cmd/go: fix problems with coverage percentage reporting w/ -coverpkg

This patch resolves a set of problems with "percent covered" metrics
reported when the "-coverpkg" is in effect; these bugs were introduced
in Go 1.22 with the rollout of CL 495452 and related changes.
Specifically, for runs with multiple packages selected but without
-coverpkg, "percent covered" metrics were generated for package P not
based just on P's statements but on the entire corpus of statements.

Fixes #65570.

Change-Id: I38d61886cb46ebd38d8c4313c326d671197c3568
Reviewed-on: https://go-review.googlesource.com/c/go/+/592205
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@golang.org>

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

@@ -683,6 +683,22 @@ func (t *testFuncs) Covered() string {
 	return " in " + strings.Join(t.Cover.Paths, ", ")
 }
 
+func (t *testFuncs) CoverSelectedPackages() string {
+	if t.Cover == nil || t.Cover.Paths == nil {
+		return `[]string{"` + t.Package.ImportPath + `"}`
+	}
+	var sb strings.Builder
+	fmt.Fprintf(&sb, "[]string{")
+	for k, p := range t.Cover.Pkgs {
+		if k != 0 {
+			sb.WriteString(", ")
+		}
+		fmt.Fprintf(&sb, `"%s"`, p.ImportPath)
+	}
+	sb.WriteString("}")
+	return sb.String()
+}
+
 // Tested returns the name of the package being tested.
 func (t *testFuncs) Tested() string {
 	return t.Package.Name
@@ -950,6 +966,7 @@ func init() {
 {{if .Cover}}
 	testdeps.CoverMode = {{printf "%q" .Cover.Mode}}
 	testdeps.Covered = {{printf "%q" .Covered}}
+	testdeps.CoverSelectedPackages = {{printf "%s" .CoverSelectedPackages}}
 	testdeps.CoverSnapshotFunc = cfile.Snapshot
 	testdeps.CoverProcessTestDirFunc = cfile.ProcessCoverTestDir
 	testdeps.CoverMarkProfileEmittedFunc = cfile.MarkProfileEmitted

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

@@ -0,0 +1,63 @@
+# Without -coverpkg, we should get the same value for a given
+# package regardless of how many other packages are selected
+# (see issue 65570).
+
+[short] skip
+
+go test -count=1 -cover ./a ./b ./main
+stdout '^ok\s+M/main\s+\S+\s+coverage: 75.0% of statements'
+go test -count=1 -cover ./main
+stdout '^ok\s+M/main\s+\S+\s+coverage: 75.0% of statements'
+
+-- go.mod --
+module M
+
+go 1.21
+-- a/a.go --
+package a
+
+func AFunc() int {
+	return 42
+}
+-- b/b.go --
+package b
+
+func BFunc() int {
+	return -42
+}
+-- main/main.go --
+package main
+
+import (
+	"M/a"
+)
+
+func MFunc() string {
+	return "42"
+}
+
+func M2Func() int {
+	return a.AFunc()
+}
+
+func init() {
+	println("package 'main' init")
+}
+
+func main() {
+	println(a.AFunc())
+}
+-- main/main_test.go --
+package main
+
+import "testing"
+
+func TestMain(t *testing.T) {
+	if MFunc() != "42" {
+		t.Fatalf("bad!")
+	}
+	if M2Func() != 42 {
+		t.Fatalf("also bad!")
+	}
+}
+

src/internal/coverage/cfile/testsupport.go

@@ -27,7 +27,7 @@ import (
 // testmain code when "go test -cover" is in effect. It is not
 // intended to be used other than internally by the Go command's
 // generated code.
-func ProcessCoverTestDir(dir string, cfile string, cm string, cpkg string, w io.Writer) error {
+func ProcessCoverTestDir(dir string, cfile string, cm string, cpkg string, w io.Writer, selpkgs []string) error {
 	cmode := coverage.ParseCounterMode(cm)
 	if cmode == coverage.CtrModeInvalid {
 		return fmt.Errorf("invalid counter mode %q", cm)
@@ -103,7 +103,7 @@ func ProcessCoverTestDir(dir string, cfile string, cm string, cpkg string, w io.
 	}
 
 	// Emit percent.
-	if err := ts.cf.EmitPercent(w, nil, cpkg, true, true); err != nil {
+	if err := ts.cf.EmitPercent(w, selpkgs, cpkg, true, true); err != nil {
 		return err
 	}
 

src/internal/coverage/cfile/ts_test.go

@@ -45,7 +45,7 @@ func TestTestSupport(t *testing.T) {
 	textfile := filepath.Join(t.TempDir(), "file.txt")
 	var sb strings.Builder
 	err := ProcessCoverTestDir(tgcd, textfile,
-		testing.CoverMode(), "", &sb)
+		testing.CoverMode(), "", &sb, nil)
 	if err != nil {
 		t.Fatalf("bad: %v", err)
 	}
@@ -168,7 +168,7 @@ func TestAuxMetaDataFiles(t *testing.T) {
 	var sb strings.Builder
 	textfile := filepath.Join(td, "file2.txt")
 	err = ProcessCoverTestDir(tgcd, textfile,
-		testing.CoverMode(), "", &sb)
+		testing.CoverMode(), "", &sb, nil)
 	if err != nil {
 		t.Fatalf("bad: %v", err)
 	}

src/testing/internal/testdeps/deps.go

@@ -203,14 +203,15 @@ func (TestDeps) SnapshotCoverage() {
 
 var CoverMode string
 var Covered string
+var CoverSelectedPackages []string
 
 // These variables below are set at runtime (via code in testmain) to point
 // to the equivalent functions in package internal/coverage/cfile; doing
 // things this way allows us to have tests import internal/coverage/cfile
 // only when -cover is in effect (as opposed to importing for all tests).
 var (
 	CoverSnapshotFunc           func() float64
-	CoverProcessTestDirFunc     func(dir string, cfile string, cm string, cpkg string, w io.Writer) error
+	CoverProcessTestDirFunc     func(dir string, cfile string, cm string, cpkg string, w io.Writer, selpkgs []string) error
 	CoverMarkProfileEmittedFunc func(val bool)
 )
 
@@ -232,7 +233,7 @@ func coverTearDown(coverprofile string, gocoverdir string) (string, error) {
 	}
 	CoverMarkProfileEmittedFunc(true)
 	cmode := CoverMode
-	if err := CoverProcessTestDirFunc(gocoverdir, coverprofile, cmode, Covered, os.Stdout); err != nil {
+	if err := CoverProcessTestDirFunc(gocoverdir, coverprofile, cmode, Covered, os.Stdout, CoverSelectedPackages); err != nil {
 		return "error generating coverage report", err
 	}
 	return "", nil