bpo-43423 Fix IndexError in subprocess _communicate function (GH-24777)

Check to make sure stdout and stderr are not empty before selecting an item from them in Windows subprocess._communicate.

Co-authored-by: Gregory P. Smith <greg@krypto.org>

Lib/subprocess.py

@@ -1535,10 +1535,8 @@ class Popen(object):
                 self.stderr.close()
 
             # All data exchanged.  Translate lists into strings.
-            if stdout is not None:
-                stdout = stdout[0]
-            if stderr is not None:
-                stderr = stderr[0]
+            stdout = stdout[0] if stdout else None
+            stderr = stderr[0] if stderr else None
 
             return (stdout, stderr)
 

Misc/NEWS.d/next/Library/2021-03-11-15-44-18.bpo-43423.rRomRD.rst

@@ -0,0 +1,2 @@
+:func:`subprocess.communicate` no longer raises an IndexError when there is an
+empty stdout or stderr IO buffer during a timeout on Windows.