Sync Protocol Style

General guidelines:

• Follow the Protocol Buffers Style Guide
• Follow the Proto Do‘s and Don’ts (sorry, Googlers only).
• Maintain local consistency.
• Use proto2 syntax.
• Always get a review from someone in the OWNERS file in this folder. Don't use owners from higher-level folders, and never TBR changes!

Some specific guidelines on top of the general ones above, or just things that often come up:

• Enum entries should be in ALL_CAPS (different from C++!), and for new code, the first entry should be a FOO_UNSPECIFIED = 0 one.
• Proto changes also require corresponding changes in proto_visitors.h and (where appropriate) in proto_enum_conversions.h/cc.
• Backwards compatibility: In general, all changes must be fully backwards-compatible - consider that a single user might be running different versions of the browser simultaneously! Also note that Sync supports clients up to a few years old, so deprecating/removing an existing field is typically a multi-year process.
• Deprecating fields:
  • If the field is still accessed: Mark it as [deprecated = true]. This is the common case, since the browser typically needs to continue supporting the old field for backwards compatibility reasons.
  • If the field is not accessed anymore (i.e. no non-ancient clients depend on the field being populated anymore, all migration code has been retired, etc): Remove the field, and add reserved entries for both its name and its tag number.
• Deprecating enum values: This is particularly tricky, especially if the default value is not a FOO_UNSPECIFIED one (see above). A common pattern is prepending DEPRECATED_ to the entry name.