 'worker.numcpus=2' \
   > test 100000.0 exc 2>&1 | grep '^Traceback'
   Traceback (most recent call last):
 
+Workers should not do cleanups in all cases
+
+  $ cat > $TESTTMP/detectcleanup.py <<EOF
+  > from __future__ import absolute_import
+  > import atexit
+  > import os
+  > import time
+  > oldfork = os.fork
+  > count = 0
+  > parentpid = os.getpid()
+  > def delayedfork():
+  >     global count
+  >     count += 1
+  >     pid = oldfork()
+  >     # make it easier to test SIGTERM hitting other workers when they have
+  >     # not set up error handling yet.
+  >     if count > 1 and pid == 0:
+  >         time.sleep(0.1)
+  >     return pid
+  > os.fork = delayedfork
+  > def cleanup():
+  >     if os.getpid() != parentpid:
+  >         os.write(1, 'should never happen\n')
+  > atexit.register(cleanup)
+  > EOF
+
+  $ hg --config "extensions.t=$abspath" --config worker.numcpus=8 --config \
+  > "extensions.d=$TESTTMP/detectcleanup.py" test 100000 abort
+  start
+  abort: known exception
+  [255]
+
 #endif