Add option for autounmount

Even though filesystem notifications aren't implemented for FUSE, I decided to
try my hand at implementing the autounmount feature (#128). I based it on the
EncFS autounmount code, which records filesystem accesses and checks every X
seconds whether it's idled long enough to unmount.

I've tested the feature locally, but I haven't added any tests for this flag.
I also haven't worked with Go before. So please let me know if there's
anything that should be done differently.

One particular concern: I worked from the assumption that the open files table
is unique per-filesystem. If that's not true, I'll need to add an open file
count and associated lock to the Filesystem type instead.

https://github.com/rfjakob/gocryptfs/pull/265

3 files changed, 19 insertions, 0 deletions

diff --git a/internal/fusefrontend/fs.go b/internal/fusefrontend/fs.go
index 1b6941e..c0d6151 100644
--- a/internal/fusefrontend/fs.go
+++ b/internal/fusefrontend/fs.go
@@ -47,6 +47,11 @@ 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
 }
 
 var _ pathfs.FileSystem = &FS{} // Verify that interface is implemented.
diff --git a/internal/fusefrontend/names.go b/internal/fusefrontend/names.go
index 5530e3e..3bf64d5 100644
--- a/internal/fusefrontend/names.go
+++ b/internal/fusefrontend/names.go
@@ -60,9 +60,15 @@ func (fs *FS) openBackingDir(relPath string) (dirfd int, cName string, err error
 
 // encryptPath - encrypt relative plaintext path
 func (fs *FS) encryptPath(plainPath string) (string, error) {
+	if plainPath != "" { // Empty path gets encrypted all the time without actual file accesses.
+		fs.AccessedSinceLastCheck = 1
+	} else { // Empty string gets encrypted as empty string
+		return plainPath, nil
+	}
 	if fs.args.PlaintextNames {
 		return plainPath, nil
 	}
+
 	fs.dirIVLock.RLock()
 	cPath, err := fs.nameTransform.EncryptPathDirIV(plainPath, fs.args.Cipherdir)
 	tlog.Debug.Printf("encryptPath '%s' -> '%s' (err: %v)", plainPath, cPath, err)
diff --git a/internal/openfiletable/open_file_table.go b/internal/openfiletable/open_file_table.go
index e21c96d..4a8ce28 100644
--- a/internal/openfiletable/open_file_table.go
+++ b/internal/openfiletable/open_file_table.go
@@ -112,3 +112,11 @@ func (c *countingMutex) Lock() {
 func WriteOpCount() uint64 {
 	return atomic.LoadUint64(&t.writeOpCount)
 }
+
+// CountOpenFiles returns how many entries are currently in the table
+// in a threadsafe manner.
+func CountOpenFiles() int {
+	t.Lock()
+	defer t.Unlock()
+	return len(t.entries)
+}