fuse: change raw API for ListXAttr/GetXAttr

The protocol is:

  request: read (with buffer 1024)
  response: data (if buffer is big enough), or (data size, ERANGE)

Before, the raw API provided

* GetXAttrSize: called if the kernel sends a 0 sized buffer (which
  never happens

* GetXAttrData: called in other cases. The return value is []byte
  forcing allocation of potentially large buffers

* ListXAttr: return value []byte, so also problematic for allocation.

Now, the raw API mirrors the kernel API more closely, and use recycled
buffers for more efficiency.

fuse/api.go

@@ -218,10 +218,21 @@ type RawFileSystem interface {
 	Access(cancel <-chan struct{}, input *AccessIn) (code Status)
 
 	// Extended attributes.
-	GetXAttrSize(cancel <-chan struct{}, header *InHeader, attr string) (sz int, code Status)
-	GetXAttrData(cancel <-chan struct{}, header *InHeader, attr string) (data []byte, code Status)
-	ListXAttr(cancel <-chan struct{}, header *InHeader) (attributes []byte, code Status)
+
+	// GetXAttr reads an extended attribute, and should return the
+	// number of bytes. If the buffer is too small, return ERANGE,
+	// with the required buffer size.
+	GetXAttr(cancel <-chan struct{}, header *InHeader, attr string, dest []byte) (sz uint32, code Status)
+
+	// ListXAttr lists extended attributes as '\0' delimited byte
+	// slice, and return the number of bytes. If the buffer is too
+	// small, return ERANGE, with the required buffer size.
+	ListXAttr(cancel <-chan struct{}, header *InHeader, dest []byte) (uint32, Status)
+
+	// SetAttr writes an extended attribute.
 	SetXAttr(cancel <-chan struct{}, input *SetXAttrIn, attr string, data []byte) Status
+
+	// RemoveXAttr removes an extended attribute.
 	RemoveXAttr(cancel <-chan struct{}, header *InHeader, attr string) (code Status)
 
 	// File handling.

fuse/defaultraw.go

@@ -81,20 +81,16 @@ func (fs *defaultRawFileSystem) Link(cancel <-chan struct{}, input *LinkIn, name
 	return ENOSYS
 }
 
-func (fs *defaultRawFileSystem) GetXAttrSize(cancel <-chan struct{}, header *InHeader, attr string) (size int, code Status) {
+func (fs *defaultRawFileSystem) GetXAttr(cancel <-chan struct{}, header *InHeader, attr string, dest []byte) (size uint32, code Status) {
 	return 0, ENOSYS
 }
 
-func (fs *defaultRawFileSystem) GetXAttrData(cancel <-chan struct{}, header *InHeader, attr string) (data []byte, code Status) {
-	return nil, ENOATTR
-}
-
 func (fs *defaultRawFileSystem) SetXAttr(cancel <-chan struct{}, input *SetXAttrIn, attr string, data []byte) Status {
 	return ENOSYS
 }
 
-func (fs *defaultRawFileSystem) ListXAttr(cancel <-chan struct{}, header *InHeader) (data []byte, code Status) {
-	return nil, ENOSYS
+func (fs *defaultRawFileSystem) ListXAttr(cancel <-chan struct{}, header *InHeader, dest []byte) (n uint32, code Status) {
+	return 0, ENOSYS
 }
 
 func (fs *defaultRawFileSystem) RemoveXAttr(cancel <-chan struct{}, header *InHeader, attr string) Status {

fuse/lockingfs.go

@@ -109,19 +109,14 @@ func (fs *lockingRawFileSystem) SetXAttr(cancel <-chan struct{}, input *SetXAttr
 	return fs.RawFS.SetXAttr(cancel, input, attr, data)
 }
 
-func (fs *lockingRawFileSystem) GetXAttrData(cancel <-chan struct{}, header *InHeader, attr string) (data []byte, code Status) {
+func (fs *lockingRawFileSystem) GetXAttr(cancel <-chan struct{}, header *InHeader, attr string, dest []byte) (uint32, Status) {
 	defer fs.locked()()
-	return fs.RawFS.GetXAttrData(cancel, header, attr)
+	return fs.RawFS.GetXAttr(cancel, header, attr, dest)
 }
 
