Add reserved option range for mypy-protobuf

[nipunn1313]

Mypy-protobuf is here
https://github.com/nipunn1313/mypy-protobuf

It currently uses extensions as explained here.
https://github.com/nipunn1313/mypy-protobuf/blob/main/proto/mypy_protobuf/extensions.proto

It's a fairly stable project for generating type stubs for protobuf.
It's been around since ~2015 (open source since 2017). Since open
sourcing it, it would make sense to reserve some extension numbers in
the global range.

See
nipunn1313/mypy-protobuf#396

[fowles]

It seems like you only need one extension number since you are using a message with subfields.

[nipunn1313]

Hi @fowles - appreciate the quick turnaround.

The proto extension (as used today) uses three option extensions (from https://github.com/nipunn1313/mypy-protobuf/blob/main/proto/mypy_protobuf/extensions.proto)

extend google.protobuf.FieldOptions {
    // Tells mypy to use a specific newtype rather than the normal type for this field.
    optional string casttype = 60000;

    // Tells mypy to use a specific type for keys; only makes sense on map fields
    optional string keytype = 60002;
    // Tells mypy to use a specific type for values; only makes sense on map fields
    optional string valuetype = 60003;
}

Example usage is like this (from https://github.com/nipunn1313/mypy-protobuf/blob/main/proto/testproto/test.proto#L70):

message Simple1 {
  optional uint32 user_id = 21 [(mypy_protobuf.casttype)="test/test_generated_mypy.UserId"];
  optional string email = 22 [(mypy_protobuf.casttype)="test/test_generated_mypy.Email"];
  map<uint32, string> email_by_uid = 23 [
    (mypy_protobuf.keytype)="test/test_generated_mypy.UserId",
    (mypy_protobuf.valuetype)="test/test_generated_mypy.Email"
  ];
}

Which generates .pyi files like this (from https://github.com/nipunn1313/mypy-protobuf/blob/main/test/generated/testproto/test_pb2.pyi#L165):

class Simple1:
    user_id: test.test_generated_mypy.UserId
    email: test.test_generated_mypy.Email
    @property
    def email_by_uid(self) -> google.protobuf.internal.containers.ScalarMap[test.test_generated_mypy.UserId, test.test_generated_mypy.Email]: ...

As you can see here, there are 3 options used (so far).

I spent some time reading through the docs and found this - https://developers.google.com/protocol-buffers/docs/proto#customoptions - specifically Usually you only need one extension number. You can declare multiple options with only one extension number by putting them in a sub-message:.

We could do this, however it would be a non-backward compatible change to mypy-protobuf.
We'd have to convert to

message Options {
    // Tells mypy to use a specific newtype rather than the normal type for this field.
    optional string casttype = 1;
    // Tells mypy to use a specific type for keys; only makes sense on map fields
    optional string keytype = 2;
    // Tells mypy to use a specific type for values; only makes sense on map fields
    optional string valuetype = 3;
}

extend google.protobuf.FieldOptions {
    optional Options options = 1151;
}

Callsite would have to switch to:

message Simple1 {
  optional uint32 user_id = 21 [(mypy_protobuf.options).casttype="test/test_generated_mypy.UserId"];
  optional string email = 22 [(mypy_protobuf.options).casttype="test/test_generated_mypy.Email"];
  map<uint32, string> email_by_uid = 23 [
    (mypy_protobuf.options).keytype="test/test_generated_mypy.UserId",
    (mypy_protobuf.options).valuetype="test/test_generated_mypy.Email"
  ];
}

I would really prefer to be able to make this switch in a code-compatible way, by reserving at least 3 numbers.

One possibility is that we could reserve 4 numbers. 3 for the legacy options and a 4th one for the Options struct approach outlined above. This has the benefit of being backward compatible, but also means that going forward, we'll only be using a single Options struct extension number, even if we add new option functionality in the future.

[nipunn1313]

I went ahead and implemented this "reserve 4 numbers" strategy.
It's now visible here https://github.com/nipunn1313/mypy-protobuf/blob/main/proto/mypy_protobuf/extensions.proto

3 legacy numbers (for backward compat) and one main number going forward.