x/tools/cmd/eg: allow multiple refactorings in a single template

[josharian]

While working on Node refactoring leading up to CL 7360, I found myself applying 8 different templates (one per relevant field) sequentially. It would be nicer to use--and I think faster to execute--to apply them all in a single pass.

The template could look like a sequence of alternating pairs of before and after:

package P

func before(s string) error { return fmt.Errorf("%s", s) }
func after(s string)  error { return errors.New(s) }

func before(msg string) { log.Fatalf("%s", msg) }
func after(msg string)  { log.Fatal(msg) }

The semantics would be that the output would be equivalent to applying each of the pairs sequentially as a template.

Thoughts? I'm happy to help with implementation.

[jbduncan]

This issue is of interest to me, as I'm probably the only person on the planet crazy enough to run eg templates as part of lint checks (./x.sh --check) and fixes (./x.sh --fix) in my personal project.

./x.sh --check runs multiple eg processes, one per template, in parallel on every Go file.

Then when a fix needs to be made, I run ./x.sh --fix to run the templates sequentially with the -w flag. I do things sequentially to avoid data races, like if two or more templates are writing to the same file.

I feel like eg could be faster if it allowed multiple templates to be run in parallel in a single pass, whilst also avoiding the aforementioned data races.

Furthermore, if eg had a command-like flag to print out which templates were going to fix which files in JSON, then I could script ./x.sh --fix to only run some templates on some files, making things much faster.