From 7d1b6226cdbc4e36aa6f5c4ce8291893999e2bda Mon Sep 17 00:00:00 2001
From: Sandy Xu <sandy@juicedata.io>
Date: Wed, 30 Oct 2024 17:41:44 +0800
Subject: [PATCH] trySplice: use one pair instead of two; add move flag to
 splice

---
 fuse/splice_linux.go | 45 +++++++++-----------------------------------
 splice/pair_linux.go |  7 ++++---
 2 files changed, 13 insertions(+), 39 deletions(-)

diff --git a/fuse/splice_linux.go b/fuse/splice_linux.go
index 70f836a24..bf331301e 100644
--- a/fuse/splice_linux.go
+++ b/fuse/splice_linux.go
@@ -29,48 +29,25 @@ func (s *Server) setSplice() {
 // This dance is neccessary because header and payload cannot be split across
 // two splices and we cannot seek in a pipe buffer.
 func (ms *Server) trySplice(header []byte, req *request, fdData *readResultFd) error {
-	var err error
-
 	// Get a pair of connected pipes
-	pair1, err := splice.Get()
-	if err != nil {
-		return err
-	}
-	defer splice.Done(pair1)
-
-	// Grow buffer pipe to requested size + one extra page
-	// Without the extra page the kernel will block once the pipe is almost full
-	pair1Sz := fdData.Size() + os.Getpagesize()
-	if err := pair1.Grow(pair1Sz); err != nil {
-		return err
-	}
-
-	// Read data from file
-	payloadLen, err := pair1.LoadFromAt(fdData.Fd, fdData.Size(), fdData.Off)
-
-	if err != nil {
-		// TODO - extract the data from splice.
-		return err
-	}
-
-	// Get another pair of connected pipes
-	pair2, err := splice.Get()
+	pair, err := splice.Get()
 	if err != nil {
 		return err
 	}
-	defer splice.Done(pair2)
+	defer splice.Done(pair)
 
 	// Grow pipe to header + actually read size + one extra page
 	// Without the extra page the kernel will block once the pipe is almost full
+	payloadLen := fdData.Size()
 	header = req.serializeHeader(payloadLen)
 	total := len(header) + payloadLen
-	pair2Sz := total + os.Getpagesize()
-	if err := pair2.Grow(pair2Sz); err != nil {
+	// FIXME: use pipes big enough to skip the Grow()?
+	if err := pair.Grow(total + os.Getpagesize()); err != nil {
 		return err
 	}
 
 	// Write header into pair2
-	n, err := pair2.Write(header)
+	n, err := pair.Write(header)
 	if err != nil {
 		return err
 	}
@@ -79,7 +56,7 @@ func (ms *Server) trySplice(header []byte, req *request, fdData *readResultFd) e
 	}
 
 	// Write data into pair2
-	n, err = pair2.LoadFrom(pair1.ReadFd(), payloadLen)
+	n, err = pair.LoadFrom(fdData.Fd, payloadLen)
 	if err != nil {
 		return err
 	}
@@ -88,10 +65,6 @@ func (ms *Server) trySplice(header []byte, req *request, fdData *readResultFd) e
 	}
 
 	// Write header + data to /dev/fuse
-	_, err = pair2.WriteTo(uintptr(ms.mountFd), total)
-	if err != nil {
-		return err
-	}
-
-	return nil
+	_, err = pair.WriteTo(uintptr(ms.mountFd), total)
+	return err
 }
diff --git a/splice/pair_linux.go b/splice/pair_linux.go
index f8923e307..c14578474 100644
--- a/splice/pair_linux.go
+++ b/splice/pair_linux.go
@@ -11,6 +11,9 @@ import (
 	"syscall"
 )
 
+const _SPLICE_F_MOVE = 0x1
+const _SPLICE_F_NONBLOCK = 0x2
+
 func (p *Pair) LoadFromAt(fd uintptr, sz int, off int64) (int, error) {
 	n, err := syscall.Splice(int(fd), &off, p.w, nil, sz, 0)
 	return int(n), err
@@ -30,15 +33,13 @@ func (p *Pair) LoadFrom(fd uintptr, sz int) (int, error) {
 }
 
 func (p *Pair) WriteTo(fd uintptr, n int) (int, error) {
-	m, err := syscall.Splice(p.r, nil, int(fd), nil, int(n), 0)
+	m, err := syscall.Splice(p.r, nil, int(fd), nil, int(n), _SPLICE_F_MOVE)
 	if err != nil {
 		err = os.NewSyscallError("Splice write", err)
 	}
 	return int(m), err
 }
 
-const _SPLICE_F_NONBLOCK = 0x2
-
 func (p *Pair) discard() {
 	_, err := syscall.Splice(p.r, nil, int(devNullFD), nil, int(p.size), _SPLICE_F_NONBLOCK)
 	if err == syscall.EAGAIN {