Add waitNext() to BlockTemplate interface

src/interfaces/mining.h

@@ -7,7 +7,7 @@
 
 #include <consensus/amount.h>       // for CAmount
 #include <interfaces/types.h>       // for BlockRef
-#include <node/types.h>             // for BlockCreateOptions
+#include <node/types.h>             // for BlockCreateOptions, BlockWaitOptions
 #include <primitives/block.h>       // for CBlock, CBlockHeader
 #include <primitives/transaction.h> // for CTransactionRef
 #include <stdint.h>                 // for int64_t
@@ -56,6 +56,16 @@ class BlockTemplate
      * @returns if the block was processed, independent of block validity
      */
     virtual bool submitSolution(uint32_t version, uint32_t timestamp, uint32_t nonce, CTransactionRef coinbase) = 0;
+
+    /**
+     * Waits for fees in the next block to rise, a new tip or the timeout.
+     *
+     * @param[in] options   Control the timeout (default forever) and by how much total fees
+     *                      for the next block should rise (default infinite).
+     *
+     * @returns a new BlockTemplate or nothing if the timeout occurs.
+     */
+    virtual std::unique_ptr<BlockTemplate> waitNext(const node::BlockWaitOptions options = {}) = 0;
 };
 
 //! Interface giving clients (RPC, Stratum v2 Template Provider in the future)

src/ipc/capnp/mining.capnp

@@ -31,6 +31,7 @@ interface BlockTemplate $Proxy.wrap("interfaces::BlockTemplate") {
     getWitnessCommitmentIndex @7 (context: Proxy.Context) -> (result: Int32);
     getCoinbaseMerklePath @8 (context: Proxy.Context) -> (result: List(Data));
     submitSolution @9 (context: Proxy.Context, version: UInt32, timestamp: UInt32, nonce: UInt32, coinbase :Data) -> (result: Bool);
+    waitNext @10 (context: Proxy.Context, options: BlockWaitOptions) -> (result: BlockTemplate);
 }
 
 struct BlockCreateOptions $Proxy.wrap("node::BlockCreateOptions") {
@@ -39,6 +40,11 @@ struct BlockCreateOptions $Proxy.wrap("node::BlockCreateOptions") {
     coinbaseOutputMaxAdditionalSigops @2 :UInt64 $Proxy.name("coinbase_output_max_additional_sigops");
 }
 
+struct BlockWaitOptions $Proxy.wrap("node::BlockWaitOptions") {
+    timeout @0 : Float64 $Proxy.name("timeout");
+    feeThreshold @1 : Int64 $Proxy.name("fee_threshold");
+}
+
 # Note: serialization of the BlockValidationState C++ type is somewhat fragile
 # and using the struct can be awkward. It would be good if testBlockValidity
 # method were changed to return validity information in a simpler format.

src/node/interfaces.cpp

@@ -81,6 +81,7 @@ using interfaces::Mining;
 using interfaces::Node;
 using interfaces::WalletLoader;
 using node::BlockAssembler;
+using node::BlockWaitOptions;
 using util::Join;
 
 namespace node {
@@ -871,7 +872,7 @@ 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(BlockAssembler::Options assemble_options, std::unique_ptr<CBlockTemplate> block_template, NodeContext& node) : m_assemble_options(std::move(assemble_options)), m_block_template(std::move(block_template)), m_node(node)
     {
         assert(m_block_template);
     }
@@ -936,9 +937,102 @@ class BlockTemplateImpl : public BlockTemplate
         return chainman().ProcessNewBlock(block_ptr, /*force_processing=*/true, /*min_pow_checked=*/true, /*new_block=*/nullptr);
     }
 
+    std::unique_ptr<BlockTemplate> waitNext(BlockWaitOptions options) override
+    {
+        // Delay calculating the current template fees, just in case a new block
+        // comes in before the next tick.
+        CAmount current_fees = -1;
+
+        // 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{NodeClock::now()};
+        const auto deadline = now + options.timeout;
+        const MillisecondsDouble tick{1000};
+
+        bool tip_changed{false};
+        // Helper to check if the tip has has changed, and also update tip_changed.
+        auto check_tip_changed = [this, &tip_changed]() EXCLUSIVE_LOCKS_REQUIRED(notifications().m_tip_block_mutex) {
+            if (tip_changed) return true;
+            AssertLockHeld(notifications().m_tip_block_mutex);
+            const auto tip_block{notifications().TipBlock()};
+            // This is an instance method on BlockTemplate and no template
+            // could have been generated before a tip exists.
+            Assume(tip_block);
+            tip_changed = notifications().TipBlock() != m_block_template->block.hashPrevBlock;
+            return tip_changed;
+        };
+
+        while (now <= deadline) {
+            {
+                WAIT_LOCK(notifications().m_tip_block_mutex, lock);
+                // First make sure the tip hasn't already changed:
+                check_tip_changed() || notifications().m_tip_block_cv.wait_until(lock, std::min(now + tick, deadline), [&]() EXCLUSIVE_LOCKS_REQUIRED(notifications().m_tip_block_mutex) {
+                    return check_tip_changed() || 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);
+
+            /**
+             * We determine if fees increased compared to the previous template by generating
+             * a fresh template. There may be more efficient ways to determine how much
+             * (approximate) fees for the next block increased, perhaps more so after
+             * Cluster Mempool.
+             *
+             * We'll also create a new template if the tip changed during the last tick.
+             */
+            if (options.fee_threshold != MAX_MONEY || tip_changed) {
+                auto block_template{std::make_unique<BlockTemplateImpl>(m_assemble_options, BlockAssembler{chainman().ActiveChainstate(), context()->mempool.get(), m_assemble_options}.CreateNewBlock(), m_node)};
+
+                // If the tip changed, return the new template regardless of its fees.
+                if (tip_changed) {
+                    return block_template;
+                }
+
+                // Calculate the original template total fees if we haven't already
+                if (current_fees == -1) {
+                    current_fees = 0;
+                    for (CAmount fee : m_block_template->vTxFees) {
+                        // Skip coinbase
+                        if (fee < 0) continue;
+                        current_fees += fee;
+                    }
+                }
+
+                CAmount new_fees = 0;
+                for (CAmount fee : block_template->m_block_template->vTxFees) {
+                    // Skip coinbase
+                    if (fee < 0) continue;
+                    new_fees += fee;
+                    Assume(options.fee_threshold != MAX_MONEY);
+                    if (new_fees >= current_fees + options.fee_threshold) return block_template;
+                }
+            }
+
+            /**
+             * Break out of while when using mock time. While functional tests
+             * can call setmocktime to move the clock and exit this loop, a
+             * unit tests would need to spawn a thread to achieve this.
+             * Making the while loop use now < deadline won't work either, because
+             * then there's no wait to test its internals.
+             */
+            if (now == deadline) break;
+            now = NodeClock::now();
+        }
+
+        return nullptr;
+    }
+
+    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;
 };
 
