Use subprocess umask support

This requires Python 3.9 or newer

Author: nanonyme

src/buildstream/utils.py

@@ -1260,14 +1260,7 @@ def _call(*popenargs, terminate=False, **kwargs):
 
     process = None
 
-    old_preexec_fn = kwargs.get("preexec_fn")
-    if "preexec_fn" in kwargs:
-        del kwargs["preexec_fn"]
-
-    def preexec_fn():
-        os.umask(stat.S_IWGRP | stat.S_IWOTH)
-        if old_preexec_fn is not None:
-            old_preexec_fn()
+    kwargs["umask"] = stat.S_IWGRP | stat.S_IWOTH
 
     # Handle termination, suspend and resume
     def kill_proc():
@@ -1313,7 +1306,7 @@ def resume_proc():
             os.killpg(group_id, signal.SIGCONT)
 
     with _signals.suspendable(suspend_proc, resume_proc), _signals.terminator(kill_proc), subprocess.Popen(
-        *popenargs, preexec_fn=preexec_fn, universal_newlines=True, **kwargs
+        *popenargs, universal_newlines=True, **kwargs
     ) as process:
         # Here, we don't use `process.communicate()` directly without a timeout
         # This is because, if we were to do that, and the process would never