Read directly into out messages for files.

connection.go

@@ -378,8 +378,10 @@ func (c *Connection) ReadOp() (ctx context.Context, op interface{}, err error) {
 		}
 
 		// Convert the message to an op.
-		op, err = convertInMessage(inMsg, c.protocol)
+		outMsg := c.getOutMessage()
+		op, err = convertInMessage(inMsg, outMsg, c.protocol)
 		if err != nil {
+			c.putOutMessage(outMsg)
 			err = fmt.Errorf("convertInMessage: %v", err)
 			return
 		}
@@ -396,9 +398,6 @@ func (c *Connection) ReadOp() (ctx context.Context, op interface{}, err error) {
 			continue
 		}
 
-		// Allocate an output message up front, to be used later when replying.
-		outMsg := c.getOutMessage()
-
 		// Set up a context that remembers information about this op.
 		ctx = c.beginOp(inMsg.Header().Opcode, inMsg.Header().Unique)
 		ctx = context.WithValue(ctx, contextKey, opState{inMsg, outMsg, op, opID})

internal/buffer/out_message.go

@@ -25,6 +25,9 @@ import (
 
 const outHeaderSize = unsafe.Sizeof(fusekernel.OutHeader{})
 
+// OutMessage structs begin life with Len() == OutMessageInitialSize.
+const OutMessageInitialSize = outHeaderSize
+
 // We size out messages to be large enough to hold a header for the response
 // plus the largest read that may come in.
 const outMessageSize = outHeaderSize + MaxReadSize
@@ -53,8 +56,8 @@ func init() {
 // Reset the message so that it is ready to be used again. Afterward, the
 // contents are solely a zeroed header.
 func (m *OutMessage) Reset() {
-	m.offset = outHeaderSize
-	memclr(unsafe.Pointer(&m.storage), outHeaderSize)
+	m.offset = OutMessageInitialSize
+	memclr(unsafe.Pointer(&m.storage), OutMessageInitialSize)
 }
 
 // Return a pointer to the header at the start of the message.
@@ -87,6 +90,15 @@ func (b *OutMessage) GrowNoZero(size uintptr) (p unsafe.Pointer) {
 	return
 }
 
+// Throw away the last n bytes. Panics if n is out of range.
+func (b *OutMessage) Shrink(n uintptr) {
+	if n > b.offset-OutMessageInitialSize {
+		panic(fmt.Sprintf("Shrink(%d) out of range for offset %d", n, b.offset))
+	}
+
+	b.offset -= n
+}
+
 // Equivalent to growing by the length of p, then copying p over the new
 // segment. Panics if there is not enough room available.
 func (b *OutMessage) Append(src []byte) {