net: handle >=2GiB files with sendfile on Windows

CL 187037 applied a fix to handle the case where
files larger than 2GiB were not being sendfile-d,
in one shot, rejecting any files whose size was
larger than the 2GiB.

This CL allows files that are larger than limit
by SendFile-ing in chunks of upto 2GiB per chunk.

The test has been excluded as testing with 3GB
requires creating a local file, flushing it
and then doing sendfile which takes a while
and could cause flakes on computers without capacity,
but the test can be retroactively accessed at:
https://go-review.googlesource.com/c/go/+/192518/8/src/net/sendfile_windows_test.go

Fixes #33193.

Change-Id: If57c25bc289aec82b748890ac1ac4f55798d6a5e
Reviewed-on: https://go-review.googlesource.com/c/go/+/192518
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

src/net/sendfile_windows.go

@@ -18,10 +18,8 @@ import (
 // non-EOF error.
 //
 // if handled == false, sendFile performed no work.
-//
-// Note that sendfile for windows does not support >2GB file.
 func sendFile(fd *netFD, r io.Reader) (written int64, err error, handled bool) {
-	var n int64 = 0 // by default, copy until EOF
+	var n int64 = 0 // by default, copy until EOF.
 
 	lr, ok := r.(*io.LimitedReader)
 	if ok {
@@ -29,26 +27,53 @@ func sendFile(fd *netFD, r io.Reader) (written int64, err error, handled bool) {
 		if n <= 0 {
 			return 0, nil, true
 		}
-		// 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 maxSendBytes = 0x7fffffff - 1
-		if n > maxSendBytes {
-			return 0, nil, false
-		}
 	}
+
 	f, ok := r.(*os.File)
 	if !ok {
 		return 0, nil, false
 	}
 
-	done, err := poll.SendFile(&fd.pfd, syscall.Handle(f.Fd()), n)
+	// 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)
+
+	switch {
+	case n <= maxChunkSizePerCall:
+		// The file is within sendfile's limits.
+		written, err = doSendFile(fd, lr, f, n)
+
+	default:
+		// Now invoke doSendFile on the file in chunks of upto 2GiB per chunk.
+		for lr.N > 0 { // lr.N is decremented in every successful invocation of doSendFile.
+			chunkSize := maxChunkSizePerCall
+			if chunkSize > lr.N {
+				chunkSize = lr.N
+			}
+			var nw int64
+			nw, err = doSendFile(fd, lr, f, chunkSize)
+			if err != nil {
+				break
+			}
+			written += nw
+		}
+	}
+
+	// If any byte was copied, regardless of any error
+	// encountered mid-way, handled must be set to true.
+	return written, err, written > 0
+}
 
+// doSendFile is a helper to invoke poll.SendFile.
+// It will decrement lr.N by the number of written bytes.
+func doSendFile(fd *netFD, lr *io.LimitedReader, f *os.File, remain int64) (written int64, err error) {
+	done, err := poll.SendFile(&fd.pfd, syscall.Handle(f.Fd()), remain)
 	if err != nil {
-		return 0, wrapSyscallError("transmitfile", err), false
+		return 0, wrapSyscallError("transmitfile", err)
 	}
 	if lr != nil {
 		lr.N -= int64(done)
 	}
-	return int64(done), nil, true
+	return int64(done), nil
 }