rpc: Avoid permanent cs_main lock in getblockheader

Author: promag

src/rpc/blockchain.cpp

@@ -747,15 +747,20 @@ static UniValue getblockheader(const JSONRPCRequest& request)
             + HelpExampleRpc("getblockheader", "\"00000000c937983704a73af28acdec37b049d214adbda81d7e2a3dd146f6ed09\"")
         );
 
-    LOCK(cs_main);
-
     uint256 hash(ParseHashV(request.params[0], "hash"));
 
     bool fVerbose = true;
     if (!request.params[1].isNull())
         fVerbose = request.params[1].get_bool();
 
-    const CBlockIndex* pblockindex = LookupBlockIndex(hash);
+    const CBlockIndex* pblockindex;
+    const CBlockIndex* tip;
+    {
+        LOCK(cs_main);
+        pblockindex = LookupBlockIndex(hash);
+        tip = chainActive.Tip();
+    }
+
     if (!pblockindex) {
         throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Block not found");
     }
@@ -768,7 +773,7 @@ static UniValue getblockheader(const JSONRPCRequest& request)
         return strHex;
     }
 
-    return blockheaderToJSON(chainActive.Tip(), pblockindex);
+    return blockheaderToJSON(tip, pblockindex);
 }
 
 static CBlock GetBlockChecked(const CBlockIndex* pblockindex)