Add waitNext() to BlockTemplate interface

src/interfaces/mining.h

@@ -56,6 +56,17 @@ class BlockTemplate
      * @returns if the block was processed, independent of block validity
      */
     virtual bool submitSolution(uint32_t version, uint32_t timestamp, uint32_t nonce, CMutableTransaction coinbase) = 0;
+
+    /**
+     * Waits for fees in the next block to rise, a new tip or the timeout.
+     *
+     * @param[in] fee_threshold By how much total fees for the next block should rise.
+     *                          Default is to not monitor fee changes and only wait for a new chaintip.
+     * @param[in] timeout       How long to wait. Default is forever.
+     *
+     * @returns a new BlockTemplate or nullptr if the timeout occurs
+     */
+    virtual std::unique_ptr<BlockTemplate> waitNext(CAmount fee_threshold = MAX_MONEY, MillisecondsDouble timeout = MillisecondsDouble::max()) = 0;
 };
 
 //! Interface giving clients (RPC, Stratum v2 Template Provider in the future)

src/ipc/capnp/mining.capnp

@@ -33,6 +33,7 @@ interface BlockTemplate $Proxy.wrap("interfaces::BlockTemplate") {
     getWitnessCommitmentIndex @6 (context: Proxy.Context) -> (result: Int32);
     getCoinbaseMerklePath @7 (context: Proxy.Context) -> (result: List(Data));
     submitSolution@8 (context: Proxy.Context, version: UInt32, timestamp: UInt32, nonce: UInt32, coinbase :Data) -> (result: Bool);
+    waitNext @9 (context: Proxy.Context, feeThreshold: Int64, timeout: Float64) -> (result: BlockTemplate);
 }
 
 struct BlockCreateOptions $Proxy.wrap("node::BlockCreateOptions") {

src/node/interfaces.cpp

@@ -871,7 +871,7 @@ class ChainImpl : public Chain
 class BlockTemplateImpl : public BlockTemplate
 {
 public:
-    explicit BlockTemplateImpl(std::unique_ptr<CBlockTemplate> block_template, NodeContext& node) : m_block_template(std::move(block_template)), m_node(node)
+    explicit BlockTemplateImpl(CScript script_pub_key, BlockAssembler::Options assemble_options, std::unique_ptr<CBlockTemplate> block_template, NodeContext& node) : m_script_pub_key(script_pub_key), m_assemble_options(std::move(assemble_options)), m_block_template(std::move(block_template)), m_node(node)
     {
         assert(m_block_template);
     }
@@ -938,9 +938,71 @@ class BlockTemplateImpl : public BlockTemplate
         return chainman().ProcessNewBlock(block_ptr, /*force_processing=*/true, /*min_pow_checked=*/true, /*new_block=*/nullptr);
     }
 
+    std::unique_ptr<BlockTemplate> waitNext(CAmount fee_threshold, MillisecondsDouble timeout) override
+    {
+        if (timeout > std::chrono::years{100}) timeout = std::chrono::years{100}; // Upper bound to avoid UB in std::chrono
+
+        CAmount current_fees = 0;
+        if (fee_threshold < MAX_MONEY) {
+            for (CAmount fee : m_block_template->vTxFees) {
+                // Skip coinbase
+                if (fee < 0) continue;
+                current_fees += fee;
+            }
+        }
+
+        // Alternate waiting for a new tip and checking if fees have risen.
+        // The latter check is expensive so we only run it once per second.
+        auto now{std::chrono::steady_clock::now()};
+        const auto deadline = now + timeout;
+        const MillisecondsDouble tick{1000};
+        std::unique_ptr<BlockTemplateImpl> block_template;
+
+        while (now < deadline) {
+            {
+                WAIT_LOCK(notifications().m_tip_block_mutex, lock);
+                notifications().m_tip_block_cv.wait_until(lock, std::min(now + tick, deadline), [&]() EXCLUSIVE_LOCKS_REQUIRED(notifications().m_tip_block_mutex) {
+                    return notifications().m_tip_block != m_block_template->block.hashPrevBlock || chainman().m_interrupt;
+                });
+            }
+
+            if (chainman().m_interrupt) return nullptr;
+
+            // Must release m_tip_block_mutex before locking cs_main, to avoid deadlocks.
+            LOCK(::cs_main);
+
+            // The only way to determine if fees increased compared to the previous template,
+            // is to generate a fresh template. Cluster Mempool may allow for a more efficient
+            // way to determine how much (approximate) fees for the next block increased.
+            block_template = std::make_unique<BlockTemplateImpl>(m_script_pub_key, m_assemble_options, BlockAssembler{chainman().ActiveChainstate(), context()->mempool.get(), m_assemble_options}.CreateNewBlock(m_script_pub_key), m_node);
+
+            // If the tip changed, return the new template regardless of its fees.
+            if (block_template->m_block_template->block.hashPrevBlock != m_block_template->block.hashPrevBlock) {
+                return block_template;
+            }
+
+            CAmount new_fees = 0;
+            for (CAmount fee : block_template->m_block_template->vTxFees) {
+                // Skip coinbase
+                if (fee < 0) continue;
+                new_fees += fee;
+                if (new_fees >= current_fees + fee_threshold) return block_template;
+            }
+            //
+            block_template.reset();
+        }
+
+        return block_template;
+    }
+
+    const CScript m_script_pub_key;
+    const BlockAssembler::Options m_assemble_options;
+
     const std::unique_ptr<CBlockTemplate> m_block_template;
 
+    NodeContext* context() { return &m_node; }
     ChainstateManager& chainman() { return *Assert(m_node.chainman); }
+    KernelNotifications& notifications() { return *Assert(m_node.notifications); }
     NodeContext& m_node;
 };
 
@@ -1008,7 +1070,7 @@ class MinerImpl : public Mining
     {
         BlockAssembler::Options assemble_options{options};
         ApplyArgsManOptions(*Assert(m_node.args), assemble_options);
-        return std::make_unique<BlockTemplateImpl>(BlockAssembler{chainman().ActiveChainstate(), context()->mempool.get(), assemble_options}.CreateNewBlock(script_pub_key), m_node);
+        return std::make_unique<BlockTemplateImpl>(script_pub_key, assemble_options, BlockAssembler{chainman().ActiveChainstate(), context()->mempool.get(), assemble_options}.CreateNewBlock(script_pub_key), m_node);
     }
 
     NodeContext* context() override { return &m_node; }