perf(rpc): rebuild the block template when the mempool changes

.changes/unreleased/zebra-rpc-Added-20260901-141500.yaml

@@ -1,6 +1,7 @@
 project: zebra-rpc
 kind: Added
-body: 'Method `RpcImpl::spawn_block_template_updater()`, which spawns a task that keeps a block
-  template precomputed for the current chain tip, and returns `None` when mining isn''t configured
+body: 'Method `RpcImpl::spawn_block_template_updater()`, which takes a
+  `MempoolTxSubscriber` and spawns a task that keeps a block template precomputed for the current
+  chain tip, and returns `None` when mining isn''t configured
   ([#11370](https://github.com/ZcashFoundation/zebra/issues/11370)).'
 time: 2026-09-01T14:15:00.000000000Z

.changes/unreleased/zebra-rpc-Changed-20260901-141501.yaml

@@ -4,7 +4,8 @@ body: 'The `getblocktemplate` RPC answers from the template precomputed by
   `RpcImpl::spawn_block_template_updater()`, so a call validates the chain tip against the state
   instead of reading the mempool, selecting transactions, and building a coinbase transaction. A
   long polling call waits on that template, and returns when the task publishes a new one, when the
-  chain tip changes, or when `max_time` is reached. A precomputed template''s transactions can be a
-  few seconds behind the mempool, but it always extends the tip the state has committed
+  chain tip changes, or when `max_time` is reached. The task rebuilds when the chain tip changes and
+  when the mempool changes, rather than on a fixed interval, so a precomputed template''s
+  transactions trail the mempool by a rebuild, and it always extends the tip the state has committed
   ([#11370](https://github.com/ZcashFoundation/zebra/issues/11370)).'
 time: 2026-09-01T14:15:01.000000000Z

.changes/unreleased/zebrad-Changed-20260901-141502.yaml

@@ -2,8 +2,8 @@ project: zebrad
 kind: Changed
 body: 'The `getblocktemplate` RPC returns a block template that Zebra keeps ready for the current
   chain tip, rather than assembling one while the miner waits. Zebra keeps one ready whenever a
-  miner address is configured and either the RPC server or the internal miner is enabled, refreshing
-  it on every chain tip change and every few seconds, so a template can be a few seconds behind the
-  mempool but is never behind the chain
+  miner address is configured and either the RPC server or the internal miner is enabled, rebuilding
+  it when the chain tip changes and when the mempool changes, instead of every few seconds, so a new
+  transaction reaches miners in the next rebuild and a template is never behind the chain
   ([#11370](https://github.com/ZcashFoundation/zebra/issues/11370)).'
 time: 2026-09-01T14:15:02.000000000Z

zebra-rpc/src/methods.rs

@@ -85,7 +85,7 @@ use zebra_consensus::{
     funding_stream_address, router::service_trait::BlockVerifierService, RouterError,
 };
 use zebra_network::{address_book_peers::AddressBookPeers, types::PeerServices, PeerSocketAddr};
-use zebra_node_services::mempool::{self, CreatedOrSpent, MempoolService};
+use zebra_node_services::mempool::{self, CreatedOrSpent, MempoolService, MempoolTxSubscriber};
 use zebra_state::{
     AnyTx, HashOrHeight, OutputLocation, ReadRequest, ReadResponse, ReadState as ReadStateService,
     State as StateService, TransactionLocation,
@@ -1008,8 +1008,14 @@ where
     /// `getblocktemplate` calls don't have to read the state and the mempool, select transactions,
     /// and build a coinbase transaction.
     ///
+    /// The task rebuilds when the chain tip changes and when `mempool_change` reports a change
+    /// that affects the template, so a new transaction reaches miners without waiting for a timer.
+    ///
     /// Returns `None` if mining isn't configured.
-    pub fn spawn_block_template_updater(&self) -> Option<JoinHandle<()>> {
+    pub fn spawn_block_template_updater(
+        &self,
+        mempool_change: MempoolTxSubscriber,
+    ) -> Option<JoinHandle<()>> {
         let miner_params = self.gbt.miner_params()?.clone();
         let template_cache = self.gbt.template_cache()?.clone();
 
@@ -1020,6 +1026,7 @@ where
                 self.gbt.coinbase_cache(),
                 template_cache,
                 self.mempool.clone(),
+                mempool_change.subscribe(),
                 self.read_state.clone(),
                 self.latest_chain_tip.clone(),
                 self.gbt.sync_status(),