Inspect protobuf plugins

Protoc find a plugin from command line arguments, for example

% protoc -I src --go_out=dest/

The above command line has an argument --go_out, protoc get it and guess that the word "go" is a plugin, then it search through the PATH environment variable to find an executable named "protoc-gen-go", if the executable program exists then it is considered a plugin to compile .proto files into golang source codes.

Once a plugin is found, protoc spawn a process of the plugin, parse input .proto files into descriptors and send them to plugin process via stdin.

Protoc gives the plugin process a chunk of data of the protobuf type CodeGeneratorRequest, which has details of parsed .proto file and all kinds of descriptors including the AST, and then expect a response of the protobuf type CodeGeneratorResponse.

Both message types are defined in the url pluin.proto.

Since plugins are normal executable programs, it's possible to write code generators of languages using another language. Some languange's protobuf stubs can be build using simpler bootstrap languages such as Python or any of the favoriate languages that already support protobuf.

There are example projects using Python, a typical one is protoc-docs-plugin.