restorer: Fix race condition in partialFile.WriteAt

The restorer can issue multiple calls to WriteAt in parallel. This can
result in unexpected orderings of the Truncate and WriteAt calls and
sometimes too short restored files.

internal/restorer/sparsewrite.go

@@ -13,7 +13,6 @@ func (f *partialFile) WriteAt(p []byte, offset int64) (n int, err error) {
 	}
 
 	n = len(p)
-	end := offset + int64(n)
 
 	// Skip the longest all-zero prefix of p.
 	// If it's long enough, we can punch a hole in the file.
@@ -22,26 +21,16 @@ func (f *partialFile) WriteAt(p []byte, offset int64) (n int, err error) {
 	offset += int64(skipped)
 
 	switch {
-	case len(p) == 0 && end > f.size:
-		// We need to do a Truncate, as WriteAt with length-0 input
-		// doesn't actually extend the file.
-		err = f.Truncate(end)
-		if err != nil {
-			return 0, err
-		}
-
 	case len(p) == 0:
 		// All zeros, file already big enough. A previous WriteAt or
 		// Truncate will have produced the zeros in f.File.
 
 	default:
-		n, err = f.File.WriteAt(p, offset)
+		var n2 int
+		n2, err = f.File.WriteAt(p, offset)
+		n = skipped + n2
 	}
 
-	end = offset + int64(n)
-	if end > f.size {
-		f.size = end
-	}
 	return n, err
 }