index: Don't commit a best block before indexing it during sync

Committing a block prior to indexing would leave the index database
in an inconsistent state until it is indexed, which could corrupt the
index in case of a unclean shutdown. Thus commit its predecessor.

Co-authored-by: Luke Dashjr <[email protected]>

Author: mzumsande

src/index/base.cpp

@@ -168,7 +168,7 @@ void BaseIndex::ThreadSync()
             }
 
             if (last_locator_write_time + SYNC_LOCATOR_WRITE_INTERVAL < current_time) {
-                SetBestBlockIndex(pindex);
+                SetBestBlockIndex(pindex->pprev);
                 last_locator_write_time = current_time;
                 // No need to handle errors in Commit. See rationale above.
                 Commit();