Proto2 possible field options

[Michael Powell]

Hello,

In the Proto2 specification, "field options" are sort of meta-tucked
away. I don't think the specification really names them. Is there a
list of them somewhere in the docs?

i.e. field = label type fieldName "=" fieldNumber [ "[" fieldOptions "]" ] ";"
fieldOptions = fieldOption { "," fieldOption }
fieldOption = optionName "=" constant

Specifically, "optionName" ? If memory serves from examples, things
like "default" ?

Thanks!

Best regards,

Michael Powell

[Josh Humphries]

Michael,

They map to field names in the options messages defined in descriptor.proto.

Also, if you define custom options (by extending those options messages), you refer to them using qualified name enclosed in parentheses. That proto file above also serves as a sort of AST for proto source files, where FileDescriptorProto is the root of the AST. You can then look at UninterpretedOption for seeing the AST structure for parsing options.

----
Josh Humphries
jh...@bluegosling.com

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[Michael Powell]

On Thu, Nov 1, 2018 at 11:08 AM Josh Humphries <jh...@bluegosling.com> wrote:
>
> Michael,
> They map to field names in the options messages defined in descriptor.proto.

This is pretty interesting. So they've used the Proto spec to describe
itself, ey?

> Also, if you define custom options (by extending those options messages), you refer to them using qualified name enclosed in parentheses. That proto file above also serves as a sort of AST for proto source files, where FileDescriptorProto is the root of the AST. You can then look at UninterpretedOption for seeing the AST structure for parsing options.

So, for instance, my field options could include something like this:

... [default_value = 3, oneof_index = 1, json_name = "myField", ...]

And, so on, ad nauseam for the possible field options?

https://github.com/protocolbuffers/protobuf/blob/master/src/google/protobuf/descriptor.proto#L136

[Josh Humphries]

On Thu, Nov 1, 2018 at 11:36 AM Michael Powell <mwpow...@gmail.com> wrote:

On Thu, Nov 1, 2018 at 11:08 AM Josh Humphries <jh...@bluegosling.com> wrote:
>
> Michael,
> They map to field names in the options messages defined in descriptor.proto.

This is pretty interesting. So they've used the Proto spec to describe
itself, ey?

> Also, if you define custom options (by extending those options messages), you refer to them using qualified name enclosed in parentheses. That proto file above also serves as a sort of AST for proto source files, where FileDescriptorProto is the root of the AST. You can then look at UninterpretedOption for seeing the AST structure for parsing options.

So, for instance, my field options could include something like this:

... [default_value = 3, oneof_index = 1, json_name = "myField", ...]

And, so on, ad nauseam for the possible field options?

https://github.com/protocolbuffers/protobuf/blob/master/src/google/protobuf/descriptor.proto#L136

Not quite. Those are fields on the descriptor, which map to other elements in the source. (The oneof index, for example, is computed based on the order of oneof declarations in the enclosing DescriptorProto and is unset for fields that are not enclosed in a oneof in the source file. You can't define that via an option.)

The options are all defined in special *Options messages, like this one: https://github.com/protocolbuffers/protobuf/blob/master/src/google/protobuf/descriptor.proto#L507

The various descriptor protos have a field for the corresponding options type (for example, this).

The two exceptions are both for field options only: default and json_name. Contrary to what I just told you, these are not fields of the FieldOptions message, but they are defined via options in the actual source. I call them "pseudo-options" -- they behave just like options, but when incorporated into the descriptor proto, they get promoted out of the options message and into the enclosing FieldDescriptorProto.