cmd/go: add mod tidy -diff

[fishy]

Please answer these questions before submitting your issue. Thanks!

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

go version go1.11rc1 darwin/amd64

Does this issue reproduce with the latest release?

N/A

What operating system and processor architecture are you using (go env)?

GOARCH="amd64"
GOOS="darwin"

What did you do?

• Remove lines from go.mod and go.sum files
• Run go mod tidy to add them back

What did you expect to see?

go mod tidy should return non-zero as it made changes. This way it could be integrated to tools like pre-commit hooks to make sure that we don't commit untidied go.mod and go.sum files into version control

What did you see instead?

go mod tidy returned zero

[mvdan]

Non-zero exit codes are usually for errors, so having it behave like this by default would be counter-intuitive.

Perhaps it could have something similar to gofmt, which has the -l and -d flags to tell if any changes were made.

[swtch1]

If the files are successfully added why wouldn't you want to see a 0 status code? This means that the following script would fail:

#!/usr/bin/env bash
set -e
go mod tidy  # <-- non 0 return code
echo 'we never get here'

[fishy]

That makes sense. How about maybe make it optional? e.g. when it's executed with a flag it should return non-zero when it made any changes.

[thepudds]

@gopherbot, please add label modules

[rsc]

We could think about adding a flag that means "don't do the update, but fail if one is needed".
Not sure exactly what to call it but I put -check in the description. Needs more thought.