plumb through Fd in setattr (#67)

this allows us to distinguish between truncate vs ftruncate

conversions.go

@@ -93,6 +93,11 @@ func convertInMessage(
 			to.Mtime = &t
 		}
 
+		if valid.Handle() {
+			t := fuseops.HandleID(in.Fh)
+			to.Handle = &t
+		}
+
 	case fusekernel.OpForget:
 		type input fusekernel.ForgetIn
 		in := (*input)(inMsg.Consume(unsafe.Sizeof(input{})))

fuseops/ops.go

@@ -145,6 +145,9 @@ type SetInodeAttributesOp struct {
 	// The inode of interest.
 	Inode InodeID
 
+	// If set, this is ftruncate(2), otherwise it's truncate(2)
+	Handle *HandleID
+
 	// The attributes to modify, or nil for attributes that don't need a change.
 	Size  *uint64
 	Mode  *os.FileMode

samples/memfs/memfs.go

@@ -247,6 +247,12 @@ func (fs *memFS) SetInodeAttributes(
 	fs.mu.Lock()
 	defer fs.mu.Unlock()
 
+	if op.Size != nil && op.Handle == nil && *op.Size != 0 {
+		// require that truncate to non-zero has to be ftruncate()
+		// but allow open(O_TRUNC)
+		err = syscall.EBADF
+	}
+
 	// Grab the inode.
 	inode := fs.getInodeOrDie(op.Inode)