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
@@ -21,7 +22,8 @@ import (
 // must not be copied after first use. The Path field must be set before first
 // use and must not be change thereafter.
 type Mutex struct {
-	Path string // The path to the well-known lock file. Must be non-empty.
+	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
 }