testing: limit TempDir name length

Fixes #71742

Change-Id: Ibef8f7f0a36b25f181062c4d2f84279a97e467a4
Reviewed-on: https://go-review.googlesource.com/c/go/+/671577
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

src/testing/testing.go

@@ -1244,6 +1244,11 @@ func (c *common) TempDir() string {
 	if nonExistent {
 		c.Helper()
 
+		pattern := c.Name()
+		// Limit length of file names on disk.
+		// Invalid runes from slicing are dropped by strings.Map below.
+		pattern = pattern[:min(len(pattern), 64)]
+
 		// Drop unusual characters (such as path separators or
 		// characters interacting with globs) from the directory name to
 		// avoid surprising os.MkdirTemp behavior.
@@ -1263,7 +1268,7 @@ func (c *common) TempDir() string {
 			}
 			return -1
 		}
-		pattern := strings.Map(mapper, c.Name())
+		pattern = strings.Map(mapper, pattern)
 		c.tempDir, c.tempDirErr = os.MkdirTemp("", pattern)
 		if c.tempDirErr == nil {
 			c.Cleanup(func() {

src/testing/testing_test.go

@@ -89,6 +89,7 @@ func TestTempDir(t *testing.T) {
 	t.Run("test[]", testTempDir)
 	t.Run("test*", testTempDir)
 	t.Run("äöüéè", testTempDir)
+	t.Run(strings.Repeat("a", 300), testTempDir)
 }
 
 func testTempDir(t *testing.T) {