runtime/poll: fix race condition in Window's SendFile

The destination of SendFile is a socket, which doesn't support file
offsets. There is no need to keep track of the file offset, and doing
so causes a race between SendFile and Read.

While here, make sure that SendFile tests do call poll.SendFile.

Fixes #78015

Cq-Include-Trybots: luci.golang.try:gotip-windows-amd64-race,gotip-linux-amd64-race
Change-Id: I8cce45c0c110e848d9bdbc5ba340b92ca041f0a4
Reviewed-on: https://go-review.googlesource.com/c/go/+/752860
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>

src/internal/poll/sendfile_windows.go

@@ -68,8 +68,8 @@ func SendFile(fd *FD, src uintptr, size int64) (written int64, err error, handle
 			chunkSize = size
 		}
 
-		fd.setOffset(startpos + written)
 		n, err := fd.execIO('w', func(o *operation) (uint32, error) {
+			o.setOffset(startpos + written)
 			err := syscall.TransmitFile(fd.Sysfd, hsrc, uint32(chunkSize), 0, &o.o, nil, syscall.TF_WRITE_BEHIND)
 			if err != nil {
 				return 0, err

src/net/sendfile.go

@@ -12,6 +12,8 @@ import (
 	"syscall"
 )
 
+var testHookSupportsSendfile func() bool
+
 // sendFile copies the contents of r to c using the sendfile
 // system call to minimize copies.
 //

src/net/sendfile_stub.go

@@ -8,6 +8,8 @@ package net
 
 import "io"
 
+var testHookSupportsSendfile func() bool
+
 func supportsSendfile() bool {
 	return false
 }

src/net/sendfile_test.go

@@ -28,6 +28,16 @@ const (
 	newtonSHA256 = "d4a9ac22462b35e7821a4f2706c211093da678620a8f9997989ee7cf8d507bbd"
 )
 
+func hookSupportsSendfile(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		origHook := testHookSupportsSendfile
+		testHookSupportsSendfile = func() bool { return true }
+		t.Cleanup(func() {
+			testHookSupportsSendfile = origHook
+		})
+	}
+}
+
 // expectSendfile runs f, and verifies that internal/poll.SendFile successfully handles
 // a write to wantConn during f's execution.
 //
@@ -35,6 +45,7 @@ const (
 // expect a call to SendFile.
 func expectSendfile(t *testing.T, wantConn Conn, f func()) {
 	t.Helper()
+	hookSupportsSendfile(t)
 	if !supportsSendfile() {
 		f()
 		return

src/net/sendfile_windows.go

@@ -12,5 +12,8 @@ import "internal/syscall/windows"
 // https://learn.microsoft.com/en-us/windows/win32/api/mswsock/nf-mswsock-transmitfile
 // https://golang.org/issue/73746
 func supportsSendfile() bool {
+	if testHookSupportsSendfile != nil {
+		return testHookSupportsSendfile()
+	}
 	return windows.SupportUnlimitedTransmitFile()
 }