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

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 <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
Run-TryBot: Andy Pan <panjf2000@gmail.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/622697

diff --git a/src/internal/poll/sendfile_bsd.go b/src/internal/poll/sendfile_bsd.go
index 0b0966815deedd1876a203ae0ad121c22fb3d809..341e07ca1fed6a33545bead6fadd791f70a0c3bf 100644 (file)
--- a/src/internal/poll/sendfile_bsd.go
+++ b/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