proposal: protoc-gen-go: generator.WriteImport()

[zellyn]

Goal

per-file import uniquification

Why?

1. It makes output deterministic. Currently, changing the order of input files, or even adding a new import to one file can result in changes to other files.
2. It's a big step towards being able to generate files one at a time, or multiple packages at a time, as protoc does for other languages. Numerous people have requested this, the ability to generate file-at-a-time was only recently broken, and it's otherwise easy to fix.
3. It cleans up the generated code when multiple plugin-plugins use the same packages.

I am happy to write this code if there's general support, and I don't believe the changes would be intrusive.

Current operation

Currently, plugins call RegisterUniquePackageName once per imported name. This happens globally (ie. once per run).

Proposed addition

Add a method WriteImport(f *FileDescriptor, desiredName string, importPath string) (prefix string)

This would generate an import statement in the output, as well as returning a prefix string to use when referring to objects from that package.

• If importPath is the same package as the FileDescriptor, no import is generated, and an empty prefix is returned.
• If importPath has already been imported in the file, no import is generated, and the previous prefix is returned.
• If desiredName has already been used by a different import in this file, or reserved by RegisterUniquePackageName, a unique import alias is generated for the import. The prefix returned is the alias followed by . The alias is added to the blacklist used by RegisterUniquePackageName.

This change would allow incremental movement from RegisterUniquePackageName towards WriteImport, and both could coexist.

@dsymonds @jhump @tamird @awalterschulze

[dsnet]

Support for plugins in general needs to be discussed. If we decide to provide first-class support for plugins, there will probably be an entire redesign.

Closing this in favor of the umbrella issue #547.