worker: raise exception instead of calling sys.exit() with child's code

When a worker process returns an error code, we would call
`sys.exit()` with that exit code on the main process. The `SystemExit`
exception would then get caught in `scmutil.callcatch()`, which would
return that error code. The comment there says "Commands shouldn't
sys.exit directly", which I agree with. This patch changes it so we
raise a specific exception when a worker fails so we can catch
instead. I think that means that `SystemExit` is now always an
internal error.

(I had earlier thought that this call to `sys.exit()` was from within
the child process until Matt Harbison made me look again, so thanks
for that!)

Differential Revision: https://phab.mercurial-scm.org/D9287

mercurial/error.py

@@ -130,6 +130,13 b' class AmbiguousCommand(Exception):'
     __bytes__ = _tobytes
 
 
+class WorkerError(Exception):
+    """Exception raised when a worker process dies."""
+
+    def __init__(self, status_code):
+        self.status_code = status_code
+
+
 class InterventionRequired(Hint, Exception):
     """Exception raised when a command requires human intervention."""
 

mercurial/scmutil.py

@@ -219,6 +219,9 b' def callcatch(ui, func):'
         ui.error(_(b"abort: %s\n") % inst.message)
         if inst.hint:
             ui.error(_(b"(%s)\n") % inst.hint)
+    except error.WorkerError as inst:
+        # Don't print a message -- the worker already should have
+        return inst.status_code
     except ImportError as inst:
         ui.error(_(b"abort: %s!\n") % stringutil.forcebytestr(inst))
         m = stringutil.forcebytestr(inst).split()[-1]

mercurial/worker.py

@@ -300,7 +300,7 b' def _posixworker(ui, func, staticargs, a'
     if status:
         if status < 0:
             os.kill(os.getpid(), -status)
-        sys.exit(status)
+        raise error.WorkerError(status)
     if hasretval:
         yield True, retval
 

tests/test-worker.t

@@ -85,11 +85,12 b' Known exception should be caught, but pr'
   [255]
 
   $ hg --config "extensions.t=$abspath" --config 'worker.numcpus=8' \
-  > test 100000.0 abort --traceback 2>&1 | egrep '(SystemExit|Abort)'
+  > test 100000.0 abort --traceback 2>&1 | egrep '(WorkerError|Abort)'
       raise error.Abort(b'known exception')
   mercurial.error.Abort: known exception (py3 !)
   Abort: known exception (no-py3 !)
-  SystemExit: 255
+      raise error.WorkerError(status)
+  mercurial.error.WorkerError: 255
 
 Traceback must be printed for unknown exceptions