Use a block that is 8 blocks in the past for SelectQuorumForSigning

Otherwise nodes might already fail to verify sigs when just 1 block away
from the tip.

Author: codablock

src/llmq/quorums_signing.cpp

@@ -565,7 +565,7 @@ CQuorumCPtr CSigningManager::SelectQuorumForSigning(Consensus::LLMQType llmqType
         if (signHeight > chainActive.Height()) {
             return nullptr;
         }
-        startBlock = chainActive[signHeight]->GetBlockHash();
+        startBlock = chainActive[signHeight - SIGN_HEIGHT_OFFSET]->GetBlockHash();
     }
 
     auto quorums = quorumManager->ScanQuorums(llmqType, startBlock, poolSize);

src/llmq/quorums_signing.h

@@ -104,6 +104,11 @@ class CSigningManager
     friend class CSigSharesManager;
     static const int64_t DEFAULT_MAX_RECOVERED_SIGS_AGE = 60 * 60 * 24 * 7; // keep them for a week
 
+    // when selecting a quorum for signing and verification, we use CQuorumManager::SelectQuorum with this offset as
+    // starting height for scanning. This is because otherwise the resulting signatures would not be verifiable by nodes
+    // which are not 100% at the chain tip.
+    static const int SIGN_HEIGHT_OFFSET = 8;
+
 private:
     CCriticalSection cs;