Add tool for generating field trial registry header

The tool will generate a C++ header with all field trials in
REGISTERED_FIELD_TRIALS. This registry will later be used while looking
up field trials from native code to ensure they have been properly
registered in accordance with the policy.

Bug: webrtc:14154
Change-Id: I29bf880735121034585c541c46ef19f617d0afb9
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/276268
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Emil Lundmark <lndmrk@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#38426}

api/DEPS

@@ -11,6 +11,7 @@ include_rules = [
   "-common_video",
   "-data",
   "-examples",
+  "-experiments",
   "-g3doc",
   "-ios",
   "-infra",

experiments/BUILD.gn

@@ -0,0 +1,26 @@
+# Copyright (c) 2022 The WebRTC project authors. All Rights Reserved.
+#
+# Use of this source code is governed by a BSD-style license
+# that can be found in the LICENSE file in the root of the source
+# tree. An additional intellectual property rights grant can be found
+# in the file PATENTS.  All contributing project authors may
+# be found in the AUTHORS file in the root of the source tree.
+
+import("../webrtc.gni")
+
+action("registered_field_trials_header") {
+  visibility = [ ":*" ]
+  script = "field_trials.py"
+  args = [
+    "header",
+    "--output",
+    rebase_path(target_gen_dir, root_build_dir) + "/registered_field_trials.h",
+  ]
+  outputs = [ "$target_gen_dir/registered_field_trials.h" ]
+}
+
+rtc_library("registered_field_trials") {
+  visibility = [ "*" ]
+  sources = get_target_outputs(":registered_field_trials_header")
+  deps = [ ":registered_field_trials_header" ]
+}

experiments/field_trials.py

@@ -0,0 +1,114 @@
+#!/usr/bin/env vpython3
+
+# Copyright (c) 2022 The WebRTC Project Authors. All Rights Reserved.
+#
+# Use of this source code is governed by a BSD-style license
+# that can be found in the LICENSE file in the root of the source
+# tree. An additional intellectual property rights grant can be found
+# in the file PATENTS.  All contributing project authors may
+# be found in the AUTHORS file in the root of the source tree.
+
+import sys
+from typing import Set
+
+import argparse
+import dataclasses
+
+
+# TODO(bugs.webrtc.org/14154): End date and bug should also be stored.
+@dataclasses.dataclass(frozen=True)
+class FieldTrial:
+  """Representation of all attributes associated with a field trial.
+
+  Attributes:
+    key: Field trial key.
+  """
+  key: str
+
+
+# As per the policy in `g3doc/field-trials.md`, all field trials should be
+# registered in the container below. Please keep the keys sorted.
+REGISTERED_FIELD_TRIALS: Set[FieldTrial] = {
+    FieldTrial(''),  # TODO(bugs.webrtc.org/14154): Populate
+}
+
+
+def RegistryHeader(field_trials: Set[FieldTrial] = None) -> str:
+  """Generates a C++ header with all field trial keys.
+
+  Args:
+    field_trials: Field trials to include in the header.
+
+  Returns:
+    String representation of a C++ header file containing all field trial keys.
+
+  >>> trials = {FieldTrial('B'), FieldTrial('A'), FieldTrial('B')}
+  >>> print(RegistryHeader(trials))
+  // This file was automatically generated. Do not edit.
+  <BLANKLINE>
+  #ifndef GEN_REGISTERED_FIELD_TRIALS_H_
+  #define GEN_REGISTERED_FIELD_TRIALS_H_
+  <BLANKLINE>
+  #include "absl/strings/string_view.h"
+  <BLANKLINE>
+  namespace webrtc {
+  <BLANKLINE>
+  inline constexpr absl::string_view kRegisteredFieldTrials[] = {
+      "A",
+      "B",
+  };
+  <BLANKLINE>
+  }  // namespace webrtc
+  <BLANKLINE>
+  #endif  // GEN_REGISTERED_FIELD_TRIALS_H_
+  <BLANKLINE>
+  """
+  if not field_trials:
+    field_trials = REGISTERED_FIELD_TRIALS
+  registered_keys = [f.key for f in field_trials]
+  keys = '\n'.join(f'    "{k}",' for k in sorted(registered_keys))
+  return ('// This file was automatically generated. Do not edit.\n'
+          '\n'
+          '#ifndef GEN_REGISTERED_FIELD_TRIALS_H_\n'
+          '#define GEN_REGISTERED_FIELD_TRIALS_H_\n'
+          '\n'
+          '#include "absl/strings/string_view.h"\n'
+          '\n'
+          'namespace webrtc {\n'
+          '\n'
+          'inline constexpr absl::string_view kRegisteredFieldTrials[] = {\n'
+          f'{keys}\n'
+          '};\n'
+          '\n'
+          '}  // namespace webrtc\n'
+          '\n'
+          '#endif  // GEN_REGISTERED_FIELD_TRIALS_H_\n')
+
+
+def CmdHeader(args: argparse.Namespace) -> None:
+  args.output.write(RegistryHeader())
+
+
+def main() -> None:
+  parser = argparse.ArgumentParser()
+  subcommand = parser.add_subparsers(dest='cmd')
+  parser_header = subcommand.add_parser(
+      'header',
+      help='generate C++ header file containing registered field trial keys')
+  parser_header.add_argument('--output',
+                             default=sys.stdout,
+                             type=argparse.FileType('w'),
+                             required=False,
+                             help='output file')
+  parser_header.set_defaults(cmd=CmdHeader)
+  args = parser.parse_args()
+
+  if not args.cmd:
+    parser.print_help(sys.stderr)
+    sys.exit(1)
+
+  args.cmd(args)
+
+
+if __name__ == '__main__':
+  main()