tests: filter leaked fds by prefix

When running

  $ go test ./tests/matrix/

in isolation, it failed like this:

  fd leak? before, after:
  [0r=/dev/null 3w=/dev/null 5r=/proc/8078/fd (hidden:4)]
  [0r=/dev/null 3w=/dev/null 5w=/tmp/go-build366655199/b001/testlog.txt 7r=/proc/8078/fd (hidden:4)]

Filter by prefix to get rid of this spurious test failure.

tests/cli/cli_test.go

@@ -22,9 +22,9 @@ var testPw = []byte("test")
 
 func TestMain(m *testing.M) {
 	test_helpers.ResetTmpDir(false)
-	before := test_helpers.ListFds(0)
+	before := test_helpers.ListFds(0, "")
 	r := m.Run()
-	after := test_helpers.ListFds(0)
+	after := test_helpers.ListFds(0, "")
 	if len(before) != len(after) {
 		fmt.Printf("fd leak in test process? before, after:\n%v\n%v\n", before, after)
 		os.Exit(1)

tests/matrix/matrix_test.go

@@ -77,11 +77,13 @@ func TestMain(m *testing.M) {
 		opts = append(opts, fmt.Sprintf("-raw64=%v", testcase.raw64))
 		opts = append(opts, testcase.extraArgs...)
 		test_helpers.MountOrExit(test_helpers.DefaultCipherDir, test_helpers.DefaultPlainDir, opts...)
-		before := test_helpers.ListFds(0)
+		before := test_helpers.ListFds(0, test_helpers.TmpDir)
 		r := m.Run()
 		// Catch fd leaks in the tests. NOTE: this does NOT catch leaks in
 		// the gocryptfs FUSE process, but only in the tests that access it!
-		after := test_helpers.ListFds(0)
+		// All fds that point outside TmpDir are not interesting (the Go test
+		// infrastucture creates temporary log files we don't care about).
+		after := test_helpers.ListFds(0, test_helpers.TmpDir)
 		if len(before) != len(after) {
 			fmt.Printf("fd leak in test process? before, after:\n%v\n%v\n", before, after)
 			os.Exit(1)

tests/test_helpers/mount_unmount.go

@@ -91,7 +91,7 @@ func Mount(c string, p string, showOutput bool, extraArgs ...string) error {
 	}
 
 	// Save PID and open FDs
-	MountInfo[p] = mountInfo{pid, ListFds(pid)}
+	MountInfo[p] = mountInfo{pid, ListFds(pid, "")}
 	return nil
 }
 
@@ -160,13 +160,13 @@ func UnmountErr(dir string) (err error) {
 				// when testing "-ctlsock" is as well. Wait a little and
 				// hope that all close commands get through to the gocryptfs
 				// process.
-				fdsNow = ListFds(pid)
+				fdsNow = ListFds(pid, "")
 				if len(fdsNow) <= len(fds)+maxCacheFds {
 					break
 				}
 				fmt.Printf("UnmountErr: fdsOld=%d fdsNow=%d, retrying\n", len(fds), len(fdsNow))
 				time.Sleep(10 * time.Millisecond)
-				fdsNow = ListFds(pid)
+				fdsNow = ListFds(pid, "")
 			}
 		}
 		cmd := exec.Command(UnmountScript, "-u", dir)
@@ -187,8 +187,8 @@ func UnmountErr(dir string) (err error) {
 }
 
 // ListFds lists the open file descriptors for process "pid". Pass pid=0 for
-// ourselves.
-func ListFds(pid int) []string {
+// ourselves. Pass a prefix to ignore all paths that do not start with "prefix".
+func ListFds(pid int, prefix string) []string {
 	// We need /proc to get the list of fds for other processes. Only exists
 	// on Linux.
 	if runtime.GOOS != "linux" && pid > 0 {
@@ -211,7 +211,7 @@ func ListFds(pid int) []string {
 		log.Panic(err)
 	}
 	var out []string
-	hidden := 0
+	var filtered []string
 	for _, n := range names {
 		fdPath := dir + "/" + n
 		fi, err := os.Lstat(fdPath)
@@ -235,11 +235,15 @@ func ListFds(pid int) []string {
 			// creates spurious test failures. Ignore all pipes.
 			// Also get rid of the "eventpoll" fd that is always there and not
 			// interesting.
-			hidden++
+			filtered = append(filtered, target)
+			continue
+		}
+		if prefix != "" && !strings.HasPrefix(target, prefix) {
+			filtered = append(filtered, target)
 			continue
 		}
 		out = append(out, n+"="+target)
 	}
-	out = append(out, fmt.Sprintf("(hidden:%d)", hidden))
+	out = append(out, fmt.Sprintf("(filtered: %s)", strings.Join(filtered, ", ")))
 	return out
 }