--- a/mercurial/utils/procutil.py	Wed Oct 03 13:54:45 2018 -0400
+++ b/mercurial/utils/procutil.py	Wed Oct 03 14:01:04 2018 -0400
@@ -10,6 +10,7 @@
 from __future__ import absolute_import
 
 import contextlib
+import errno
 import imp
 import io
 import os
@@ -467,3 +468,74 @@
             signal.signal(signal.SIGINT, oldsiginthandler[0])
         if shouldbail:
             raise KeyboardInterrupt
+
+if pycompat.iswindows:
+    # no fork on Windows, but we can create a detached process
+    # https://msdn.microsoft.com/en-us/library/windows/desktop/ms684863.aspx
+    # No stdlib constant exists for this value
+    DETACHED_PROCESS = 0x00000008
+    _creationflags = DETACHED_PROCESS | subprocess.CREATE_NEW_PROCESS_GROUP
+
+    def runbgcommand(script, env, shell=False, stdout=None, stderr=None):
+        '''Spawn a command without waiting for it to finish.'''
+        # we can't use close_fds *and* redirect stdin. I'm not sure that we
+        # need to because the detached process has no console connection.
+        subprocess.Popen(
+            script, shell=shell, env=env, close_fds=True,
+            creationflags=_creationflags, stdout=stdout, stderr=stderr)
+else:
+    def runbgcommand(cmd, env, shell=False, stdout=None, stderr=None):
+        '''Spawn a command without waiting for it to finish.'''
+        # double-fork to completely detach from the parent process
+        # based on http://code.activestate.com/recipes/278731
+        pid = os.fork()
+        if pid:
+            # Parent process
+            (_pid, status) = os.waitpid(pid, 0)
+            if os.WIFEXITED(status):
+                returncode = os.WEXITSTATUS(status)
+            else:
+                returncode = -os.WTERMSIG(status)
+            if returncode != 0:
+                # The child process's return code is 0 on success, an errno
+                # value on failure, or 255 if we don't have a valid errno
+                # value.
+                #
+                # (It would be slightly nicer to return the full exception info
+                # over a pipe as the subprocess module does.  For now it
+                # doesn't seem worth adding that complexity here, though.)
+                if returncode == 255:
+                    returncode = errno.EINVAL
+                raise OSError(returncode, 'error running %r: %s' %
+                              (cmd, os.strerror(returncode)))
+            return
+
+        returncode = 255
+        try:
+            # Start a new session
+            os.setsid()
+
+            stdin = open(os.devnull, 'r')
+            if stdout is None:
+                stdout = open(os.devnull, 'w')
+            if stderr is None:
+                stderr = open(os.devnull, 'w')
+
+            # connect stdin to devnull to make sure the subprocess can't
+            # muck up that stream for mercurial.
+            subprocess.Popen(
+                cmd, shell=shell, env=env, close_fds=True,
+                stdin=stdin, stdout=stdout, stderr=stderr)
+            returncode = 0
+        except EnvironmentError as ex:
+            returncode = (ex.errno & 0xff)
+            if returncode == 0:
+                # This shouldn't happen, but just in case make sure the
+                # return code is never 0 here.
+                returncode = 255
+        except Exception:
+            returncode = 255
+        finally:
+            # mission accomplished, this child needs to exit and not
+            # continue the hg process here.
+            os._exit(returncode)