Try to make running background commands more responsive

Calling communicate() blocks the process; but since we're writing the
output directly to a file and not sending any input we don't actually
need to call communicate(), just poll() (so that we can check
the returncode attribute). Subjectively this does appear to improve
performance although it has not fixed the ConnectionResetError issues.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>

layerindex/utils.py

@@ -309,7 +309,10 @@ def runcmd(cmd, destdir=None, printerr=True, outfile=None, logger=None, shell=Fa
         proc = subprocess.Popen(cmd, stdout=out, stderr=out, cwd=destdir, shell=shell)
         global child_pid
         child_pid = proc.pid
-        proc.communicate()
+        proc.poll()
+        while proc.returncode is None:
+            proc.poll()
+            time.sleep(0.05)
         if proc.returncode:
             out.seek(0)
             output = out.read()