Fix windows mmap on zero length file.

pkg/mmap/mmap_windows.go

@@ -8,26 +8,29 @@ import (
 
 // Map memory-maps a file.
 func Map(path string, sz int64) ([]byte, error) {
+	fi, err := os.Stat(path)
+	if err != nil {
+		return nil, err
+	}
+
+	// Truncate file to size if too small.
+	if fi.Size() < sz {
+		if err := os.Truncate(path, sz); err != nil {
+			return nil, err
+		}
+	} else {
+		sz = fi.Size()
+	}
+	if sz == 0 {
+		return nil, nil
+	}
+
 	f, err := os.Open(path)
 	if err != nil {
 		return nil, err
 	}
 	defer f.Close()
 
-	fi, err := f.Stat()
-	if err != nil {
-		return nil, err
-	}
-
-	// Use file size if map size is not passed in.
-	// TODO(edd): test.
-	// if sz == 0 {
-	// }
-	sz = fi.Size()
-	if fi.Size() == 0 {
-		return nil, nil
-	}
-
 	lo, hi := uint32(sz), uint32(sz>>32)
 	fmap, err := syscall.CreateFileMapping(syscall.Handle(f.Fd()), nil, syscall.PAGE_READONLY, hi, lo, nil)
 	if err != nil {