Fix SV2 reserved weight floor calculation

Use std::max so block_reserved_weight enforces the 2000 weight
minimum rather than capping at 2000.

Add a focused regression test that covers the floor behavior and
documents the min-vs-cap failure mode.

Reported by xyephy:
#80 (comment)

Assisted-by: OpenAI GPT-5.3-Codex

Author: Sjors

src/sv2/block_options.h

@@ -14,6 +14,8 @@ namespace node {
 
 //! Default reserved weight for block assembly scaffolding (header, coinbase, etc).
 static constexpr unsigned int DEFAULT_BLOCK_RESERVED_WEIGHT{8000};
+//! Minimum reserved weight enforced by block assembly.
+static constexpr size_t MIN_BLOCK_RESERVED_WEIGHT{2000};
 
 struct BlockCreateOptions {
     /** Set false to omit mempool transactions from templates. */

src/sv2/template_provider.cpp

@@ -266,14 +266,13 @@ void Sv2TemplateProvider::ThreadSv2ClientHandler(size_t client_id)
                 const size_t block_reserved_floor{1168};
                 // Reserve a little more so that if the above calculation is
                 // wrong or there's an implementation error, we don't produce
-                // an invalid bock when the template is completely full.
+                // an invalid block when the template is completely full.
                 const size_t block_reserved_padding{400};
 
-                // Bitcoin Core enforces a mimimum block reserved weight of 2000.
-                options.block_reserved_weight = std::min(size_t(2000),
-                                                         block_reserved_floor +
-                                                             block_reserved_padding +
-                                                             client->m_coinbase_tx_outputs_size * 4);
+                // Bitcoin Core enforces a minimum block reserved weight of 2000.
+                options.block_reserved_weight = std::max(
+                    node::MIN_BLOCK_RESERVED_WEIGHT,
+                    block_reserved_floor + block_reserved_padding + client->m_coinbase_tx_outputs_size * 4);
             }
             return true;
         };

src/test/sv2_template_provider_tests.cpp

@@ -1,5 +1,6 @@
 #include <boost/test/unit_test.hpp>
 #include <interfaces/mining.h>
+#include <sv2/block_options.h>
 #include <interfaces/init.h>
 #include <sv2/messages.h>
 #include <test/sv2_test_setup.h>
@@ -16,6 +17,7 @@
 #include <test/sv2_mock_mining.h>
 
 #include <future>
+#include <algorithm>
 #include <memory>
 #include <string>
 #include <thread>
@@ -27,6 +29,15 @@
 
 BOOST_FIXTURE_TEST_SUITE(sv2_template_provider_tests, Sv2BasicTestingSetup)
 
+BOOST_AUTO_TEST_CASE(block_reserved_weight_floor)
+{
+    node::BlockCreateOptions options{};
+    // Guard against regressions where the reserved weight floor is treated as a cap.
+    options.block_reserved_weight = 1800;
+    options.block_reserved_weight = std::max(node::MIN_BLOCK_RESERVED_WEIGHT, options.block_reserved_weight);
+    BOOST_REQUIRE_EQUAL(options.block_reserved_weight, node::MIN_BLOCK_RESERVED_WEIGHT);
+}
+
 BOOST_AUTO_TEST_CASE(client_tests)
 {
     TPTester tester{};