@@ -985,7 +1079,7 @@ 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(), m_node);
+        return std::make_unique<BlockTemplateImpl>(assemble_options, BlockAssembler{chainman().ActiveChainstate(), context()->mempool.get(), assemble_options}.CreateNewBlock(), m_node);
     }
 
     NodeContext* context() override { return &m_node; }

src/node/types.h

@@ -14,7 +14,9 @@
 #define BITCOIN_NODE_TYPES_H
 
 #include <cstddef>
+#include <consensus/amount.h>
 #include <script/script.h>
+#include <util/time.h>
 
 namespace node {
 enum class TransactionError {
@@ -61,6 +63,28 @@ struct BlockCreateOptions {
      */
     CScript coinbase_output_script{CScript() << OP_TRUE};
 };
+
+struct BlockWaitOptions {
+    /**
+     * How long to wait before returning nullptr instead of a new template.
+     * Default is to wait forever.
+     */
+    MillisecondsDouble timeout{MillisecondsDouble::max()};
+
+    /**
+     * Wait until total fees in the new template exceed fees in the origal
+     * template by at least this amount (in sats). The default is to ignore
+     * fee increases and only wait for a tip change.
+     *
+     * This is not an std::optional<CAmount> because (the current version of)
+     * libmultiprocess maps missing values to 0.
+     * https://github.com/bitcoin/bitcoin/pull/31283#discussion_r1937622086
+     *
+     * Use a magic value instead.
+     */
+    CAmount fee_threshold{MAX_MONEY};
+};
+
 } // namespace node
 
 #endif // BITCOIN_NODE_TYPES_H

src/test/miner_tests.cpp

@@ -180,8 +180,11 @@ void MinerTestingSetup::TestPackageSelection(const CScript& scriptPubKey, const
     tx.vout[0].nValue = 5000000000LL - 1000 - 50000 - feeToUse;
     Txid hashLowFeeTx = tx.GetHash();
     AddToMempool(tx_mempool, entry.Fee(feeToUse).FromTx(tx));
-    block_template = mining->createNewBlock(options);
-    BOOST_REQUIRE(block_template);
+
+    // waitNext() should return nullptr because there is no better template
+    auto should_be_nullptr = block_template->waitNext({.timeout = MillisecondsDouble{0}, .fee_threshold = 1});
+    BOOST_REQUIRE(should_be_nullptr == nullptr);
+
     block = block_template->getBlock();
     // Verify that the free tx and the low fee tx didn't get selected
     for (size_t i=0; i<block.vtx.size(); ++i) {
@@ -196,7 +199,9 @@ void MinerTestingSetup::TestPackageSelection(const CScript& scriptPubKey, const
     tx.vout[0].nValue -= 2; // Now we should be just over the min relay fee
     hashLowFeeTx = tx.GetHash();
     AddToMempool(tx_mempool, entry.Fee(feeToUse + 2).FromTx(tx));
-    block_template = mining->createNewBlock(options);
+
+    // waitNext() should return if fees for the new template are at least 1 sat up
+    block_template = block_template->waitNext({.fee_threshold = 1});
     BOOST_REQUIRE(block_template);
     block = block_template->getBlock();
     BOOST_REQUIRE_EQUAL(block.vtx.size(), 6U);
@@ -672,8 +677,10 @@ BOOST_AUTO_TEST_CASE(CreateNewBlock_validity)
         const int current_height{mining->getTip()->height};
 
         // Simple block creation, nothing special yet:
-        block_template = mining->createNewBlock(options);
-        BOOST_REQUIRE(block_template);
+        if (current_height % 2 == 0) {
+            block_template = mining->createNewBlock(options);
+            BOOST_REQUIRE(block_template);
+        } // for odd heights the next template is created at the end of this loop
 
         CBlock block{block_template->getBlock()};
         CMutableTransaction txCoinbase(*block.vtx[0]);
@@ -710,7 +717,12 @@ BOOST_AUTO_TEST_CASE(CreateNewBlock_validity)
             BOOST_REQUIRE_EQUAL(maybe_new_tip->GetBlockHash(), block.GetHash());
         }
         // This just adds coverage
-        mining->waitTipChanged(block.hashPrevBlock);
+        if (current_height % 2 == 0) {
+            block_template = block_template->waitNext();
+            BOOST_REQUIRE(block_template);
+        } else {
+            mining->waitTipChanged(block.hashPrevBlock);
+        }
     }
 
     LOCK(cs_main);