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, 116 insertions, 0 deletions

diff --git a/internal/fusefrontend/node_prepare_syscall.go b/internal/fusefrontend/node_prepare_syscall.go
new file mode 100644
index 0000000..fa558e8
--- /dev/null
+++ b/internal/fusefrontend/node_prepare_syscall.go
@@ -0,0 +1,116 @@
+package fusefrontend
+
+import (
+	"sync/atomic"
+	"syscall"
+
+	"github.com/rfjakob/gocryptfs/internal/tlog"
+
+	"github.com/hanwen/go-fuse/v2/fs"
+
+	"github.com/rfjakob/gocryptfs/internal/nametransform"
+	"github.com/rfjakob/gocryptfs/internal/syscallcompat"
+)
+
+// prepareAtSyscall returns a (dirfd, cName) pair that can be used
+// with the "___at" family of system calls (openat, fstatat, unlinkat...) to
+// access the backing encrypted child file.
+func (n *Node) prepareAtSyscall(child string) (dirfd int, cName string, errno syscall.Errno) {
+	if child == "" {
+		tlog.Warn.Printf("BUG: prepareAtSyscall: child=%q, should have called prepareAtSyscallMyself", child)
+		return n.prepareAtSyscallMyself()
+	}
+
+	rn := n.rootNode()
+
+	// All filesystem operations go through here, so this is a good place
+	// to reset the idle marker.
+	atomic.StoreUint32(&rn.IsIdle, 0)
+
+	if n.IsRoot() && rn.isFiltered(child) {
+		return -1, "", syscall.EPERM
+	}
+
+	var encryptName func(int, string, []byte) (string, error)
+	if !rn.args.PlaintextNames {
+		encryptName = func(dirfd int, child string, iv []byte) (cName string, err error) {
+			// Badname allowed, try to determine filenames
+			if rn.nameTransform.HaveBadnamePatterns() {
+				return rn.nameTransform.EncryptAndHashBadName(child, iv, dirfd)
+			}
+			return rn.nameTransform.EncryptAndHashName(child, iv)
+		}
+	}
+
+	// Cache lookup
+	var iv []byte
+	dirfd, iv = rn.dirCache.Lookup(n)
+	if dirfd > 0 {
+		if rn.args.PlaintextNames {
+			return dirfd, child, 0
+		}
+		var err error
+		cName, err = encryptName(dirfd, child, iv)
+		if err != nil {
+			syscall.Close(dirfd)
+			return -1, "", fs.ToErrno(err)
+		}
+		return
+	}
+
+	// Slowpath: Open ourselves & read diriv
+	parentDirfd, myCName, errno := n.prepareAtSyscallMyself()
+	if errno != 0 {
+		return
+	}
+	defer syscall.Close(parentDirfd)
+
+	dirfd, err := syscallcompat.Openat(parentDirfd, myCName, syscall.O_NOFOLLOW|syscall.O_DIRECTORY|syscallcompat.O_PATH, 0)
+
+	// Cache store
+	if !rn.args.PlaintextNames {
+		var err error
+		iv, err = nametransform.ReadDirIVAt(dirfd)
+		if err != nil {
+			syscall.Close(dirfd)
+			return -1, "", fs.ToErrno(err)
+		}
+	}
+	rn.dirCache.Store(n, dirfd, iv)
+
+	if rn.args.PlaintextNames {
+		return dirfd, child, 0
+	}
+
+	cName, err = encryptName(dirfd, child, iv)
+	if err != nil {
+		syscall.Close(dirfd)
+		return -1, "", fs.ToErrno(err)
+	}
+
+	return
+}
+
+func (n *Node) prepareAtSyscallMyself() (dirfd int, cName string, errno syscall.Errno) {
+	dirfd = -1
+
+	// Handle root node
+	if n.IsRoot() {
+		var err error
+		rn := n.rootNode()
+		dirfd, cName, err = rn.openBackingDir("")
+		if err != nil {
+			errno = fs.ToErrno(err)
+		}
+		return
+	}
+
+	// Otherwise convert to prepareAtSyscall of parent node
+	myName, p1 := n.Parent()
+	if p1 == nil || myName == "" {
+		errno = syscall.ENOENT
+		return
+	}
+	parent := toNode(p1.Operations())
+	return parent.prepareAtSyscall(myName)
+}