mining: enforce minimum reserved weight for IPC

Previously a lower value was silently clamped to MINIMUM_BLOCK_RESERVED_WEIGHT.

src/node/interfaces.cpp

@@ -67,6 +67,7 @@
 #include <any>
 #include <memory>
 #include <optional>
+#include <stdexcept>
 #include <utility>
 
 #include <boost/signals2/signal.hpp>
@@ -969,6 +970,16 @@ public:
 
     std::unique_ptr<BlockTemplate> createNewBlock(const BlockCreateOptions& options) override
     {
+        // Reject too-small values instead of clamping so callers don't silently
+        // end up mining with different options than requested. This matches the
+        // behavior of the `-blockreservedweight` startup option, which rejects
+        // values below MINIMUM_BLOCK_RESERVED_WEIGHT.
+        if (options.block_reserved_weight && options.block_reserved_weight < MINIMUM_BLOCK_RESERVED_WEIGHT) {
+            throw std::runtime_error(strprintf("block_reserved_weight (%zu) must be at least %u weight units",
+                                               *options.block_reserved_weight,
+                                               MINIMUM_BLOCK_RESERVED_WEIGHT));
+        }
+
         // Ensure m_tip_block is set so consumers of BlockTemplate can rely on that.
         if (!waitTipChanged(uint256::ZERO, MillisecondsDouble::max())) return {};
 

src/node/types.h

@@ -43,7 +43,8 @@ struct BlockCreateOptions {
     bool use_mempool{true};
     /**
      * The default reserved weight for the fixed-size block header,
-     * transaction count and coinbase transaction.
+     * transaction count and coinbase transaction. Minimum: 2000 weight units
+     * (MINIMUM_BLOCK_RESERVED_WEIGHT).
      *
      * Providing a value overrides the `-blockreservedweight` startup setting.
      * Cap'n Proto IPC clients currently cannot leave this field unset, so they