miner: empty mempool special case for waitNext()

Block template fees are calculated by looping over new_tmpl->vTxFees
and return (early) once the fee_threshold is exceeded.

This left an edge case when the mempool is empty, which this commit
fixes and adds a test for.

Also update interface_ipc.py to account for the new behavior.

src/node/miner.cpp

@@ -528,6 +528,9 @@ std::unique_ptr<CBlockTemplate> WaitAndCreateNewBlock(ChainstateManager& chainma
                 }
             }
 
+            // Special case for when fee threshold is zero and the mempool is and was empty
+            if (current_fees == 0 && options.fee_threshold == 0 && new_tmpl->vTxFees.empty()) return new_tmpl;
+
             CAmount new_fees = 0;
             for (CAmount fee : new_tmpl->vTxFees) {
                 new_fees += fee;

src/test/miner_tests.cpp

@@ -113,6 +113,22 @@ void MinerTestingSetup::TestPackageSelection(const CScript& scriptPubKey, const
     options.coinbase_output_script = scriptPubKey;
 
     LOCK(tx_mempool.cs);
+    BOOST_CHECK(tx_mempool.size() == 0);
+
+    // Block template should only have a coinbase when there's nothing in the mempool
+    std::unique_ptr<BlockTemplate> block_template = mining->createNewBlock(options);
+    BOOST_REQUIRE(block_template);
+    CBlock block{block_template->getBlock()};
+    BOOST_REQUIRE_EQUAL(block.vtx.size(), 1U);
+
+    // waitNext() on an empty mempool 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);
+
+    // Unless fee_threshold is 0
+    block_template = block_template->waitNext({.timeout = MillisecondsDouble{0}, .fee_threshold = 0});
+    BOOST_REQUIRE(block_template);
+
     // Test the ancestor feerate transaction selection.
     TestMemPoolEntryHelper entry;
 
@@ -144,9 +160,9 @@ void MinerTestingSetup::TestPackageSelection(const CScript& scriptPubKey, const
     const auto high_fee_tx{entry.Fee(50000).Time(Now<NodeSeconds>()).SpendsCoinbase(false).FromTx(tx)};
     AddToMempool(tx_mempool, high_fee_tx);
 
-    std::unique_ptr<BlockTemplate> block_template = mining->createNewBlock(options);
+    block_template = mining->createNewBlock(options);
     BOOST_REQUIRE(block_template);
-    CBlock block{block_template->getBlock()};
+    block = block_template->getBlock();
     BOOST_REQUIRE_EQUAL(block.vtx.size(), 4U);
     BOOST_CHECK(block.vtx[1]->GetHash() == hashParentTx);
     BOOST_CHECK(block.vtx[2]->GetHash() == hashHighFeeTx);
@@ -184,7 +200,7 @@ void MinerTestingSetup::TestPackageSelection(const CScript& scriptPubKey, const
     AddToMempool(tx_mempool, entry.Fee(feeToUse).FromTx(tx));
 
     // waitNext() should return nullptr because there is no better template
-    auto should_be_nullptr = block_template->waitNext({.timeout = MillisecondsDouble{0}, .fee_threshold = 1});
+    should_be_nullptr = block_template->waitNext({.timeout = MillisecondsDouble{0}, .fee_threshold = 1});
     BOOST_REQUIRE(should_be_nullptr == nullptr);
 
     block = block_template->getBlock();

test/functional/interface_ipc.py

@@ -153,6 +153,7 @@ class IPCInterfaceTest(BitcoinTestFramework):
             self.log.debug("Wait for a new template")
             waitoptions = self.capnp_modules['mining'].BlockWaitOptions()
             waitoptions.timeout = timeout
+            waitoptions.feeThreshold = 1
             waitnext = template.result.waitNext(ctx, waitoptions)
             self.generate(self.nodes[0], 1)
             template2 = await waitnext
@@ -168,6 +169,7 @@ class IPCInterfaceTest(BitcoinTestFramework):
             block3 = await self.parse_and_deserialize_block(template4, ctx)
             assert_equal(len(block3.vtx), 2)
             self.log.debug("Wait again, this should return the same template, since the fee threshold is zero")
+            waitoptions.feeThreshold = 0
             template5 = await template4.result.waitNext(ctx, waitoptions)
             block4 = await self.parse_and_deserialize_block(template5, ctx)
             assert_equal(len(block4.vtx), 2)