-func (fs *lockingRawFileSystem) GetXAttrSize(cancel <-chan struct{}, header *InHeader, attr string) (sz int, code Status) {
+func (fs *lockingRawFileSystem) ListXAttr(cancel <-chan struct{}, header *InHeader, data []byte) (uint32, Status) {
 	defer fs.locked()()
-	return fs.RawFS.GetXAttrSize(cancel, header, attr)
-}
-
-func (fs *lockingRawFileSystem) ListXAttr(cancel <-chan struct{}, header *InHeader) (data []byte, code Status) {
-	defer fs.locked()()
-	return fs.RawFS.ListXAttr(cancel, header)
+	return fs.RawFS.ListXAttr(cancel, header, data)
 }
 
 func (fs *lockingRawFileSystem) RemoveXAttr(cancel <-chan struct{}, header *InHeader, attr string) Status {

fuse/nodefs/fsops.go

@@ -8,7 +8,6 @@ package nodefs
 // RawFileSystem
 
 import (
-	"bytes"
 	"fmt"
 	"log"
 	"strings"
@@ -377,11 +376,15 @@ func (c *rawBridge) ReleaseDir(input *fuse.ReleaseIn) {
 		node.mount.unregisterFileHandle(input.Fh, node)
 	}
 }
-
-func (c *rawBridge) GetXAttrSize(cancel <-chan struct{}, header *fuse.InHeader, attribute string) (sz int, code fuse.Status) {
+func (c *rawBridge) GetXAttr(cancel <-chan struct{}, header *fuse.InHeader, attribute string, dest []byte) (sz uint32, code fuse.Status) {
 	node := c.toInode(header.NodeId)
 	data, errno := node.fsInode.GetXAttr(attribute, &fuse.Context{Caller: header.Caller, Cancel: cancel})
-	return len(data), errno
+
+	if len(data) > len(dest) {
+		return uint32(len(data)), fuse.ERANGE
+	}
+	copy(dest, data)
+	return uint32(len(data)), errno
 }
 
 func (c *rawBridge) GetXAttrData(cancel <-chan struct{}, header *fuse.InHeader, attribute string) (data []byte, code fuse.Status) {
@@ -399,20 +402,29 @@ func (c *rawBridge) SetXAttr(cancel <-chan struct{}, input *fuse.SetXAttrIn, att
 	return node.fsInode.SetXAttr(attr, data, int(input.Flags), &fuse.Context{Caller: input.Caller, Cancel: cancel})
 }
 
-func (c *rawBridge) ListXAttr(cancel <-chan struct{}, header *fuse.InHeader) (data []byte, code fuse.Status) {
+func (c *rawBridge) ListXAttr(cancel <-chan struct{}, header *fuse.InHeader, dest []byte) (uint32, fuse.Status) {
 	node := c.toInode(header.NodeId)
 	attrs, code := node.fsInode.ListXAttr(&fuse.Context{Caller: header.Caller, Cancel: cancel})
 	if code != fuse.OK {
-		return nil, code
+		return 0, code
+	}
+
+	var sz uint32
+	for _, v := range attrs {
+		sz += uint32(len(v)) + 1
+	}
+
+	if int(sz) > len(dest) {
+		return sz, fuse.ERANGE
 	}
 
-	b := bytes.NewBuffer([]byte{})
+	dest = dest[:0]
 	for _, v := range attrs {
-		b.Write([]byte(v))
-		b.WriteByte(0)
+		dest = append(dest, v...)
+		dest = append(dest, 0)
 	}
 
-	return b.Bytes(), code
+	return sz, code
 }
 
 ////////////////

fuse/opcode.go

@@ -239,48 +239,29 @@ func doGetXAttr(server *Server, req *request) {
 
 	input := (*GetXAttrIn)(req.inData)
 
