Made the message shrinking API less confusing.

conversions.go

@@ -448,7 +448,7 @@ func (c *Connection) kernelResponse(
 		// the header, because on OS X the kernel otherwise returns EINVAL when we
 		// attempt to write an error response with a length that extends beyond the
 		// header.
-		m.Shrink(uintptr(m.Len() - int(buffer.OutMessageInitialSize)))
+		m.ShrinkTo(buffer.OutMessageInitialSize)
 	}
 
 	// Otherwise, fill in the rest of the response.
@@ -522,7 +522,7 @@ func (c *Connection) kernelResponseForOp(
 		// convertInMessage already set up the destination buffer to be at the end
 		// of the out message. We need only shrink to the right size based on how
 		// much the user read.
-		m.Shrink(uintptr(m.Len() - (int(buffer.OutMessageInitialSize) + o.BytesRead)))
+		m.ShrinkTo(buffer.OutMessageInitialSize + uintptr(o.BytesRead))
 
 	case *fuseops.ReleaseDirHandleOp:
 		// Empty response
@@ -539,7 +539,7 @@ func (c *Connection) kernelResponseForOp(
 		// convertInMessage already set up the destination buffer to be at the end
 		// of the out message. We need only shrink to the right size based on how
 		// much the user read.
-		m.Shrink(uintptr(m.Len() - (int(buffer.OutMessageInitialSize) + o.BytesRead)))
+		m.ShrinkTo(buffer.OutMessageInitialSize + uintptr(o.BytesRead))
 
 	case *fuseops.WriteFileOp:
 		out := (*fusekernel.WriteOut)(m.Grow(unsafe.Sizeof(fusekernel.WriteOut{})))

internal/buffer/out_message.go

@@ -90,13 +90,14 @@ 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))
+// Shrink to the supplied size. Panic if the size is greater than Len() or less
+// than OutMessageInitialSize.
+func (b *OutMessage) ShrinkTo(n uintptr) {
+	if n < OutMessageInitialSize || n > b.offset {
+		panic(fmt.Sprintf("ShrinkTo(%d) out of range for offset %d", n, b.offset))
 	}
 
-	b.offset -= n
+	b.offset = n
 }
 
 // Equivalent to growing by the length of p, then copying p over the new