[release-branch.go1.23] internal/poll: handle the special case of sendfile(2) sending the full chunk

CL 622235 would fix #70000 while resulting in one extra sendfile(2) system
call when sendfile(2) returns (>0, EAGAIN).
That's also why I left sendfile_bsd.go behind, and didn't make it line up
with other two implementations: sendfile_linux.go and sendfile_solaris.go.

Unlike sendfile(2)'s on Linux and Solaris that always return (0, EAGAIN),
sendfile(2)'s on *BSD and macOS may return (>0, EAGAIN) when using a socket
marked for non-blocking I/O. In that case, the current code will try to re-call
sendfile(2) immediately, which will most likely get us a (0, EAGAIN).
After that, it goes to `dstFD.pd.waitWrite(dstFD.isFile)` below,
which should have been done in the first place.

Thus, the real problem that leads to #70000 is that the old code doesn't handle
the special case of sendfile(2) sending the exact number of bytes the caller requested.

Fixes #70000
Fixes #70020

Change-Id: I6073d6b9feb58b3d7e114ec21e4e80d9727bca66
Reviewed-on: https://go-review.googlesource.com/c/go/+/622255
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Damien Neil <[email protected]>
Run-TryBot: Andy Pan <[email protected]>
Reviewed-on: https://go-review.googlesource.com/c/go/+/622697

Author: panjf2000

src/internal/poll/sendfile_bsd.go

@@ -32,13 +32,28 @@ func SendFile(dstFD *FD, src int, pos, remain int64) (written int64, err error,
 		if int64(n) > remain {
 			n = int(remain)
 		}
+		m := n
 		pos1 := pos
 		n, err = syscall.Sendfile(dst, src, &pos1, n)
 		if n > 0 {
 			pos += int64(n)
 			written += int64(n)
 			remain -= int64(n)
-			continue
+			// (n, nil) indicates that sendfile(2) has transferred
+			// the exact number of bytes we requested, or some unretryable
+			// error have occurred with partial bytes sent. Either way, we
+			// don't need to go through the following logic to check EINTR
+			// or fell into dstFD.pd.waitWrite, just continue to send the
+			// next chunk or break the loop.
+			if n == m {
+				continue
+			} else if err != syscall.EAGAIN &&
+				err != syscall.EINTR &&
+				err != syscall.EBUSY {
+				// Particularly, EPIPE. Errors like that would normally lead
+				// the subsequent sendfile(2) call to (-1, EBADF).
+				break
+			}
 		} else if err != syscall.EAGAIN && err != syscall.EINTR {
 			// This includes syscall.ENOSYS (no kernel
 			// support) and syscall.EINVAL (fd types which