From 53c911860cc07e9fa7270b3ccae446676a0a007f Mon Sep 17 00:00:00 2001
From: Sjors Provoost <sjors@sprovoost.nl>
Date: Mon, 12 Aug 2024 15:55:44 +0200
Subject: [PATCH] rpc: add optional blockhash to waitfornewblock

---
 doc/release-30635.md              |  4 ++++
 src/rpc/blockchain.cpp            | 16 ++++++++++++++++
 test/functional/rpc_blockchain.py |  2 +-
 3 files changed, 21 insertions(+), 1 deletion(-)
 create mode 100644 doc/release-30635.md

diff --git a/doc/release-30635.md b/doc/release-30635.md
new file mode 100644
index 00000000000000..085112dcb27f82
--- /dev/null
+++ b/doc/release-30635.md
@@ -0,0 +1,4 @@
+Updated RPCs
+------------
+
+- The waitfornewblock RPC takes an optional `blockhash` argument. (#30635)
diff --git a/src/rpc/blockchain.cpp b/src/rpc/blockchain.cpp
index 9d7ad99f6387da..105e82dde53e23 100644
--- a/src/rpc/blockchain.cpp
+++ b/src/rpc/blockchain.cpp
@@ -259,6 +259,7 @@ static RPCHelpMan waitfornewblock()
                 "\nMake sure to use no RPC timeout (bitcoin-cli -rpcclienttimeout=0)",
                 {
                     {"timeout", RPCArg::Type::NUM, RPCArg::Default{0}, "Time in milliseconds to wait for a response. 0 indicates no timeout."},
+                    {"blockhash", RPCArg::Type::STR_HEX, RPCArg::Optional::OMITTED, "Last known block hash. Returns immediately if the tip is different."},
                 },
                 RPCResult{
                     RPCResult::Type::OBJ, "", "",
@@ -280,7 +281,22 @@ static RPCHelpMan waitfornewblock()
     NodeContext& node = EnsureAnyNodeContext(request.context);
     Mining& miner = EnsureMining(node);
 
+   // Use tip as reference block, unless a block hash was provided.
     auto block{CHECK_NONFATAL(miner.getTip()).value()};
+    if (!request.params[1].isNull()) {
+        uint256 hash(ParseHashV(request.params[1], "blockhash"));
+        {
+            ChainstateManager& chainman = EnsureAnyChainman(request.context);
+            LOCK(cs_main);
+            const CBlockIndex* block_index{chainman.m_blockman.LookupBlockIndex(hash)};
+            if (!block_index) {
+                throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Block not found");
+            }
+            block.hash = block_index->GetBlockHash();
+            block.height = block_index->nHeight;
+        }
+    }
+
     if (IsRPCRunning()) {
         block = timeout ? miner.waitTipChanged(block.hash, std::chrono::milliseconds(timeout)) : miner.waitTipChanged(block.hash);
     }
diff --git a/test/functional/rpc_blockchain.py b/test/functional/rpc_blockchain.py
index 82c67256717012..5fc166b54c0bb3 100755
--- a/test/functional/rpc_blockchain.py
+++ b/test/functional/rpc_blockchain.py
@@ -536,7 +536,7 @@ def _test_waitforblock(self):
         node.reconsiderblock(rollback_hash)
         # The chain has probably already been restored by the time reconsiderblock returns,
         # but poll anyway.
-        self.wait_until(lambda: node.waitfornewblock(timeout=100)['hash'] == current_hash)
+        self.wait_until(lambda: node.waitfornewblock(blockhash=rollback_header['previousblockhash'])['hash'] == current_hash)
 
     def _test_waitforblockheight(self):
         self.log.info("Test waitforblockheight")