conv_linker_config handling existing output file

`proto` sub command now fails when the output file exists.
This is to avoid accidental overwrite.

To handle the case properly, it support --force and --append.

This is to support amending /{partition}/etc/linker.config.pb in the
build process.

Bug: 244531518
Test: manual testing
Change-Id: I0af8c83015e485f2c7533221cae8caf6143403c8

scripts/conv_linker_config.py

@@ -19,6 +19,7 @@ import argparse
 import collections
 import json
 import os
+import sys
 
 import linker_config_pb2 #pylint: disable=import-error
 from google.protobuf.descriptor import FieldDescriptor
@@ -27,7 +28,24 @@ from google.protobuf.text_format import MessageToString
 
 
 def Proto(args):
+    """
+    Merges input json files (--source) into a protobuf message (--output).
+    Fails if the output file exists. Set --force or --append to deal with the existing
+    output file.
+    --force to overwrite the output file with the input (.json files).
+    --append to append the input to the output file.
+    """
     pb = linker_config_pb2.LinkerConfig()
+    if os.path.isfile(args.output):
+        if args.force:
+            pass
+        elif args.append:
+            with open(args.output, 'rb') as f:
+                pb.ParseFromString(f.read())
+        else:
+            sys.stderr.write(f'Error: {args.output} exists. Use --force or --append.\n')
+            sys.exit(1)
+
     if args.source:
         for input in args.source.split(':'):
             json_content = ''
@@ -114,6 +132,17 @@ def GetArgParser():
         required=True,
         type=str,
         help='Target path to create protobuf file.')
+    option_for_existing_output = parser_proto.add_mutually_exclusive_group()
+    option_for_existing_output.add_argument(
+        '-f',
+        '--force',
+        action='store_true',
+        help='Overwrite if the output file exists.')
+    option_for_existing_output.add_argument(
+        '-a',
+        '--append',
+        action='store_true',
+        help='Append the input to the output file if the output file exists.')
     parser_proto.set_defaults(func=Proto)
 
     print_proto = subparsers.add_parser(
@@ -195,8 +224,12 @@ def GetArgParser():
 
 
 def main():
-    args = GetArgParser().parse_args()
-    args.func(args)
+    parser = GetArgParser()
+    args = parser.parse_args()
+    if 'func' in args:
+        args.func(args)
+    else:
+        parser.print_help()
 
 
 if __name__ == '__main__':