Fix -idle unmounting despite activity

Fixes https://github.com/rfjakob/gocryptfs/issues/421

internal/fusefrontend/fs.go

@@ -6,6 +6,7 @@ package fusefrontend
 import (
 	"os"
 	"sync"
+	"sync/atomic"
 	"syscall"
 	"time"
 
@@ -50,11 +51,11 @@ type FS struct {
 	// "gocryptfs -fsck" reads from the channel to also catch these transparently-
 	// mitigated corruptions.
 	MitigatedCorruptions chan string
-	// Track accesses to the filesystem so that we can know when to autounmount.
-	// An access is considered to have happened on every call to encryptPath,
-	// which is called as part of every filesystem operation.
-	// (This flag uses a uint32 so that it can be reset with CompareAndSwapUint32.)
-	AccessedSinceLastCheck uint32
+	// This flag is set to zero each time fs.isFiltered() is called
+	// (uint32 so that it can be reset with CompareAndSwapUint32).
+	// When -idle was used when mounting, idleMonitor() sets it to 1
+	// periodically.
+	IsIdle uint32
 
 	dirCache dirCacheStruct
 }
@@ -662,6 +663,8 @@ func (fs *FS) reportMitigatedCorruption(item string) {
 //
 // Prevents name clashes with internal files when file names are not encrypted
 func (fs *FS) isFiltered(path string) bool {
+	atomic.StoreUint32(&fs.IsIdle, 0)
+
 	if !fs.args.PlaintextNames {
 		return false
 	}

mount.go

@@ -181,18 +181,18 @@ func idleMonitor(idleTimeout time.Duration, fs *fusefrontend.FS, srv *fuse.Serve
 	timeoutCycles := int(math.Ceil(float64(idleTimeout) / float64(sleepTimeBetweenChecks)))
 	idleCount := 0
 	for {
-		// Atomically check whether the access flag is set and reset it to 0 if so.
-		recentAccess := atomic.CompareAndSwapUint32(&fs.AccessedSinceLastCheck, 1, 0)
+		// Atomically check whether the flag is 0 and reset it to 1 if so.
+		isIdle := !atomic.CompareAndSwapUint32(&fs.IsIdle, 0, 1)
 		// Any form of current or recent access resets the idle counter.
 		openFileCount := openfiletable.CountOpenFiles()
-		if recentAccess || openFileCount > 0 {
+		if !isIdle || openFileCount > 0 {
 			idleCount = 0
 		} else {
 			idleCount++
 		}
 		tlog.Debug.Printf(
-			"Checking for idle (recentAccess = %t, open = %d): %s",
-			recentAccess, openFileCount, time.Now().String())
+			"Checking for idle (isIdle = %t, open = %d): %s",
+			isIdle, openFileCount, time.Now().String())
 		if idleCount > 0 && idleCount%timeoutCycles == 0 {
 			tlog.Info.Printf("Filesystem idle; unmounting: %s", mountpoint)
 			unmount(srv, mountpoint)