fuse: request explicit control over data cache if filesystem asks for it

This complements commit "fuse: allow filesystems to disable
CAP_AUTO_INVAL_DATA" and teaches go-fuse to request explicit data cache
invalidation mode if fuse.MountOptions.ExplicitDataCacheControl is set.

See https://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/fuse.git/commit/?id=ad2ba64dd489
and https://lwn.net/ml/linux-fsdevel/20190315212556.9315-1-kirr%40nexedi.com/ for rationale and details.

fuse/api.go

@@ -157,12 +157,6 @@ type MountOptions struct {
 
 	// If set, ask kernel not to do automatic data cache invalidation.
 	// The filesystem is fully responsible for invalidating data cache.
-	//
-	// XXX for Linux ExplicitDataCacheControl currently disables data cache
-	// to be automatically invalidated only on file mtime change. If file size
-	// changes, Linux currently unconditionally discards whole cache of the
-	// file. See https://github.com/hanwen/go-fuse/pull/273 for kernel +
-	// go-fuse patches with corresponding fixes.
 	ExplicitDataCacheControl bool
 }
 

fuse/opcode.go

@@ -98,14 +98,13 @@ func doInit(server *Server, req *request) {
 
 	dataCacheMode := input.Flags & CAP_AUTO_INVAL_DATA
 	if server.opts.ExplicitDataCacheControl {
-		// XXX this only disables automatic invalidations on mtime
-		// change, but not on size change.
-		//
-		// TODO explicitly disable invalidations on size change when
-		// kernel has proper support. Details:
-		//
-		//	https://github.com/hanwen/go-fuse/pull/273
+		// we don't want CAP_AUTO_INVAL_DATA even if we cannot go into fully explicit mode
 		dataCacheMode = 0
+
+		explicit := input.Flags & CAP_EXPLICIT_INVAL_DATA
+		if explicit != 0 {
+			dataCacheMode = explicit
+		}
 	}
 	server.kernelSettings.Flags |= dataCacheMode
 

fuse/print.go

@@ -45,6 +45,7 @@ var (
 		CAP_MAX_PAGES:           "MAX_PAGES",
 		CAP_CACHE_SYMLINKS:      "CACHE_SYMLINKS",
 		CAP_NO_OPENDIR_SUPPORT:  "NO_OPENDIR_SUPPORT",
+		CAP_EXPLICIT_INVAL_DATA: "EXPLICIT_INVAL_DATA",
 	}
 	releaseFlagNames = map[int64]string{
 		RELEASE_FLUSH: "FLUSH",

fuse/types.go

@@ -290,6 +290,7 @@ const (
 	CAP_MAX_PAGES           = (1 << 22)
 	CAP_CACHE_SYMLINKS      = (1 << 23)
 	CAP_NO_OPENDIR_SUPPORT  = (1 << 24)
+	CAP_EXPLICIT_INVAL_DATA = (1 << 25)
 )
 
 type InitIn struct {