cmd/go: add sync/atomic dependency when needed by test coverage

Fixes #18486.

Change-Id: I359dc4169e04b4123bd41679ea939b06fa754ac2
Reviewed-on: https://go-review.googlesource.com/34830
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/go_test.go

@@ -2247,6 +2247,28 @@ func TestCoverageImportMainLoop(t *testing.T) {
 	tg.grepStderr("not an importable package", "did not detect import main")
 }
 
+func TestTestEmpty(t *testing.T) {
+	if !canRace {
+		t.Skip("no race detector")
+	}
+
+	wd, _ := os.Getwd()
+	testdata := filepath.Join(wd, "testdata")
+
+	for _, dir := range []string{"pkg", "test", "xtest", "pkgtest", "pkgxtest", "pkgtestxtest", "testxtest"} {
+		t.Run(dir, func(t *testing.T) {
+			tg := testgo(t)
+			defer tg.cleanup()
+			tg.setenv("GOPATH", testdata)
+			tg.cd(filepath.Join(testdata, "src/empty/"+dir))
+			tg.run("test", "-cover", "-coverpkg=.", "-race")
+		})
+		if testing.Short() {
+			break
+		}
+	}
+}
+
 func TestBuildDryRunWithCgo(t *testing.T) {
 	if !canCgo {
 		t.Skip("skipping because cgo not enabled")

src/cmd/go/pkg.go

@@ -955,6 +955,10 @@ func (p *Package) load(stk *importStack, bp *build.Package, err error) *Package
 		if p.Name == "main" && goarch == "arm" {
 			importPaths = append(importPaths, "math")
 		}
+		// In coverage atomic mode everything depends on sync/atomic.
+		if 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/testdata/src/empty/pkg/pkg.go

+package p

src/cmd/go/testdata/src/empty/pkgtest/pkg.go

+package p

src/cmd/go/testdata/src/empty/pkgtest/test_test.go

+package p

src/cmd/go/testdata/src/empty/pkgtestxtest/pkg.go

+package p

src/cmd/go/testdata/src/empty/pkgtestxtest/test_test.go

+package p

src/cmd/go/testdata/src/empty/pkgtestxtest/xtest_test.go

+package p_test

src/cmd/go/testdata/src/empty/pkgxtest/pkg.go

+package p

src/cmd/go/testdata/src/empty/pkgxtest/xtest_test.go

+package p_test

src/cmd/go/testdata/src/empty/test/test_test.go

+package p

src/cmd/go/testdata/src/empty/testxtest/test_test.go

+package p

src/cmd/go/testdata/src/empty/testxtest/xtest_test.go

+package p_test

src/cmd/go/testdata/src/empty/xtest/xtest_test.go

+package p_test