worker: POSIX only supports workers from main thread (issue6460)

The POSIX backend sets signal handlers for SIGINT (maybe avoidable) and
SIGCHLD (necessary for waitpid). Python up to 3.9 only allow this from
the main thread, so disable the worker feature otherwise.

Differential Revision: https://phab.mercurial-scm.org/D9660

mercurial/worker.py

@@ -67,6 +67,9 b' def _numworkers(ui):'
 
 if pycompat.ispy3:
 
+    def ismainthread():
+        return threading.current_thread() == threading.main_thread()
+
     class _blockingreader(object):
         def __init__(self, wrapped):
             self._wrapped = wrapped
@@ -100,6 +103,9 b' if pycompat.ispy3:'
 
 else:
 
+    def ismainthread():
+        return isinstance(threading.current_thread(), threading._MainThread)
+
     def _blockingreader(wrapped):
         return wrapped
 
@@ -155,6 +161,11 b' def worker('
     release the GIL.
     """
     enabled = ui.configbool(b'worker', b'enabled')
+    if enabled and _platformworker is _posixworker and not ismainthread():
+        # The POSIX worker has to install a handler for SIGCHLD.
+        # Python up to 3.9 only allows this in the main thread.
+        enabled = False
+
     if enabled and worthwhile(ui, costperarg, len(args), threadsafe=threadsafe):
         return _platformworker(ui, func, staticargs, args, hasretval)
     return func(*staticargs + (args,))