buffer: give OutMessage's public API a makeover.

Use int where appropriate, fix up documentation, and clarify. Delete the
implementation where it will soon need to change.

internal/buffer/out_message.go

@@ -23,14 +23,9 @@ import (
 	"github.com/jacobsa/fuse/internal/fusekernel"
 )
 
-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
+// OutMessageHeaderSize is the size of the leading header in every
+// properly-constructed OutMessage. Reset brings the message back to this size.
+const OutMessageHeaderSize = unsafe.Sizeof(fusekernel.OutHeader{})
 
 // OutMessage provides a mechanism for constructing a single contiguous fuse
 // message from multiple segments, where the first segment is always a
@@ -38,8 +33,6 @@ const outMessageSize = outHeaderSize + MaxReadSize
 //
 // Must be initialized with Reset.
 type OutMessage struct {
-	offset  uintptr
-	storage [outMessageSize]byte
 }
 
 // Make sure alignment works out correctly, at least for the header.
@@ -53,57 +46,30 @@ 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 = OutMessageInitialSize
-	memclr(unsafe.Pointer(&m.storage), OutMessageInitialSize)
-}
+// Reset resets m so that it's ready to be used again. Afterward, the contents
+// are solely a zeroed fusekernel.OutHeader struct.
+func (m *OutMessage) Reset()
 
-// Return a pointer to the header at the start of the message.
-func (b *OutMessage) OutHeader() (h *fusekernel.OutHeader) {
-	h = (*fusekernel.OutHeader)(unsafe.Pointer(&b.storage))
-	return
-}
+// OutHeader returns a pointer to the header at the start of the message.
+func (m *OutMessage) OutHeader() (h *fusekernel.OutHeader)
 
-// Grow the buffer by the supplied number of bytes, returning a pointer to the
-// start of the new segment, which is zeroed. If there is no space left, return
-// the nil pointer.
-func (b *OutMessage) Grow(size uintptr) (p unsafe.Pointer) {
-	p = b.GrowNoZero(size)
-	if p != nil {
-		memclr(p, size)
-	}
+// Grow grows m's buffer by the given number of bytes, returning a pointer to
+// the start of the new segment, which is guaranteed to be zeroed. If there is
+// insufficient space, it returns nil.
+func (b *OutMessage) Grow(n int) (p unsafe.Pointer)
 
-	return
-}
+// GrowNoZero is equivalent to Grow, except the new segment is not zeroed. Use
+// with caution!
+func (b *OutMessage) GrowNoZero(n int) (p unsafe.Pointer)
 
-// Equivalent to Grow, except the new segment is not zeroed. Use with caution!
-func (b *OutMessage) GrowNoZero(size uintptr) (p unsafe.Pointer) {
-	if outMessageSize-b.offset < size {
-		return
-	}
+// ShrinkTo shrinks m to the given size. It panics if the size is greater than
+// Len() or less than OutMessageHeaderSize.
+func (b *OutMessage) ShrinkTo(n int)
 
-	p = unsafe.Pointer(uintptr(unsafe.Pointer(&b.storage)) + b.offset)
-	b.offset += size
-
-	return
-}
-
-// 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
-}
-
-// Equivalent to growing by the length of p, then copying p over the new
-// segment. Panics if there is not enough room available.
+// Append is equivalent to growing by len(src), then copying src over the new
+// segment. Int panics if there is not enough room available.
 func (b *OutMessage) Append(src []byte) {
-	p := b.GrowNoZero(uintptr(len(src)))
+	p := b.GrowNoZero(len(src))
 	if p == nil {
 		panic(fmt.Sprintf("Can't grow %d bytes", len(src)))
 	}
@@ -114,10 +80,9 @@ func (b *OutMessage) Append(src []byte) {
 	return
 }
 
-// Equivalent to growing by the length of s, then copying s over the new
-// segment. Panics if there is not enough room available.
+// AppendString is like Append, but accepts string input.
 func (b *OutMessage) AppendString(src string) {
-	p := b.GrowNoZero(uintptr(len(src)))
+	p := b.GrowNoZero(len(src))
 	if p == nil {
 		panic(fmt.Sprintf("Can't grow %d bytes", len(src)))
 	}
@@ -128,12 +93,13 @@ func (b *OutMessage) AppendString(src string) {
 	return
 }
 
-// Return the current size of the buffer.
+// Len returns the current size of the message, including the leading header.
 func (b *OutMessage) Len() int {
 	return int(b.offset)
 }
 
-// Return a reference to the current contents of the buffer.
+// Bytes returns a reference to the current contents of the buffer, including
+// the leading header.
 func (b *OutMessage) Bytes() []byte {
 	return b.storage[:int(b.offset)]
 }