miner: init: add -blockreservedweight startup option

- Prevent setting the value of `-blockreservedweight` below
  a safety value of 2000.

src/init.cpp

@@ -649,6 +649,7 @@ void SetupServerArgs(ArgsManager& argsman, bool can_listen_ipc)
 
 
     argsman.AddArg("-blockmaxweight=<n>", strprintf("Set maximum BIP141 block weight (default: %d)", DEFAULT_BLOCK_MAX_WEIGHT), ArgsManager::ALLOW_ANY, OptionsCategory::BLOCK_CREATION);
+    argsman.AddArg("-blockreservedweight=<n>", strprintf("Reserve space for the fixed-size block header plus the largest coinbase transaction the mining software may add to the block. (default: %d).", DEFAULT_BLOCK_RESERVED_WEIGHT), ArgsManager::ALLOW_ANY, OptionsCategory::BLOCK_CREATION);
     argsman.AddArg("-blockmintxfee=<amt>", strprintf("Set lowest fee rate (in %s/kvB) for transactions to be included in block creation. (default: %s)", CURRENCY_UNIT, FormatMoney(DEFAULT_BLOCK_MIN_TX_FEE)), ArgsManager::ALLOW_ANY, OptionsCategory::BLOCK_CREATION);
     argsman.AddArg("-blockversion=<n>", "Override block version to test forking scenarios", ArgsManager::ALLOW_ANY | ArgsManager::DEBUG_ONLY, OptionsCategory::BLOCK_CREATION);
 
@@ -1023,6 +1024,16 @@ bool AppInitParameterInteraction(const ArgsManager& args)
         }
     }
 
+    if (args.IsArgSet("-blockreservedweight")) {
+        const auto block_reserved_weight = args.GetIntArg("-blockreservedweight", DEFAULT_BLOCK_RESERVED_WEIGHT);
+        if (block_reserved_weight > MAX_BLOCK_WEIGHT) {
+            return InitError(strprintf(_("Specified -blockreservedweight (%d) exceeds consensus maximum block weight (%d)"), block_reserved_weight, MAX_BLOCK_WEIGHT));
+        }
+        if (block_reserved_weight < MINIMUM_BLOCK_RESERVED_WEIGHT) {
+            return InitError(strprintf(_("Specified -blockreservedweight (%d) is lower than minimum safety value of (%d)"), block_reserved_weight, MINIMUM_BLOCK_RESERVED_WEIGHT));
+        }
+    }
+
     nBytesPerSigOp = args.GetIntArg("-bytespersigop", nBytesPerSigOp);
 
     if (!g_wallet_init_interface.ParameterInteraction()) return false;

src/node/miner.cpp

@@ -68,6 +68,7 @@ void RegenerateCommitments(CBlock& block, ChainstateManager& chainman)
 static BlockAssembler::Options ClampOptions(BlockAssembler::Options options)
 {
     Assert(options.block_reserved_weight <= MAX_BLOCK_WEIGHT);
+    Assert(options.block_reserved_weight >= MINIMUM_BLOCK_RESERVED_WEIGHT);
     Assert(options.coinbase_output_max_additional_sigops <= MAX_BLOCK_SIGOPS_COST);
     // Limit weight to between block_reserved_weight and MAX_BLOCK_WEIGHT for sanity:
     // block_reserved_weight can safely exceed -blockmaxweight, but the rest of the block template will be empty.
@@ -91,6 +92,7 @@ void ApplyArgsManOptions(const ArgsManager& args, BlockAssembler::Options& optio
         if (const auto parsed{ParseMoney(*blockmintxfee)}) options.blockMinFeeRate = CFeeRate{*parsed};
     }
     options.print_modified_fee = args.GetBoolArg("-printpriority", options.print_modified_fee);
+    options.block_reserved_weight = args.GetIntArg("-blockreservedweight", options.block_reserved_weight);
 }
 
 void BlockAssembler::resetBlock()

src/policy/policy.h

@@ -21,7 +21,13 @@ class CScript;
 
