lib/scanner: When scanning a file, stick to the size given by Lstat (fixes #3440)

Otherwise if the file grows during scanning the block list will be out
of sync with the stated size and things get confused. We could fixup the
size afterwards based on the block list, but then we might see other
inconsistencies as the mtime should have changed to reflect the new size
etc. Better stick to the original state and let the next scan pick up
the change.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3442

lib/scanner/blocks.go

@@ -31,7 +31,8 @@ func Blocks(r io.Reader, blocksize int, sizehint int64, counter Counter) ([]prot
 
 	if sizehint > 0 {
 		// Allocate contiguous blocks for the BlockInfo structures and their
-		// hashes once and for all.
+		// hashes once and for all, and stick to the specified size.
+		r = io.LimitReader(r, sizehint)
 		numBlocks := int(sizehint / int64(blocksize))
 		blocks = make([]protocol.BlockInfo, 0, numBlocks)
 		hashes = make([]byte, 0, hashLength*numBlocks)