stage: don't ignore SIGINT in the child (#2271)

Signal mask is inherited by the child, so when we set it up before
Popen, our child ignores SIGINT, which is wrong. What we should've
done ideally, is to setup the signal handler after fork() but before
exec(). That is pretty much what preexec_fn is used for in Popen, but
unfortunately for us, it is not available on windows, so we can't use
it. What we will do instead is just setup signal handler after Popen()
but before communicate(), which would leave a slight gap where we might
also get affected by SIGINTS meant for the child, but that is the best
we can do.

Signed-off-by: Ruslan Kuprieiev <[email protected]>

dvc/stage.py

@@ -775,9 +775,6 @@ class Stage(object):
         main_thread = isinstance(
             threading.current_thread(), threading._MainThread
         )
-        if main_thread:
-            old_handler = signal.signal(signal.SIGINT, signal.SIG_IGN)
-
         p = None
 
         try:
@@ -789,6 +786,8 @@ class Stage(object):
                 executable=executable,
                 close_fds=True,
             )
+            if main_thread:
+                old_handler = signal.signal(signal.SIGINT, signal.SIG_IGN)
             p.communicate()
         finally:
             if main_thread: