cmd/go: go fmt shouldn't send network request in module mode

[crvv]

What version of Go are you using (go version)?

master branch, 0fe1986

Does this issue reproduce with the latest release?

Partially.

What did you do?

mkdir ~/test
cd ~/test
go mod init github.com/a/b
echo 'package main\nimport "github.com/a/b/c"\n' > main.go

And then there are three situations.

1. cd ~/test; go fmt
2. cd ~/test; go fmt ./main.go
3. cd ~; go fmt ~/test/main.go

What did you expect to see?

go fmt reformats the file main.go in the three situations.
If I didn't run go mod init github.com/a/b, everything works fine.

What did you see instead?

On master branch, with commit eca5c83, go fmt runs git ls-remote -q https://github.com/a/b and fails in the three situations.

build github.com/a/b: cannot load github.com/a/b/c: cannot find module providing package github.com/a/b/c

Go1.12 fails in situation 1 and 2, and it succeeds in situation 3.

I think there are two things to be fixed.

1. go fmt doesn't need to find package github.com/a/b/c and it shouldn't run git. It can just reformat the file.
2. If the module github.com/a/b is in the directory ~/test and I am doing something with ~/test(such as cd ~/test; go build), go should look for github.com/a/b and github.com/a/b/c in ~/test. It shouldn't use git or request github.com to find the current module.

[mvdan]

Duplicate of #30577? Also, consider using gofmt instead, if you want a lower-level tool which will never resolve or fetch any packages.

[jayconrod]

Hmm, we could probably improve behavior here by ignoring imports when loading packages and rejecting package paths outside of the main module. We may be able to avoid loading the build list and going out to network that way.

#30577 is a dup that points to #27841: adding a -mod flag for go fmt. I don't think it makes sense to format sources in other modules though, so we don't need to load them.

[crvv]

I don't understand why go fmt needs to load packages.
If it just does the following things, what will be wrong?

go fmt path/to/file.go => gofmt -l -w path/to/file.go
go fmt . => for f in ./*.go; do gofmt -l -w $f; done
go fmt ./... => for f in **/*.go; do gofmt -l -w $f; done
go fmt github.com/a/b => for f in $GOPATH/src/github.com/a/b/*.go; do gofmt -l -w $f; done

[jayconrod]

go fmt arguments should be interpreted the same way go build interprets arguments for consistency. For example, go fmt ./... should not recurse into testdata directories or process hidden files.

[crvv]

This is not reproducible on 1.18.3.