for-all-repos: Add option to keep current env

Extend support for preserving the current environment to the
for-all-repos plugin with the --preserve-env flag.
This eases the usage of dynamic configuration done via environment
variables within the for-all-repos plugin, e.g. when calling a script.

Signed-off-by: Jasper Orschulko <jasper@fancydomain.eu>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

kas/libkas.py

@@ -391,3 +391,30 @@ def setup_parser_common_args(parser):
     parser.add_argument('--update', action='store_true',
                         help='Pull new upstream changes to the desired '
                         'refspec even if it is already checked out locally')
+
+
+def setup_parser_preserve_env_arg(parser):
+    parser.add_argument('-E', '--preserve-env',
+                        help='Keep current user enviornment block',
+                        action='store_true')
+
+
+def run_handle_preserve_env_arg(ctx, os, args, SetupHome):
+    if args.preserve_env:
+        # Warn if there's any settings that setup_home would apply
+        # but are now ignored
+        for var in SetupHome.ENV_VARS:
+            if var in os.environ:
+                logging.warning('Environment variable "%s" ignored '
+                                'because user environment is being used',
+                                var)
+
+        if not os.isatty(sys.stdout.fileno()):
+            logging.error("Error: --preserve-env can only be "
+                          "run from a tty")
+            sys.exit(1)
+
+        ctx.environ = os.environ.copy()
+
+        logging.warning("Preserving the current environment block may "
+                        "have unintended side effects on the build.")