-	if input.Size == 0 {
-		out := (*GetXAttrOut)(req.outData())
-		switch req.inHeader.Opcode {
-		case _OP_GETXATTR:
-			// TODO(hanwen): double check this. For getxattr, input.Size
-			// field refers to the size of the attribute, so it usually
-			// is not 0.
-			sz, code := server.fileSystem.GetXAttrSize(req.cancel, req.inHeader, req.filenames[0])
-			if code.Ok() {
-				out.Size = uint32(sz)
-			}
-			req.status = code
-			return
-		case _OP_LISTXATTR:
-			data, code := server.fileSystem.ListXAttr(req.cancel, req.inHeader)
-			if code.Ok() {
-				out.Size = uint32(len(data))
-			}
-			req.status = code
-			return
-		}
-	}
-	var data []byte
+	req.flatData = server.allocOut(req, input.Size)
+	out := (*GetXAttrOut)(req.outData())
+
+	var n uint32
 	switch req.inHeader.Opcode {
 	case _OP_GETXATTR:
-		data, req.status = server.fileSystem.GetXAttrData(req.cancel, req.inHeader, req.filenames[0])
+		n, req.status = server.fileSystem.GetXAttr(req.cancel, req.inHeader, req.filenames[0], req.flatData)
 	case _OP_LISTXATTR:
-		data, req.status = server.fileSystem.ListXAttr(req.cancel, req.inHeader)
+		n, req.status = server.fileSystem.ListXAttr(req.cancel, req.inHeader, req.flatData)
 	default:
-		log.Panicf("xattr opcode %v", req.inHeader.Opcode)
 		req.status = ENOSYS
 	}
 
-	if len(data) > int(input.Size) {
-		req.status = ERANGE
-	}
-
-	if !req.status.Ok() {
-		return
+	if input.Size == 0 && req.status == ERANGE {
+		// For input.size==0, returning ERANGE is an error.
+		req.status = OK
+		out.Size = n
+	} else if req.status.Ok() {
+		req.flatData = req.flatData[:n]
+		out.Size = n
+	} else {
+		req.flatData = req.flatData[:0]
 	}
-
-	req.flatData = data
 }
 
 func doGetAttr(server *Server, req *request) {

fuse/upgrade.go

@@ -228,16 +228,21 @@ func (fs *wrappingFS) Link(cancel <-chan struct{}, input *LinkIn, name string, o
 	return ENOSYS
 }
 
-func (fs *wrappingFS) GetXAttrSize(cancel <-chan struct{}, header *InHeader, attr string) (size int, code Status) {
+func (fs *wrappingFS) GetXAttr(cancel <-chan struct{}, header *InHeader, attr string, dest []byte) (size uint32, code Status) {
 	if s, ok := fs.fs.(interface {
-		GetXAttrSize(header *InHeader, attr string) (size int, code Status)
+		// Old signature.
+		GetXAttrData(header *InHeader, attr string) ([]byte, Status)
 	}); ok {
-		return s.GetXAttrSize(header, attr)
+		d, s := s.GetXAttrData(header, attr)
+		if len(d) > len(dest) {
+			return uint32(len(d)), ERANGE
+		}
+		return uint32(copy(dest, d)), s
 	}
 	if s, ok := fs.fs.(interface {
-		GetXAttrSize(cancel <-chan struct{}, header *InHeader, attr string) (size int, code Status)
+		GetXAttr(cancel <-chan struct{}, header *InHeader, attr string, dest []byte) (size uint32, code Status)
 	}); ok {
-		return s.GetXAttrSize(cancel, header, attr)
+		return s.GetXAttr(cancel, header, attr, dest)
 	}
 	return 0, ENOSYS
 }
@@ -270,18 +275,24 @@ func (fs *wrappingFS) SetXAttr(cancel <-chan struct{}, input *SetXAttrIn, attr s
 	return ENOSYS
 }
 
-func (fs *wrappingFS) ListXAttr(cancel <-chan struct{}, header *InHeader) (data []byte, code Status) {
+func (fs *wrappingFS) ListXAttr(cancel <-chan struct{}, header *InHeader, dest []byte) (n uint32, code Status) {
 	if s, ok := fs.fs.(interface {
+		// Old signature.
 		ListXAttr(header *InHeader) (data []byte, code Status)
 	}); ok {
-		return s.ListXAttr(header)
+		d, s := s.ListXAttr(header)
+		if len(d) > len(dest) {
+			return uint32(len(d)), ERANGE
+		}
+
+		return uint32(copy(dest, d)), s
 	}
 	if s, ok := fs.fs.(interface {
-		ListXAttr(cancel <-chan struct{}, header *InHeader) (data []byte, code Status)
+		ListXAttr(cancel <-chan struct{}, header *InHeader, data []byte) (uint32, Status)
 	}); ok {
-		return s.ListXAttr(cancel, header)
+		return s.ListXAttr(cancel, header, dest)
 	}
-	return nil, ENOSYS
+	return 0, ENOSYS
 }
 
 func (fs *wrappingFS) RemoveXAttr(cancel <-chan struct{}, header *InHeader, attr string) Status {