rpc: define and use new RPC_LIMIT_EXCEEDED error code

The server isn't running out of memory when the private broadcast
transaction queue is full. Add a new RPC_LIMIT_EXCEEDED code that
can be used whenever a resource is bound and currently at capacity.

Github-Pull: #35678
Rebased-From: 82a02a2a22

src/rpc/protocol.h

@@ -49,6 +49,7 @@ enum RPCErrorCode
     RPC_VERIFY_ALREADY_IN_UTXO_SET  = -27, //!< Transaction already in utxo set
     RPC_IN_WARMUP                   = -28, //!< Client still warming up
     RPC_METHOD_DEPRECATED           = -32, //!< RPC method is deprecated
+    RPC_LIMIT_EXCEEDED              = -37, //!< A bounded resource is currently at capacity
 
     //! Aliases for backward compatibility
     RPC_TRANSACTION_ERROR           = RPC_VERIFY_ERROR,

src/rpc/util.cpp

@@ -396,7 +396,7 @@ RPCErrorCode RPCErrorFromTransactionError(TransactionError terr)
         case TransactionError::ALREADY_IN_UTXO_SET:
             return RPC_VERIFY_ALREADY_IN_UTXO_SET;
         case TransactionError::PRIVATE_BROADCAST_FULL:
-            return RPC_OUT_OF_MEMORY;
+            return RPC_LIMIT_EXCEEDED;
         default: break;
     }
     return RPC_TRANSACTION_ERROR;

test/functional/p2p_private_broadcast_cap.py

@@ -71,7 +71,7 @@ class PrivateBroadcastCapTest(BitcoinTestFramework):
         # queue is left unchanged (nothing evicted to make room).
         self.log.info(f"Submitting {OVER_CAP} more; each should be rejected (queue full)")
         for child in children[MAX_TRANSACTIONS:]:
-            assert_raises_rpc_error(-7, "Private broadcast queue is full",
+            assert_raises_rpc_error(-37, "Private broadcast queue is full",
                                     node.sendrawtransaction, child["hex"])
 
         assert_equal(pbinfo["transactions"], node.getprivatebroadcastinfo()["transactions"])