tests: matrix: check for fd leaks

And fix two in test_helpers.Mount().

Leftover fds can cause an unmount failure like this later:
fusermount: failed to unmount /tmp/gocryptfs-test-parent/873632270/default-plain: Device or resource busy
so try to catch them early.

tests/matrix/matrix_test.go

@@ -72,7 +72,15 @@ func TestMain(m *testing.M) {
 		opts = append(opts, fmt.Sprintf("-aessiv=%v", testcase.aessiv))
 		opts = append(opts, fmt.Sprintf("-raw64=%v", testcase.raw64))
 		test_helpers.MountOrExit(test_helpers.DefaultCipherDir, test_helpers.DefaultPlainDir, opts...)
+		before := test_helpers.ListFds()
 		r := m.Run()
+		after := test_helpers.ListFds()
+		if len(before) != len(after) {
+			fmt.Printf("fd leak? before, after:\n")
+			fmt.Printf("%v\n", before)
+			fmt.Printf("%v\n", after)
+			os.Exit(1)
+		}
 		test_helpers.UnmountPanic(test_helpers.DefaultPlainDir)
 		if r != 0 {
 			os.Exit(r)

tests/test_helpers/helpers.go

@@ -178,9 +178,14 @@ func Mount(c string, p string, showOutput bool, extraArgs ...string) error {
 		if err != nil {
 			return err
 		}
+		// We can close the fd after cmd.Run() has executed
+		defer pw.Close()
 		cmd.Stderr = pw
 		cmd.Stdout = pw
-		go func() { io.Copy(os.Stdout, pr) }()
+		go func() {
+			io.Copy(os.Stdout, pr)
+			pr.Close()
+		}()
 	}
 
 	return cmd.Run()
@@ -412,3 +417,23 @@ func ExtractCmdExitCode(err error) int {
 	code := err2.Sys().(syscall.WaitStatus).ExitStatus()
 	return code
 }
+
+// ListFds lists our open file descriptors.
+// We use /dev/fd because it exists on both Linux and MacOS.
+func ListFds() []string {
+	f, err := os.Open("/dev/fd")
+	if err != nil {
+		log.Panic(err)
+	}
+	defer f.Close()
+	names, err := f.Readdirnames(0)
+	if err != nil {
+		log.Panic(err)
+	}
+	for i, n := range names {
+		// Note: Readdirnames filters "." and ".."
+		target, _ := os.Readlink("/dev/fd/" + n)
+		names[i] = n + "=" + target
+	}
+	return names
+}