cmd/link: call syscall.FlushFileBuffers on outbuf Unmap

In the windows version of OutBuf.munmap, call syscall.FlushFileBuffers
after the call to syscall.FlushViewOfFile, on the theory that this
will help flush all associated meta-data for the file the linker is
writing.

Updates golang#44817.

Change-Id: Ibff7d05008a91eeed7634d2760153851e15e1c18
Reviewed-on: https://go-review.googlesource.com/c/go/+/406814
Reviewed-by: Cherry Mui <[email protected]>
Reviewed-by: Alex Brainman <[email protected]>

src/cmd/link/internal/ld/outbuf_windows.go

@@ -59,6 +59,18 @@ func (out *OutBuf) munmap() {
 	if err != nil {
 		Exitf("FlushViewOfFile failed: %v", err)
 	}
+	// Issue 44817: apparently the call below may be needed (according
+	// to the Windows docs) in addition to the FlushViewOfFile call
+	// above, " ... to flush all the dirty pages plus the metadata for
+	// the file and ensure that they are physically written to disk".
+	// Windows DOC links:
+	//
+	// https://docs.microsoft.com/en-us/windows/win32/api/memoryapi/nf-memoryapi-flushviewoffile
+	// https://docs.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-flushfilebuffers
+	err = syscall.FlushFileBuffers(syscall.Handle(out.f.Fd()))
+	if err != nil {
+		Exitf("FlushFileBuffers failed: %v", err)
+	}
 	err = syscall.UnmapViewOfFile(uintptr(unsafe.Pointer(&out.buf[0])))
 	out.buf = nil
 	if err != nil {