 /** Default for -blockmaxweight, which controls the range of block weights the mining code will create **/
 static constexpr unsigned int DEFAULT_BLOCK_MAX_WEIGHT{MAX_BLOCK_WEIGHT};
+/** Default for -blockreservedweight **/
 static constexpr unsigned int DEFAULT_BLOCK_RESERVED_WEIGHT{8000};
+/** This accounts for the block header, var_int encoding of the transaction count and a minimally viable
+ * coinbase transaction. It adds an additional safety margin, because even with a thorough understanding
+ * of block serialization, it's easy to make a costly mistake when trying to squeeze every last byte.
+ * Setting a lower value is prevented at startup. */
+static constexpr unsigned int MINIMUM_BLOCK_RESERVED_WEIGHT{2000};
 /** Default for -blockmintxfee, which sets the minimum feerate for a transaction in blocks created by mining code **/
 static constexpr unsigned int DEFAULT_BLOCK_MIN_TX_FEE{1000};
 /** The maximum weight for transactions we're willing to relay/mine */

test/functional/mining_basic.py

@@ -28,6 +28,7 @@ from test_framework.messages import (
     COIN,
     DEFAULT_BLOCK_RESERVED_WEIGHT,
     MAX_BLOCK_WEIGHT,
+    MINIMUM_BLOCK_RESERVED_WEIGHT,
     ser_uint256,
     WITNESS_SCALE_FACTOR
 )
@@ -276,6 +277,28 @@ class MiningTest(BitcoinTestFramework):
             expected_weight=MAX_BLOCK_WEIGHT - DEFAULT_BLOCK_RESERVED_WEIGHT,
         )
 
+        self.log.info("Test -blockreservedweight startup option.")
+        # Lowering the -blockreservedweight by 4000 will allow for two more transactions.
+        self.restart_node(0, extra_args=[f"-datacarriersize={LARGE_VSIZE}", "-blockreservedweight=4000"])
+        self.verify_block_template(
+            expected_tx_count=12,
+            expected_weight=MAX_BLOCK_WEIGHT - 4000,
+        )
+
+        self.log.info("Test that node will fail to start when user provide invalid -blockreservedweight")
+        self.stop_node(0)
+        self.nodes[0].assert_start_raises_init_error(
+            extra_args=[f"-blockreservedweight={MAX_BLOCK_WEIGHT + 1}"],
+            expected_msg=f"Error: Specified -blockreservedweight ({MAX_BLOCK_WEIGHT + 1}) exceeds consensus maximum block weight ({MAX_BLOCK_WEIGHT})",
+        )
+
+        self.log.info(f"Test that node will fail to start when user provide -blockreservedweight below {MINIMUM_BLOCK_RESERVED_WEIGHT}")
+        self.stop_node(0)
+        self.nodes[0].assert_start_raises_init_error(
+            extra_args=[f"-blockreservedweight={MINIMUM_BLOCK_RESERVED_WEIGHT - 1}"],
+            expected_msg=f"Error: Specified -blockreservedweight ({MINIMUM_BLOCK_RESERVED_WEIGHT - 1}) is lower than minimum safety value of ({MINIMUM_BLOCK_RESERVED_WEIGHT})",
+        )
+
         self.log.info("Test that node will fail to start when user provide invalid -blockmaxweight")
         self.stop_node(0)
         self.nodes[0].assert_start_raises_init_error(

test/functional/test_framework/messages.py

@@ -34,6 +34,7 @@ from test_framework.util import assert_equal
 MAX_LOCATOR_SZ = 101
 MAX_BLOCK_WEIGHT = 4000000
 DEFAULT_BLOCK_RESERVED_WEIGHT = 8000
+MINIMUM_BLOCK_RESERVED_WEIGHT = 2000
 MAX_BLOOM_FILTER_SIZE = 36000
 MAX_BLOOM_HASH_FUNCS = 50