cmd/go: fix race libraries rebuilding by `go test -i`

`go test -i -race` adds the "sync/atomic" package to every package dependency tree
that makes buildIDs different from packages installed with `go install -race`
and causes cache rebuilding.

Fixes #19133
Fixes #19151

Change-Id: I0536c6fa41b0d20fe361b5d35b3c0937b146d07d
Reviewed-on: https://go-review.googlesource.com/37598Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/internal/cfg/cfg.go

@@ -41,9 +41,6 @@ func init() {
 	BuildToolchainLinker = func() string { return "missing-linker" }
 }
 
-// The test coverage mode affects package loading. Sigh.
-var TestCoverMode string // -covermode flag
-
 // An EnvVar is an environment variable Name=Value.
 type EnvVar struct {
 	Name  string

src/cmd/go/internal/load/pkg.go

@@ -959,10 +959,6 @@ func (p *Package) load(stk *ImportStack, bp *build.Package, err error) *Package
 		if p.Name == "main" && cfg.Goarch == "arm" {
 			ImportPaths = append(ImportPaths, "math")
 		}
-		// In coverage atomic mode everything depends on sync/atomic.
-		if cfg.TestCoverMode == "atomic" && (!p.Standard || (p.ImportPath != "runtime/cgo" && p.ImportPath != "runtime/race" && p.ImportPath != "sync/atomic")) {
-			ImportPaths = append(ImportPaths, "sync/atomic")
-		}
 	}
 
 	// Runtime and its internal packages depend on runtime/internal/sys,

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

@@ -388,9 +388,9 @@ See the documentation of the testing package for more information.
 }
 
 var (
-	testC     bool // -c flag
-	testCover bool // -cover flag
-	// Note: testCoverMode is cfg.TestCoverMode (-covermode)
+	testC            bool            // -c flag
+	testCover        bool            // -cover flag
+	testCoverMode    string          // -covermode flag
 	testCoverPaths   []string        // -coverpkg flag
 	testCoverPkgs    []*load.Package // -coverpkg flag
 	testO            string          // -o flag
@@ -548,7 +548,7 @@ func runTest(cmd *base.Command, args []string) {
 			p.Stale = true // rebuild
 			p.StaleReason = "rebuild for coverage"
 			p.Internal.Fake = true // do not warn about rebuild
-			p.Internal.CoverMode = cfg.TestCoverMode
+			p.Internal.CoverMode = testCoverMode
 			var coverFiles []string
 			coverFiles = append(coverFiles, p.GoFiles...)
 			coverFiles = append(coverFiles, p.CgoFiles...)
@@ -559,6 +559,11 @@ func runTest(cmd *base.Command, args []string) {
 
 	// Prepare build + run + print actions for all packages being tested.
 	for _, p := range pkgs {
+		// sync/atomic import is inserted by the cover tool. See #18486
+		if testCover && testCoverMode == "atomic" {
+			ensureImport(p, "sync/atomic")
+		}
+
 		buildTest, runTest, printTest, err := builderTest(&b, p)
 		if err != nil {
 			str := err.Error()
@@ -650,6 +655,23 @@ func runTest(cmd *base.Command, args []string) {
 	b.Do(root)
 }
 
+// ensures that package p imports the named package
+func ensureImport(p *load.Package, pkg string) {
+	for _, d := range p.Internal.Deps {
+		if d.Name == pkg {
+			return
+		}
+	}
+
+	a := load.LoadPackage(pkg, &load.ImportStack{})
+	if a.Error != nil {
+		base.Fatalf("load %s: %v", pkg, a.Error)
+	}
+	load.ComputeStale(a)
+
+	p.Internal.Imports = append(p.Internal.Imports, a)
+}
+
 var windowsBadWords = []string{
 	"install",
 	"patch",
@@ -788,7 +810,7 @@ func builderTest(b *work.Builder, p *load.Package) (buildAction, runAction, prin
 		ptest.Internal.Build.ImportPos = m
 
 		if localCover {
-			ptest.Internal.CoverMode = cfg.TestCoverMode
+			ptest.Internal.CoverMode = testCoverMode
 			var coverFiles []string
 			coverFiles = append(coverFiles, ptest.GoFiles...)
 			coverFiles = append(coverFiles, ptest.CgoFiles...)
@@ -1361,7 +1383,7 @@ type testFuncs struct {
 }
 
 func (t *testFuncs) CoverMode() string {
-	return cfg.TestCoverMode
+	return testCoverMode
 }
 
 func (t *testFuncs) CoverEnabled() bool {

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

@@ -174,7 +174,7 @@ func testFlags(args []string) (packageNames, passToTest []string) {
 			case "covermode":
 				switch value {
 				case "set", "count", "atomic":
-					cfg.TestCoverMode = value
+					testCoverMode = value
 				default:
 					base.Fatalf("invalid flag argument for -covermode: %q", value)
 				}
@@ -191,11 +191,11 @@ func testFlags(args []string) (packageNames, passToTest []string) {
 		}
 	}
 
-	if cfg.TestCoverMode == "" {
-		cfg.TestCoverMode = "set"
+	if testCoverMode == "" {
+		testCoverMode = "set"
 		if cfg.BuildRace {
 			// Default coverage mode is atomic when -race is set.
-			cfg.TestCoverMode = "atomic"
+			testCoverMode = "atomic"
 		}
 	}