Expose max_tx_weight API (#3407)

Add API for getting max transaction weight

Co-authored-by: Nym Seddon <unseddd@shh.xyz>

core/src/global.rs

@@ -17,10 +17,10 @@
 //! should be used sparingly.
 
 use crate::consensus::{
-	graph_weight, valid_header_version, HeaderInfo, BASE_EDGE_BITS, BLOCK_TIME_SEC,
-	COINBASE_MATURITY, CUT_THROUGH_HORIZON, DAY_HEIGHT, DEFAULT_MIN_EDGE_BITS,
-	DIFFICULTY_ADJUST_WINDOW, INITIAL_DIFFICULTY, MAX_BLOCK_WEIGHT, PROOFSIZE,
-	SECOND_POW_EDGE_BITS, STATE_SYNC_THRESHOLD,
+	graph_weight, valid_header_version, HeaderInfo, BASE_EDGE_BITS, BLOCK_KERNEL_WEIGHT,
+	BLOCK_OUTPUT_WEIGHT, BLOCK_TIME_SEC, COINBASE_MATURITY, CUT_THROUGH_HORIZON, DAY_HEIGHT,
+	DEFAULT_MIN_EDGE_BITS, DIFFICULTY_ADJUST_WINDOW, INITIAL_DIFFICULTY, MAX_BLOCK_WEIGHT,
+	PROOFSIZE, SECOND_POW_EDGE_BITS, STATE_SYNC_THRESHOLD,
 };
 use crate::core::block::HeaderVersion;
 use crate::pow::{
@@ -320,6 +320,12 @@ pub fn max_block_weight() -> usize {
 	}
 }
 
+/// Maximum allowed transaction weight (1 weight unit ~= 32 bytes)
+pub fn max_tx_weight() -> u64 {
+	let coinbase_weight = BLOCK_OUTPUT_WEIGHT + BLOCK_KERNEL_WEIGHT;
+	max_block_weight().saturating_sub(coinbase_weight) as u64
+}
+
 /// Horizon at which we can cut-through and do full local pruning
 pub fn cut_through_horizon() -> u32 {
 	match get_chain_type() {