From de73ed90a5baa4e292dec45713f977bface45d5c Mon Sep 17 00:00:00 2001
From: xtaci <daniel820313@gmail.com>
Date: Wed, 18 Dec 2019 21:23:00 +0800
Subject: [PATCH] add comments to rawCopy

---
 generic/rawcopy_unix.go | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/generic/rawcopy_unix.go b/generic/rawcopy_unix.go
index bc50218..2de72d3 100644
--- a/generic/rawcopy_unix.go
+++ b/generic/rawcopy_unix.go
@@ -9,6 +9,7 @@ import (
 	"syscall"
 )
 
+// rawCopy can fan in N src into 1 dst with only 1 shared buffer
 func rawCopy(dst io.Writer, src *net.TCPConn, ctrl *CopyControl) (written int64, err error) {
 	c, err := src.SyscallConn()
 	if err != nil {
@@ -21,7 +22,11 @@ func rawCopy(dst io.Writer, src *net.TCPConn, ctrl *CopyControl) (written int64,
 		var er error
 		var nr int
 		rr := c.Read(func(s uintptr) bool {
-			ctrl.Lock() // acquire rights to read & write
+			// if the 'src' readable, acquire the shared lock first
+			// to make sure no other writers to 'dst' are blocked on dst.Write.
+			// With such design, we only need 1 buffer for a specific 'dst',
+			// especially when 'dst' is a multiplexed connection.
+			ctrl.Lock()
 			locked = true
 			nr, er = syscall.Read(int(s), buf)
 			if er == syscall.EAGAIN {
@@ -29,7 +34,9 @@ func rawCopy(dst io.Writer, src *net.TCPConn, ctrl *CopyControl) (written int64,
 				locked = false
 				return false
 			}
-			return true // keep lock
+			// keep the lock on the shared buffer
+			// for the following dst.Write
+			return true
 		})
 
 		// read EOF