Fix TT write in MultiPV case.

AndyGrant · vondele · commit 2e1369d0302a · 2020-02-25T21:10:10.000+01:00
fixes an error reported earlier as #2404 by @AndyGrant. MultiPV at root shouldn't write to the TT for later lines, as that is neither the eval nor the bestmove for that position. Fixing this error doesn't matter for playing games (http://tests.stockfishchess.org/tests/view/5dcdbd810ebc590256324a11). However, it can lead to wrong mate announcements as reported by @uriblass. In particular the following testcase gives wrong results for the second search, prior to this patch: ``` setoption name MultiPV value 2 position fen 5R2/2kB2p1/p2bR3/8/3p1B2/8/PPP5/2K5 b - - 0 49 go depth 40 position fen 2B2R2/3r2p1/p1kbR3/8/3p1B2/8/PPP5/2K5 b - - 8 48 go depth 40 ``` fixes #2561 closes #2562 Only affects MultiPV search. Bench: 4697493
diff --git a/src/search.cpp b/src/search.cpp
@@ -1339,7 +1339,7 @@ namespace {
     if (PvNode)
         bestValue = std::min(bestValue, maxValue);
 
-    if (!excludedMove)
+    if (!excludedMove && !(rootNode && thisThread->pvIdx))
         tte->save(posKey, value_to_tt(bestValue, ss->ply), ttPv,
                   bestValue >= beta ? BOUND_LOWER :
                   PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
