loader: be more robust when creating the cached GOROOT

This commit fixes two issues:

  * Do not try to create the cached GOROOT multiple times in parallel.
    This may happen in tests and is a waste of resources (and thus
    speed).
  * Check for an "access denied" error when trying to rename a directory
    over an existing directory. On *nix systems, this results in the
    expected "file exists" error. Unfortunately, Windows gives an access
    denied. This commit fixes the Windows behavior.

loader/goroot.go

@@ -15,11 +15,14 @@ import (
 	"path"
 	"path/filepath"
 	"runtime"
+	"sync"
 
 	"github.com/tinygo-org/tinygo/compileopts"
 	"github.com/tinygo-org/tinygo/goenv"
 )
 
+var gorootCreateMutex sync.Mutex
+
 // GetCachedGoroot creates a new GOROOT by merging both the standard GOROOT and
 // the GOROOT from TinyGo using lots of symbolic links.
 func GetCachedGoroot(config *compileopts.Config) (string, error) {
@@ -54,6 +57,15 @@ func GetCachedGoroot(config *compileopts.Config) (string, error) {
 		cachedgoroot += "-syscall"
 	}
 
+	// Do not try to create the cached GOROOT in parallel, that's only a waste
+	// of I/O bandwidth and thus speed. Instead, use a mutex to make sure only
+	// one goroutine does it at a time.
+	// This is not a way to ensure atomicity (a different TinyGo invocation
+	// could be creating the same directory), but instead a way to avoid
+	// creating it many times in parallel when running tests in parallel.
+	gorootCreateMutex.Lock()
+	defer gorootCreateMutex.Unlock()
+
 	if _, err := os.Stat(cachedgoroot); err == nil {
 		return cachedgoroot, nil
 	}
@@ -88,6 +100,15 @@ func GetCachedGoroot(config *compileopts.Config) (string, error) {
 			// deleted by the defer above.
 			return cachedgoroot, nil
 		}
+		if runtime.GOOS == "windows" && os.IsPermission(err) {
+			// On Windows, a rename with a destination directory that already
+			// exists does not result in an IsExist error, but rather in an
+			// access denied error. To be sure, check for this case by checking
+			// whether the target directory exists.
+			if _, err := os.Stat(cachedgoroot); err == nil {
+				return cachedgoroot, nil
+			}
+		}
 		return "", err
 	}
 	return cachedgoroot, nil