diff --git a/hg b/hg
--- a/hg
+++ b/hg
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # mercurial - scalable distributed SCM
 #
diff --git a/mercurial/utils/procutil.py b/mercurial/utils/procutil.py
--- a/mercurial/utils/procutil.py
+++ b/mercurial/utils/procutil.py
@@ -16,6 +16,7 @@
 import signal
 import subprocess
 import sys
+import threading
 import time
 
 from ..i18n import _
@@ -604,6 +605,14 @@
             pid = os.fork()
             if pid:
                 if not ensurestart:
+                    # Even though we're not waiting on the child process,
+                    # we still must call waitpid() on it at some point so
+                    # it's not a zombie/defunct. This is especially relevant for
+                    # chg since the parent process won't die anytime soon.
+                    # We use a thread to make the overhead tiny.
+                    def _do_wait():
+                        os.waitpid(pid, 0)
+                    threading.Thread(target=_do_wait, daemon=True).start()
                     return
                 # Parent process
                 (_pid, status) = os.waitpid(pid, 0)