net: reenable sendfile on Windows

Windows sendfile optimization is skipped since CL 472475, which started
passing an os.fileWithoutWriteTo instead of an os.File to sendfile,
and that function was only implemented for os.File.

This CL fixes the issue by asserting against an interface rather than
a concrete type.

Some tests have been reenabled, triggering bugs in poll.SendFile which
have been fixed in this CL.

Fixes #67042.

Cq-Include-Trybots: luci.golang.try:gotip-windows-amd64-longtest
Change-Id: Id6f7a0e1e0f34a72216fa9d00c5bf36f5a994219
Reviewed-on: https://go-review.googlesource.com/c/go/+/664055
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

src/internal/poll/sendfile_windows.go

@@ -10,78 +10,80 @@ import (
 )
 
 // SendFile wraps the TransmitFile call.
-func SendFile(fd *FD, src syscall.Handle, n int64) (written int64, err error) {
+func SendFile(fd *FD, src syscall.Handle, size int64) (written int64, err error, handled bool) {
 	defer func() {
 		TestHookDidSendFile(fd, 0, written, err, written > 0)
 	}()
 	if fd.kind == kindPipe {
 		// TransmitFile does not work with pipes
-		return 0, syscall.ESPIPE
+		return 0, syscall.ESPIPE, false
 	}
 	if ft, _ := syscall.GetFileType(src); ft == syscall.FILE_TYPE_PIPE {
-		return 0, syscall.ESPIPE
+		return 0, syscall.ESPIPE, false
 	}
 
 	if err := fd.writeLock(); err != nil {
-		return 0, err
+		return 0, err, false
 	}
 	defer fd.writeUnlock()
 
-	o := &fd.wop
-	o.handle = src
-
-	// TODO(brainman): skip calling syscall.Seek if OS allows it
-	curpos, err := syscall.Seek(o.handle, 0, io.SeekCurrent)
+	// Get the file size so we don't read past the end of the file.
+	var fi syscall.ByHandleFileInformation
+	if err := syscall.GetFileInformationByHandle(src, &fi); err != nil {
+		return 0, err, false
+	}
+	fileSize := int64(fi.FileSizeHigh)<<32 + int64(fi.FileSizeLow)
+	startpos, err := syscall.Seek(src, 0, io.SeekCurrent)
 	if err != nil {
-		return 0, err
+		return 0, err, false
+	}
+	maxSize := fileSize - startpos
+	if size <= 0 {
+		size = maxSize
+	} else {
+		size = min(size, maxSize)
 	}
 
-	if n <= 0 { // We don't know the size of the file so infer it.
-		// Find the number of bytes offset from curpos until the end of the file.
-		n, err = syscall.Seek(o.handle, -curpos, io.SeekEnd)
-		if err != nil {
-			return
+	defer func() {
+		if written > 0 {
+			// Some versions of Windows (Windows 10 1803) do not set
+			// file position after TransmitFile completes.
+			// So just use Seek to set file position.
+			_, serr := syscall.Seek(src, startpos+written, io.SeekStart)
+			if err != nil {
+				err = serr
+			}
 		}
-		// Now seek back to the original position.
-		if _, err = syscall.Seek(o.handle, curpos, io.SeekStart); err != nil {
-			return
-		}
-	}
+	}()
 
 	// TransmitFile can be invoked in one call with at most
 	// 2,147,483,646 bytes: the maximum value for a 32-bit integer minus 1.
 	// See https://docs.microsoft.com/en-us/windows/win32/api/mswsock/nf-mswsock-transmitfile
 	const maxChunkSizePerCall = int64(0x7fffffff - 1)
 
-	for n > 0 {
+	o := &fd.wop
+	o.handle = src
+	for size > 0 {
 		chunkSize := maxChunkSizePerCall
-		if chunkSize > n {
-			chunkSize = n
+		if chunkSize > size {
+			chunkSize = size
 		}
 
-		o.o.Offset = uint32(curpos)
-		o.o.OffsetHigh = uint32(curpos >> 32)
+		off := startpos + written
+		o.o.Offset = uint32(off)
+		o.o.OffsetHigh = uint32(off >> 32)
 
