Sync New Data Type

Add New Sync Data Type

This skill provides step-by-step guidance for adding the initial scaffold (CL #1) for a new Sync data type in Chromium.

Overview

Introducing a new Sync data type involves wiring up protocol buffers, enum definitions, type info tables, feature flags, controller builders, unit/integration tests, and histogram metrics.

This skill automates creating the scaffold CL by dynamically inspecting the codebase for current static_assert counters and enum values.

Inputs Required

Before starting, obtain the following inputs from the user:

1. Data Type Name (<DATA_TYPE> in UPPER_SNAKE_CASE): e.g., FOO_BAR, PASSKEY_METADATA
2. Bug Number (<BUG_NUMBER>): e.g., 123456789

Derived Casing Conventions

From <DATA_TYPE> (e.g., FOO_BAR), compute the following variations:

• UPPER_SNAKE_CASE: <DATA_TYPE> (e.g., FOO_BAR)
• lower_snake_case: <lower_snake_case> (e.g., foo_bar)
• CamelCase / PascalCase: <CamelCase> (e.g., FooBar)
• Title Case: <Title Case> (e.g., Foo Bar)

---

Instructions

Inspect the existing codebase dynamically to discover current static_assert counters and enum values, then make all necessary scaffold changes across the following files:

1. Protocol Buffers & Build Rules

1. components/sync/protocol/<lower_snake_case>_specifics.proto (Create New File): Create this file with standard Chromium boilerplate:

   // Copyright 2026 The Chromium Authors
   // Use of this source code is governed by a BSD-style license that can be
   // found in the LICENSE file.
   
   // If you change or add any fields in this file, update proto_visitors.h and
   // potentially proto_enum_conversions.{h, cc}.
   
   syntax = "proto2";
   
   option java_multiple_files = true;
   option java_package = "org.chromium.components.sync.protocol";
   
   option optimize_for = LITE_RUNTIME;
   
   package sync_pb;
   
   // FIXME (in this CL): document.
   message <CamelCase>Specifics {
     // TODO(crbug.com/<BUG_NUMBER>): In CL #2, add fields that you wish to sync, then
     // update proto_visitors.h and potentially proto_enum_conversions.*.
   }

2. components/sync/protocol/entity_specifics.proto:

   • Add import in alphabetical order: import "components/sync/protocol/<lower_snake_case>_specifics.proto";
   • Per the guidelines in entity_specifics.proto, calculate <FIELD_NUMBER> by selecting a Cr-Commit-Position of a past commit authored by the developer (e.g., query git log -1 --author="$(git config user.email)" --grep="Cr-Commit-Position" origin/main and extract the commit position number). If the developer has no prior landed commits, ask the user or pick an available position from origin/main.
   • Verify that <FIELD_NUMBER> does not collide with any existing field tags or reserved numbers in entity_specifics.proto.
   • In message EntitySpecifics under oneof specifics_variant, add the field right before the "When adding a new type" comment with a FIXME comment:

     // FIXME (in this CL): Verify tag number matches a valid Cr-Commit-Position.
     <CamelCase>Specifics <lower_snake_case> = <FIELD_NUMBER>;

3. components/sync/protocol/protocol_sources.gni:

   • Add "<lower_snake_case>_specifics.proto", in alphabetical order to sync_protocol_sources.

4. components/sync/protocol/proto_visitors.h:

   • Add #include "components/sync/protocol/<lower_snake_case>_specifics.pb.h" in alphabetical order to the list of includes at the top.
   • Increment static_assert(N == GetNumDataTypes(), ...) in VISIT_PROTO_FIELDS(const sync_pb::EntitySpecifics& proto) from N to N + 1.
   • Add VISIT(<lower_snake_case>); to VISIT_PROTO_FIELDS(const sync_pb::EntitySpecifics& proto).
   • Add the empty visitor function:

     VISIT_PROTO_FIELDS(const sync_pb::<CamelCase>Specifics& proto) {
       // TODO(crbug.com/<BUG_NUMBER>): In CL #2, VISIT fields added to specifics.
     }

5. components/sync/protocol/proto_value_conversions_unittest.cc:

   • Increment static_assert(N == syncer::GetNumDataTypes(), ...) from N to N + 1.
   • Add DEFINE_SPECIFICS_TO_VALUE_TEST(<lower_snake_case>) within the keep-sorted macro list in alphabetical order.

---

2. Core DataType & Feature Declarations

6. components/sync/base/data_type.h:

   • In enum DataType:
     • Add // FIXME (in this CL): document. and <DATA_TYPE>, right before LAST_USER_DATA_TYPE.
     • Update LAST_USER_DATA_TYPE = <DATA_TYPE>,.
   • In enum class DataTypeForHistograms:
     • Inspect the current maximum integer value M assigned to the last entry before kMaxValue.
     • Add k<CamelCase> = <M + 1>, and update kMaxValue = k<CamelCase>,.

7. components/sync/base/data_type.cc:

   • In kDataTypeInfoTable: Add the new entry struct:

     {
         .type = <DATA_TYPE>,
         .specifics_field_number =
             sync_pb::EntitySpecifics::k<CamelCase>FieldNumber,
         .debug_string = "<Title Case>",
         .histogram_suffix = "<DATA_TYPE>",
         .stable_lowercase_string = "<lower_snake_case>",
         // FIXME (in this CL): Verify encryption_policy (e.g.,
         // kEncryptedIfCustomPassphraseSet vs. kAlwaysEncrypted vs.
         // kNeverEncrypted) with the sync champion.
         .encryption_policy =
             EncryptionPolicy::kEncryptedIfCustomPassphraseSet,
         .priority = DataTypePriority::kRegular,
         // FIXME (in this CL): Verify communication_direction (e.g.,
         // kRegularTwoWay vs. kCommitOnly) with the sync champion.
         .communication_direction = CommunicationDirection::kRegularTwoWay,
         .apply_updates_batch_policy = ApplyUpdatesBatchPolicy::kStandard,
         .unsynced_data_check_on_signout_policy =
             UnsyncedDataCheckOnSignoutPolicy::kNone,
         .cross_user_sharing_policy = CrossUserSharingPolicy::kNone,
         .local_sync_support_policy = LocalSyncSupportPolicy::kUnsupported,
     },

   • Increment static_assert(GetNumDataTypes() == N, ...) from N to N + 1.
   • In AddDefaultFieldValue(): Add case <DATA_TYPE>: specifics->mutable_<lower_snake_case>(); break;.
   • In DataTypeHistogramValue(): Add case <DATA_TYPE>: return DataTypeForHistograms::k<CamelCase>;.

8. components/sync/base/features.h & components/sync/base/features.cc:

   • In features.h:

     // FIXME (in this CL): If you already have a flag, delete this and use yours.
     // Otherwise, document.
     BASE_DECLARE_FEATURE(kSync<CamelCase>);

   • In features.cc:

     BASE_FEATURE(kSync<CamelCase>, base::FEATURE_DISABLED_BY_DEFAULT);

9. components/sync/engine/cycle/data_type_tracker.cc:

   • In GetDefaultLocalChangeNudgeDelay(): Add case <DATA_TYPE>: (returning kMediumLocalChangeNudgeDelay).
   • In CanGetCommitsFromExtensions(): Add case <DATA_TYPE>: (returning false).

---