cmd/go: go test -cover & go test -coverprofile should always output a coverage

[gustafj]

Description

Now in 1.10 when go test -cover supports multiple packages, I would expect it to print out a percentage for all packages (including those missing tests).
And for go test -coverprofile, I would expect all packages to be included in the calculated total.

Currently only packages that have at least one test (can be a *_test.go with only the package declaration) is included, see pkg2 below.

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

go version go1.10 linux/amd64

Does this issue reproduce with the latest release?

yes

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

Linux, amd64

What did you do?

go test ./... -cover
go test ./... -coverprofile cover.out; go tool cover -func cover.out

What did you expect to see?

?      path/to/pkg1   0.001s  coverage:   0.0% of statements [no test files]
ok     path/to/pkg2   0.019s  coverage:   0.0% of statements [no tests to run]
ok     path/to/pkg3   0.371s  coverage: 100.0% of statements

path/to/pkg1/pkg1.go:5: String 0.0%
path/to/pkg2/pkg2.go:5: String 0.0%
path/to/pkg3/pkg3.go:5: String 100.0%
total: (statements) 33.3%

What did you see instead?

?      path/to/pkg1   [no test files]
ok     path/to/pkg2   0.019s  coverage:   0.0% of statements [no tests to run]
ok     path/to/pkg3   0.371s  coverage: 100.0% of statements

path/to/pkg2/pkg2.go:5: String 0.0%
path/to/pkg3/pkg3.go:5: String 100.0%
total: (statements) 50.0%

[gopherbot]

Change https://golang.org/cl/115095 mentions this issue: cmd/go/internal/test: always output a coverage

[mvdan]

@kyroy noticed that this issue is very similar to #25492. It seems to me like both should be fixed at once, as they suggest changes in opposite directions. The other issue wants to change a 0.0% with a [no statements], and this one wants to change [no test files] with 0.0%.

/cc @bcmills @egonk

[bcmills]

they suggest changes in opposite directions.

#25492 is about the case where there are no statements to cover (and thus the percentage is undefined). That's materially different from the case here, where pkg1 does have a statement and it is not covered: 0.0% coverage is well-defined and correct for a package with one statement and no tests.

[mvdan]

My reading of this issue is that go test -cover should always output a percentage if it succeeded. The other issue is precisely about removing a 0.0% output in favor of something else.

[bcmills]

My reading of this issue is that go test -cover should always output a percentage if it succeeded.

That's the current issue title, but that behavior seems clearly wrong if there are no statements to cover: 0/0 is not a well-defined percentage.

(I guess we could output NaN%, but that seems strictly less helpful than [no statements].)

[kyroy]

I like the idea of a consistent output that would be created by just implementing this issue.

Another approach is, as in #25492, for all statements it is true that they are covered by the tests. So one could argue for 100% which is obviously mathematically incorrect.

I am also the creator of the CL. Happy to implement a solution that results from the discussion :)