-		nw, err := execIO(o, func(o *operation) error {
+		n, err := execIO(o, func(o *operation) error {
 			o.qty = uint32(chunkSize)
 			return syscall.TransmitFile(o.fd.Sysfd, o.handle, o.qty, 0, &o.o, nil, syscall.TF_WRITE_BEHIND)
 		})
 		if err != nil {
-			return written, err
+			return written, err, written > 0
 		}
 
-		curpos += int64(nw)
-
-		// Some versions of Windows (Windows 10 1803) do not set
-		// file position after TransmitFile completes.
-		// So just use Seek to set file position.
-		if _, err = syscall.Seek(o.handle, curpos, io.SeekStart); err != nil {
-			return written, err
-		}
-
-		n -= int64(nw)
-		written += int64(nw)
+		size -= int64(n)
+		written += int64(n)
 	}
 
-	return
+	return written, nil, written > 0
 }

src/net/sendfile_test.go

@@ -126,23 +126,16 @@ func testSendfile(t *testing.T, filePath, fileHash string, size, limit int64) {
 			// Return file data using io.Copy, which should use
 			// sendFile if available.
 			var sbytes int64
-			switch runtime.GOOS {
-			case "windows":
-				// Windows is not using sendfile for some reason:
-				// https://go.dev/issue/67042
-				sbytes, err = io.Copy(conn, f)
-			default:
-				expectSendfile(t, conn, func() {
-					if limit > 0 {
-						sbytes, err = io.CopyN(conn, f, limit)
-						if err == io.EOF && limit > size {
-							err = nil
-						}
-					} else {
-						sbytes, err = io.Copy(conn, f)
+			expectSendfile(t, conn, func() {
+				if limit > 0 {
+					sbytes, err = io.CopyN(conn, f, limit)
+					if err == io.EOF && limit > size {
+						err = nil
 					}
-				})
-			}
+				} else {
+					sbytes, err = io.Copy(conn, f)
+				}
+			})
 			if err != nil {
 				errc <- err
 				return

src/net/sendfile_windows.go

@@ -7,43 +7,55 @@ package net
 import (
 	"internal/poll"
 	"io"
-	"os"
 	"syscall"
 )
 
 const supportsSendfile = true
 
-// sendFile copies the contents of r to c using the TransmitFile
+// TODO: deduplicate this file with sendfile_linux.go and sendfile_unix_alt.go.
+
+// sendFile copies the contents of r to c using the sendfile
 // system call to minimize copies.
 //
-// if handled == true, sendFile returns the number of bytes copied and any
-// non-EOF error.
+// if handled == true, sendFile returns the number (potentially zero) of bytes
+// copied and any non-EOF error.
 //
 // if handled == false, sendFile performed no work.
-func sendFile(fd *netFD, r io.Reader) (written int64, err error, handled bool) {
-	var n int64 = 0 // by default, copy until EOF.
+func sendFile(c *netFD, r io.Reader) (written int64, err error, handled bool) {
+	var remain int64 = 0 // by default, copy until EOF.
 
 	lr, ok := r.(*io.LimitedReader)
 	if ok {
-		n, r = lr.N, lr.R
-		if n <= 0 {
+		remain, r = lr.N, lr.R
+		if remain <= 0 {
 			return 0, nil, true
 		}
 	}
 
-	f, ok := r.(*os.File)
+	// r might be an *os.File or an os.fileWithoutWriteTo.
+	// Type assert to an interface rather than *os.File directly to handle the latter case.
+	f, ok := r.(syscall.Conn)
 	if !ok {
 		return 0, nil, false
 	}
 
-	written, err = poll.SendFile(&fd.pfd, syscall.Handle(f.Fd()), n)
+	sc, err := f.SyscallConn()
 	if err != nil {
-		err = wrapSyscallError("transmitfile", err)
+		return 0, nil, false
 	}
 
-	// If any byte was copied, regardless of any error
-	// encountered mid-way, handled must be set to true.
-	handled = written > 0
+	var werr error
+	err = sc.Read(func(fd uintptr) bool {
+		written, werr, handled = poll.SendFile(&c.pfd, syscall.Handle(fd), remain)
+		return true
+	})
+	if err == nil {
+		err = werr
+	}
 
-	return
+	if lr != nil {
+		lr.N = remain - written
+	}
+
+	return written, wrapSyscallError("sendfile", err), handled
 }