Add a converter for classpaths.proto between proto and json/text formats.

The proto is used at build time to define classpath fragments to be read
and parsed at runtime by `derive_classpath` service; in order to define
and set BOOTCLASSPATH, DEX2OATCLASSPATH, and SYSTEMSERVERCLASSPATH
environ variables.

The tool is used for debugging and generation of binary protos.

Bug: 180105615
Test: m && launch_cvd with a follow up changes
Change-Id: I7e8b084e5f63e9aaad11da3221b909818e69e235

scripts/Android.bp

@@ -254,3 +254,22 @@ python_binary_host {
         "linker_config_proto",
     ],
 }
+
+python_binary_host {
+    name: "conv_classpaths_proto",
+    srcs: [
+        "conv_classpaths_proto.py",
+    ],
+    version: {
+        py2: {
+            enabled: false,
+        },
+        py3: {
+            enabled: true,
+            embedded_launcher: true,
+        },
+    },
+    libs: [
+        "classpaths_proto_python",
+    ],
+}

scripts/conv_classpaths_proto.py

@@ -0,0 +1,76 @@
+#  Copyright (C) 2021 The Android Open Source Project
+#
+#  Licensed under the Apache License, Version 2.0 (the "License");
+#  you may not use this file except in compliance with the License.
+#  You may obtain a copy of the License at
+#
+#       http://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing, software
+#  distributed under the License is distributed on an "AS IS" BASIS,
+#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#  See the License for the specific language governing permissions and
+#  limitations under the License.
+
+import argparse
+
+import classpaths_pb2
+
+import google.protobuf.json_format as json_format
+import google.protobuf.text_format as text_format
+
+
+def encode(args):
+    pb = classpaths_pb2.ExportedClasspathsJars()
+    if args.format == 'json':
+        json_format.Parse(args.input.read(), pb)
+    else:
+        text_format.Parse(args.input.read(), pb)
+    args.output.write(pb.SerializeToString())
+    args.input.close()
+    args.output.close()
+
+
+def decode(args):
+    pb = classpaths_pb2.ExportedClasspathsJars()
+    pb.ParseFromString(args.input.read())
+    if args.format == 'json':
+        args.output.write(json_format.MessageToJson(pb))
+    else:
+        args.output.write(text_format.MessageToString(pb).encode('utf_8'))
+    args.input.close()
+    args.output.close()
+
+
+def main():
+    parser = argparse.ArgumentParser('Convert classpaths.proto messages between binary and '
+                                     'human-readable formats.')
+    parser.add_argument('-f', '--format', default='textproto',
+                        help='human-readable format, either json or text(proto), '
+                             'defaults to textproto')
+    parser.add_argument('-i', '--input',
+                        nargs='?', type=argparse.FileType('rb'), default=sys.stdin.buffer)
+    parser.add_argument('-o', '--output',
+                        nargs='?', type=argparse.FileType('wb'),
+                        default=sys.stdout.buffer)
+
+    subparsers = parser.add_subparsers()
+
+    parser_encode = subparsers.add_parser('encode',
+                                          help='convert classpaths protobuf message from '
+                                               'JSON to binary format',
+                                          parents=[parser], add_help=False)
+
+    parser_encode.set_defaults(func=encode)
+
+    parser_decode = subparsers.add_parser('decode',
+                                          help='print classpaths config in JSON format',
+                                          parents=[parser], add_help=False)
+    parser_decode.set_defaults(func=decode)
+
+    args = parser.parse_args()
+    args.func(args)
+
+
+if __name__ == '__main__':
+    main()