mount, cmount: add --attr-timeout to control attribute caching in kernel

This flag allows the attribute caching in the kernel to be controlled.
The default is 0s - no caching - which is recommended for filesystems
which can change outside the control of the kernel.

Previously this was at the default meaning it was 60s for mount and 1s
for cmount.  This showed strange effects when files changed on the
remote not via the kernel.  For instance Caddy would serve corrupted
files for a while when serving from an rclone mount when a file
changed on the remote.

cmd/cmount/mount.go

@@ -40,6 +40,7 @@ func mountOptions(device string, mountpoint string) (options []string) {
 		"-o", "fsname=" + device,
 		"-o", "subtype=rclone",
 		"-o", fmt.Sprintf("max_readahead=%d", mountlib.MaxReadAhead),
+		"-o", fmt.Sprintf("attr_timeout=%g", mountlib.AttrTimeout.Seconds()),
 		// This causes FUSE to supply O_TRUNC with the Open
 		// call which is more efficient for cmount.  However
 		// it does not work with cgofuse on Windows with

cmd/mount/dir.go

@@ -8,6 +8,7 @@ import (
 
 	"bazil.org/fuse"
 	fusefs "bazil.org/fuse/fs"
+	"github.com/ncw/rclone/cmd/mountlib"
 	"github.com/ncw/rclone/fs/log"
 	"github.com/ncw/rclone/vfs"
 	"github.com/pkg/errors"
@@ -25,6 +26,7 @@ var _ fusefs.Node = (*Dir)(nil)
 // Attr updates the attributes of a directory
 func (d *Dir) Attr(ctx context.Context, a *fuse.Attr) (err error) {
 	defer log.Trace(d, "")("attr=%+v, err=%v", a, &err)
+	a.Valid = mountlib.AttrTimeout
 	a.Gid = d.VFS().Opt.GID
 	a.Uid = d.VFS().Opt.UID
 	a.Mode = os.ModeDir | d.VFS().Opt.DirPerms
@@ -72,6 +74,7 @@ func (d *Dir) Lookup(ctx context.Context, req *fuse.LookupRequest, resp *fuse.Lo
 	if err != nil {
 		return nil, translateError(err)
 	}
+	resp.EntryValid = mountlib.AttrTimeout
 	switch x := mnode.(type) {
 	case *vfs.File:
 		return &File{x}, nil

cmd/mount/file.go

@@ -8,6 +8,7 @@ import (
 
 	"bazil.org/fuse"
 	fusefs "bazil.org/fuse/fs"
+	"github.com/ncw/rclone/cmd/mountlib"
 	"github.com/ncw/rclone/fs/log"
 	"github.com/ncw/rclone/vfs"
 	"golang.org/x/net/context"
@@ -24,6 +25,7 @@ var _ fusefs.Node = (*File)(nil)
 // Attr fills out the attributes for the file
 func (f *File) Attr(ctx context.Context, a *fuse.Attr) (err error) {
 	defer log.Trace(f, "")("a=%+v, err=%v", a, &err)
+	a.Valid = mountlib.AttrTimeout
 	modTime := f.File.ModTime()
 	Size := uint64(f.File.Size())
 	Blocks := (Size + 511) / 512

cmd/mountlib/mount.go

@@ -5,6 +5,7 @@ import (
 	"log"
 	"os"
 	"runtime"
+	"time"
 
 	"github.com/ncw/rclone/cmd"
 	"github.com/ncw/rclone/fs"
@@ -27,6 +28,7 @@ var (
 	MaxReadAhead       fs.SizeSuffix = 128 * 1024
 	ExtraOptions       []string
 	ExtraFlags         []string
+	AttrTimeout        = 0 * time.Second // how long the kernel caches attribute for
 )
 
 // Check is folder is empty
@@ -143,6 +145,20 @@ can't use retries in the same way without making local copies of the
 uploads. Look at the **EXPERIMENTAL** [file caching](#file-caching)
 for solutions to make ` + commandName + ` mount more reliable.
 
+### Attribute caching
+
+You can use the flag --attr-timeout to set the time the kernel caches
+the attributes (size, modification time etc) for directory entries.
+
+The default is 0s - no caching - which is recommended for filesystems
+which can change outside the control of the kernel.
+
+If you set it higher ('1s' or '1m' say) then the kernel will call back
+to rclone less often making it more efficient, however there may be
+strange effects when files change on the remote.
+
+This is the same as setting the attr_timeout option in mount.fuse.
+
 ### Filters
 
 Note that all the rclone filters can be used to select a subset of the
@@ -203,6 +219,7 @@ will see all files and folders immediately in this mode.
 	flags.BoolVarP(flagSet, &DefaultPermissions, "default-permissions", "", DefaultPermissions, "Makes kernel enforce access control based on the file mode.")
 	flags.BoolVarP(flagSet, &WritebackCache, "write-back-cache", "", WritebackCache, "Makes kernel buffer writes before sending them to rclone. Without this, writethrough caching is used.")
 	flags.FVarP(flagSet, &MaxReadAhead, "max-read-ahead", "", "The number of bytes that can be prefetched for sequential reads.")
+	flags.DurationVarP(flagSet, &AttrTimeout, "attr-timeout", "", AttrTimeout, "Time for which file/directory attributes are cached.")
 	flags.StringArrayVarP(flagSet, &ExtraOptions, "option", "o", []string{}, "Option for libfuse/WinFsp. Repeat if required.")
 	flags.StringArrayVarP(flagSet, &ExtraFlags, "fuse-flag", "", []string{}, "Flags or arguments to be passed direct to libfuse/WinFsp. Repeat if required.")
 	flags.BoolVarP(flagSet, &Daemon, "daemon", "", Daemon, "Run mount as a daemon (background mode).")