[Feature Request] Support call external commands as transformation

[hcoona]

I'd like to run code formatting in a copybara workflow, which needs to call external commands as transformation.

[denian]

Hi Shuai,

The general approach that we've been following is to implement specific transformations (that can delegate on another tool/sub-process) instead of a general "black-box" transform where you can run any command.

There are several reasons for this, for example it provides a much better API in terms of the configuration. If you see a transform like (just making this up):

python.format(paths = glob(['**.py']),

it's much clear and scoped than something like:

core.run_binary('path/to/some/script.sh arg1 arg2...'),

and there's also the problem of, how do you specify the binary, how it's resolved, etc.

But the main issue is reversibility. Having a blind transform that runs some command does not offer any control on that, unless you make this transform explicitly non-reversible, or you have a reverse field.

We do have some format transformations internally, and they are implemented in this way:

public class MyFormatToolTransform implements Transformation {

  MyFormatToolTransform(Glob glob, MyFormatToolOptions formatOptions) {
    ...
  }

  @Override
  public void transform(TransformWork work) throws IOException, ValidationException {
    Iterable<FileState> files = work.getTreeState().find(glob.relativeTo(work.getCheckoutDir()));
    // Run your format tool on the files
    formatOptions.run(files);
    ...
  }

  @Override
  public Transformation reverse() {
    return new ExplicitReversal(IntentionalNoop.INSTANCE, this);
  }
}

where the MyFormatToolOptions resolves the binary to a default location (for example /usr/bin/format) but also exposes a flag so users can overwrite that.

Note that in this case we set explicitly that the format transformation reverse is noop.

Could you say more about the format tool that you want to run? Is it open source?

Also, are happy to accept contributions :)

[hcoona]

I want support for:

• google-java-format
• yapf
• clang-format
• dotnet format

Would you like to give more details about MyFormatToolTransform? I'd like to take it as an example for contributing.