cmd/go: respect -coverpkg for unselected no-test packages

This patch fixes a bug in the code that reports coverage percentages
and/or profiles for packages without tests. Specifically, the code
added as part of the fix for issue 24570 (in CL 495447) didn't
properly consider the -coverpkg selection and would look for the build
action meta-data file for a package that wasn't actually selected for
coverage.

Fixes #65653.

Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest
Change-Id: I66ffac11783c00a8cbd855fd05b9a90e4e0ed402
Reviewed-on: https://go-review.googlesource.com/c/go/+/568835
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>

src/cmd/go/internal/test/test.go

@@ -1404,7 +1404,7 @@ func (r *runTestActor) Act(b *work.Builder, ctx context.Context, a *work.Action)
 
 	if p := a.Package; len(p.TestGoFiles)+len(p.XTestGoFiles) == 0 {
 		reportNoTestFiles := true
-		if cfg.BuildCover && cfg.Experiment.CoverageRedesign {
+		if cfg.BuildCover && cfg.Experiment.CoverageRedesign && p.Internal.Cover.GenMeta {
 			if err := sh.Mkdir(a.Objdir); err != nil {
 				return err
 			}

src/cmd/go/testdata/script/cover_coverpkg_partial.txt

@@ -39,6 +39,14 @@ go test -coverprofile=baz.p -coverpkg=./a,./d,./f ./b ./f
 stdout '^ok\s+M/b\s+\S+\s+coverage: 83.3% of statements in ./a, ./d, ./f'
 stdout '^\s*M/f\s+coverage: 0.0% of statements'
 
+# This sub-test inspired by issue 65653: if package P is is matched
+# via the package pattern supplied as the argument to "go test -cover"
+# but P is not part of "-coverpkg", then we don't want coverage for P
+# (including the specific case where P has no test files).
+go test -coverpkg=./a ./...
+stdout '^ok\s+M/a\s+\S+\s+coverage: 100.0% of statements in ./a'
+stdout '^\s*\?\s+M/f\s+\[no test files\]'
+
 -- a/a.go --
 package a