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/fileswriter.go

@@ -24,8 +24,7 @@ type filesWriterBucket struct {
 
 type partialFile struct {
 	*os.File
-	size   int64 // File size, tracked for sparse writes (not on Windows).
-	users  int   // Reference count.
+	users  int // Reference count.
 	sparse bool
 }
 
@@ -64,24 +63,24 @@ func (w *filesWriter) writeToFile(path string, blob []byte, offset int64, create
 		}
 
 		wr := &partialFile{File: f, users: 1, sparse: sparse}
-		if createSize < 0 {
-			info, err := f.Stat()
-			if err != nil {
-				return nil, err
-			}
-			wr.size = info.Size()
-		}
 		bucket.files[path] = wr
 
-		if createSize >= 0 && !sparse {
-			err := preallocateFile(wr.File, createSize)
-			if err != nil {
-				// Just log the preallocate error but don't let it cause the restore process to fail.
-				// Preallocate might return an error if the filesystem (implementation) does not
-				// support preallocation or our parameters combination to the preallocate call
-				// This should yield a syscall.ENOTSUP error, but some other errors might also
-				// show up.
-				debug.Log("Failed to preallocate %v with size %v: %v", path, createSize, err)
+		if createSize >= 0 {
+			if sparse {
+				err = f.Truncate(createSize)
+				if err != nil {
+					return nil, err
+				}
+			} else {
+				err := preallocateFile(wr.File, createSize)
+				if err != nil {
+					// Just log the preallocate error but don't let it cause the restore process to fail.
+					// Preallocate might return an error if the filesystem (implementation) does not
+					// support preallocation or our parameters combination to the preallocate call
+					// This should yield a syscall.ENOTSUP error, but some other errors might also
+					// show up.
+					debug.Log("Failed to preallocate %v with size %v: %v", path, createSize, err)
+				}
 			}
 		}
 

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
 }