lib/portage/gpg.py: use temporary gpg out file instead of /dev/null

GnuPG removes the --output file on failure. Removing /dev/null breaks all kinds
of things. So lets use a temporary file for the output instead.

If the file already exists then we remove it to prevent GnuPG from complaining
about the file already existing and asking us if we want to choose a different
--output file.

Closes: https://bugs.gentoo.org/912808
Signed-off-by: Andrew Ammerlaan <andrewammerlaan@gentoo.org>

lib/portage/gpg.py

@@ -1,6 +1,7 @@
 # Copyright 2001-2020 Gentoo Authors
 # Distributed under the terms of the GNU General Public License v2
 
+import os
 import subprocess
 import sys
 import threading
@@ -22,6 +23,7 @@ def __init__(self, settings):
         """
         Portage settings are needed to run GPG unlock command.
         """
+        self.GPG_tmp_file="/tmp/portage_gpg_output"
         self.settings = settings
         self.thread = None
         self.GPG_signing_base_command = self.settings.get(
@@ -35,7 +37,7 @@ def __init__(self, settings):
             f"--homedir {self.signing_gpg_home} "
             f"--digest-algo {self.digest_algo} "
             f"--local-user {self.signing_gpg_key} "
-            "--output /dev/null /dev/null",
+            f"--output {self.GPG_tmp_file} /dev/null",
         )
 
         if "gpg-keepalive" in self.settings.features:
@@ -60,6 +62,11 @@ def unlock(self):
                 # GPG does not need to ask password, so can be ignored.
                 writemsg(f"{colorize('WARN', str(e))}\n")
 
+            try:
+                os.remove(self.GPG_tmp_file)
+            except FileNotFoundError:
+                pass
+
             cmd = shlex_split(varexpand(self.GPG_unlock_command, mydict=self.settings))
             return_code = subprocess.Popen(cmd).wait()