fuse: add `SyncRead` flag to MountOptions to ensure in-order reads

We enable FUSE_CAP_ASYNC_READ per default, which means that
the kernel can (and does) submit multiple concurrent out-of-order
read requests to service userspace reads and kernel readahead.

For some backing storages, like Amazon Cloud Drive, out-of-order
reads are expensive.

gocryptfs has implemented a delay-based workaround with its
`-serialize_reads` flag for this case
(see https://github.com/rfjakob/gocryptfs/issues/92 for details).

Not enabling FUSE_CAP_ASYNC_READ makes the kernel do this for us,
as verified by adding debug output to gocryptfs, so expose it as
a mount flag in MountOptions.

Fixes: https://github.com/hanwen/go-fuse/issues/140
Graphs-at: https://github.com/hanwen/go-fuse/issues/395
Related: https://github.com/rfjakob/gocryptfs/issues/92
Change-Id: I10f947d71e1453989c4a9b66fbb0407f7163994f

fuse/api.go

@@ -165,6 +165,23 @@ type MountOptions struct {
 	// The filesystem is fully responsible for invalidating data cache.
 	ExplicitDataCacheControl bool
 
+	// SyncRead is off by default, which means that go-fuse enable the
+	// FUSE_CAP_ASYNC_READ capability.
+	// The kernel then submits multiple concurrent reads to service
+	// userspace requests and kernel readahead.
+	//
+	// Setting SyncRead disables the FUSE_CAP_ASYNC_READ capability.
+	// The kernel then only sends one read request per file handle at a time,
+	// and orders the requests by offset.
+	//
+	// This is useful if reading out of order or concurrently is expensive for
+	// (example: Amazon Cloud Drive).
+	//
+	// See the comment to FUSE_CAP_ASYNC_READ in
+	// https://github.com/libfuse/libfuse/blob/master/include/fuse_common.h
+	// for more details.
+	SyncRead bool
+
 	// If set, fuse will first attempt to use syscall.Mount instead of
 	// fusermount to mount the filesystem. This will not update /etc/mtab
 	// but might be needed if fusermount is not available.

fuse/opcode.go

@@ -98,6 +98,10 @@ func doInit(server *Server, req *request) {
 	if server.opts.EnableAcl {
 		server.kernelSettings.Flags |= CAP_POSIX_ACL
 	}
+	if server.opts.SyncRead {
+		// Clear CAP_ASYNC_READ
+		server.kernelSettings.Flags &= ^uint32(CAP_ASYNC_READ)
+	}
 
 	dataCacheMode := input.Flags & CAP_AUTO_INVAL_DATA
 	if server.opts.ExplicitDataCacheControl {