fusefrontend: implement recursive diriv caching

The new contrib/maxlen.bash showed that we have exponential
runtime with respect to directory depth.

The new recursive diriv caching is a lot smarter as it caches
intermediate lookups. maxlen.bash now completes in a few seconds.

xfstests results same as
https://github.com/rfjakob/fuse-xfstests/blob/2d158e4c82be85c15269af77498e353f928f4fab/screenlog.0 :

  Failures: generic/035 generic/062 generic/080 generic/093 generic/099 generic/215 generic/285 generic/319 generic/426 generic/444 generic/467 generic/477 generic/523
  Failed 13 of 580 tests

benchmark.bash results are identical:

  $ ./benchmark.bash
  Testing gocryptfs at /tmp/benchmark.bash.BdQ: gocryptfs v2.0.1-17-g6b09bc0; go-fuse v2.1.1-0.20210611132105-24a1dfe6b4f8; 2021-06-25 go1.16.5 linux/amd64
  /tmp/benchmark.bash.BdQ.mnt is a mountpoint
  WRITE: 262144000 bytes (262 MB, 250 MiB) copied, 0,4821 s, 544 MB/s
  READ:  262144000 bytes (262 MB, 250 MiB) copied, 0,266061 s, 985 MB/s
  UNTAR: 8,280
  MD5:   4,564
  LS:    1,745
  RM:    2,244

1 files changed, 5 insertions, 3 deletions

diff --git a/internal/fusefrontend/dircache.go b/internal/fusefrontend/dircache.go
index 6732de1..8c285fa 100644
--- a/internal/fusefrontend/dircache.go
+++ b/internal/fusefrontend/dircache.go
@@ -7,7 +7,6 @@ import (
 	"syscall"
 	"time"
 
-	"github.com/rfjakob/gocryptfs/internal/nametransform"
 	"github.com/rfjakob/gocryptfs/internal/tlog"
 )
 
@@ -50,6 +49,9 @@ func (e *dirCacheEntry) Clear() {
 
 type dirCache struct {
 	sync.Mutex
+	// Expected length of the stored IVs. Only used for sanity checks.
+	// Usually set to 16, but 0 in plaintextnames mode.
+	ivLen int
 	// Cache entries
 	entries [dirCacheSize]dirCacheEntry
 	// Where to store the next entry (index into entries)
@@ -77,7 +79,7 @@ func (d *dirCache) Clear() {
 func (d *dirCache) Store(node *Node, fd int, iv []byte) {
 	// Note: package ensurefds012, imported from main, guarantees that dirCache
 	// can never get fds 0,1,2.
-	if fd <= 0 || len(iv) != nametransform.DirIVLen {
+	if fd <= 0 || len(iv) != d.ivLen {
 		log.Panicf("Store sanity check failed: fd=%d len=%d", fd, len(iv))
 	}
 	d.Lock()
@@ -139,7 +141,7 @@ func (d *dirCache) Lookup(node *Node) (fd int, iv []byte) {
 	if enableStats {
 		d.hits++
 	}
-	if fd <= 0 || len(iv) != nametransform.DirIVLen {
+	if fd <= 0 || len(iv) != d.ivLen {
 		log.Panicf("Lookup sanity check failed: fd=%d len=%d", fd, len(iv))
 	}
 	d.dbg("dirCache.Lookup %p hit fd=%d dup=%d iv=%x\n", node, e.fd, fd, iv)