fix: make SelectQuorumForSigning resolve strictly via active chain under cs_main

Always derive pindexStart from the authoritative active chain (with
SIGN_HEIGHT_OFFSET) for both signing and verification. This removes
cached-view variability that led to missed ChainLocks in rotation tests,
while keeping other paths cs_main-free.

Author: PastaPastaPasta

src/llmq/quorums.cpp

@@ -1325,54 +1325,17 @@ CQuorumCPtr SelectQuorumForSigning(const Consensus::LLMQParams& llmq_params, con
     size_t poolSize = llmq_params.signingActiveQuorumCount;
 
     CBlockIndex* pindexStart;
-    // If caller provided an explicit signHeight, always resolve relative to the active chain
-    // to guarantee correctness during rapid tip changes and rotations.
-    if (signHeight != -1) {
-        LOCK(::cs_main);
-        int startBlockHeight = signHeight - signOffset;
-        if (startBlockHeight > active_chain.Height() || startBlockHeight < 0) {
-            return {};
-        }
-        pindexStart = active_chain[startBlockHeight];
-    } else {
-        // No explicit height: prefer cs_main-free path via active chain view
-        auto view = qman.GetActiveChainView();
-        if (view.tip) {
-            signHeight = view.height;
-            int startBlockHeight = signHeight - signOffset;
-            if (startBlockHeight < 0) {
-                return {};
-            }
-            if (startBlockHeight <= view.height) {
-                const CBlockIndex* ancestor = qman.FindAncestorFast(view, startBlockHeight);
-                if (!ancestor) {
-                    // Fallback to cs_main if cache missed (e.g. during reorg/initialization)
-                    LOCK(::cs_main);
-                    if (startBlockHeight > active_chain.Height()) {
-                        return {};
-                    }
-                    pindexStart = active_chain[startBlockHeight];
-                } else {
-                    pindexStart = const_cast<CBlockIndex*>(ancestor);
-                }
-            } else {
-                // View is behind requested height; fallback to cs_main
-                LOCK(::cs_main);
-                if (startBlockHeight > active_chain.Height()) {
-                    return {};
-                }
-                pindexStart = active_chain[startBlockHeight];
-            }
-        } else {
-        LOCK(::cs_main);
-            signHeight = active_chain.Height();
-            int startBlockHeight = signHeight - signOffset;
-            if (startBlockHeight > active_chain.Height() || startBlockHeight < 0) {
-                return {};
-            }
-            pindexStart = active_chain[startBlockHeight];
-        }
+    // Resolve strictly relative to the authoritative active chain to avoid any
+    // cross-tip inconsistencies during rotations or rapid tip changes.
+    LOCK(::cs_main);
+    if (signHeight == -1) {
+        signHeight = active_chain.Height();
+    }
+    int startBlockHeight = signHeight - signOffset;
+    if (startBlockHeight > active_chain.Height() || startBlockHeight < 0) {
+        return {};
     }
+    pindexStart = active_chain[startBlockHeight];
 
     if (IsQuorumRotationEnabled(llmq_params, pindexStart)) {
         auto quorums = qman.ScanQuorums(llmq_params.type, pindexStart, poolSize);