cmd/go/internal/lockedfile: add a sync.Mutex to lockedfile.Mutex

The compiler (and race detector) don't interpret locking a file as a
synchronization operation, so we add an explicit (and redundant)
sync.Mutex to make that property clear.

The additional synchronization makes it safe to parallelize the tests
in cmd/go/internal/modfetch/coderepo_test.go, which cuts the wall time
of that test by around 50%.

Updates #30550

Change-Id: Ief3479020ebf9e0fee524a4aae5568697727c683
Reviewed-on: https://go-review.googlesource.com/c/go/+/170597
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/internal/lockedfile/mutex.go

@@ -7,6 +7,7 @@ package lockedfile
 import (
 	"fmt"
 	"os"
+	"sync"
 )
 
 // A Mutex provides mutual exclusion within and across processes by locking a
@@ -22,6 +23,7 @@ import (
 // use and must not be change thereafter.
 type Mutex struct {
 	Path string     // The path to the well-known lock file. Must be non-empty.
+	mu   sync.Mutex // A redundant mutex. The race detector doesn't know about file locking, so in tests we may need to lock something that it understands.
 }
 
 // MutexAt returns a new Mutex with Path set to the given non-empty path.
@@ -56,5 +58,10 @@ func (mu *Mutex) Lock() (unlock func(), err error) {
 	if err != nil {
 		return nil, err
 	}
-	return func() { f.Close() }, nil
+	mu.mu.Lock()
+
+	return func() {
+		mu.mu.Unlock()
+		f.Close()
+	}, nil
 }

src/cmd/go/internal/modfetch/coderepo_test.go

@@ -45,7 +45,7 @@ var altVgotests = []string{
 	vgotest1hg,
 }
 
-var codeRepoTests = []struct {
+type codeRepoTest struct {
 	path     string
 	lookerr  string
 	mpath    string
@@ -59,7 +59,9 @@ var codeRepoTests = []struct {
 	gomoderr string
 	zip      []string
 	ziperr   string
-}{
+}
+
+var codeRepoTests = []codeRepoTest{
 	{
 		path:    "github.com/rsc/vgotest1",
 		rev:     "v0.0.0",
@@ -339,13 +341,18 @@ var codeRepoTests = []struct {
 func TestCodeRepo(t *testing.T) {
 	testenv.MustHaveExternalNetwork(t)
 
-	tmpdir, err := ioutil.TempDir("", "vgo-modfetch-test-")
+	tmpdir, err := ioutil.TempDir("", "modfetch-test-")
 	if err != nil {
 		t.Fatal(err)
 	}
 	defer os.RemoveAll(tmpdir)
+
+	t.Run("parallel", func(t *testing.T) {
 		for _, tt := range codeRepoTests {
-		f := func(t *testing.T) {
+			f := func(tt codeRepoTest) func(t *testing.T) {
+				return func(t *testing.T) {
+					t.Parallel()
+
 					repo, err := Lookup(tt.path)
 					if tt.lookerr != "" {
 						if err != nil && err.Error() == tt.lookerr {
@@ -440,7 +447,8 @@ func TestCodeRepo(t *testing.T) {
 						}
 					}
 				}
-		t.Run(strings.ReplaceAll(tt.path, "/", "_")+"/"+tt.rev, f)
+			}
+			t.Run(strings.ReplaceAll(tt.path, "/", "_")+"/"+tt.rev, f(tt))
 			if strings.HasPrefix(tt.path, vgotest1git) {
 				for _, alt := range altVgotests {
 					// Note: Communicating with f through tt; should be cleaned up.
@@ -459,11 +467,12 @@ func TestCodeRepo(t *testing.T) {
 					tt.rev = remap(tt.rev, m)
 					tt.gomoderr = remap(tt.gomoderr, m)
 					tt.ziperr = remap(tt.ziperr, m)
-				t.Run(strings.ReplaceAll(tt.path, "/", "_")+"/"+tt.rev, f)
+					t.Run(strings.ReplaceAll(tt.path, "/", "_")+"/"+tt.rev, f(tt))
 					tt = old
 				}
 			}
 		}
+	})
 }
 
 var hgmap = map[string]string{
@@ -538,8 +547,13 @@ func TestCodeRepoVersions(t *testing.T) {
 		t.Fatal(err)
 	}
 	defer os.RemoveAll(tmpdir)
+
+	t.Run("parallel", func(t *testing.T) {
 		for _, tt := range codeRepoVersionsTests {
 			t.Run(strings.ReplaceAll(tt.path, "/", "_"), func(t *testing.T) {
+				tt := tt
+				t.Parallel()
+
 				repo, err := Lookup(tt.path)
 				if err != nil {
 					t.Fatalf("Lookup(%q): %v", tt.path, err)
@@ -553,6 +567,7 @@ func TestCodeRepoVersions(t *testing.T) {
 				}
 			})
 		}
+	})
 }
 
 var latestTests = []struct {
@@ -586,9 +601,14 @@ func TestLatest(t *testing.T) {
 		t.Fatal(err)
 	}
 	defer os.RemoveAll(tmpdir)
+
+	t.Run("parallel", func(t *testing.T) {
 		for _, tt := range latestTests {
 			name := strings.ReplaceAll(tt.path, "/", "_")
 			t.Run(name, func(t *testing.T) {
+				tt := tt
+				t.Parallel()
+
 				repo, err := Lookup(tt.path)
 				if err != nil {
 					t.Fatalf("Lookup(%q): %v", tt.path, err)
@@ -611,6 +631,7 @@ func TestLatest(t *testing.T) {
 				}
 			})
 		}
+	})
 }
 
 // fixedTagsRepo is a fake codehost.Repo that returns a fixed list of tags