piecrust: price event bytes at the same rate as storage

Each byte in an event, be it topic or data, is priced at the same rate
as each byte in a storage instruction. This ensures each event is priced
more than a simple host function, since they require a larger processing
window.

This should be the final commit in pricing imports and host functions,
apart from adding additional functionality such as external
configuration.

Resolves #359

piecrust/CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Changed
+
+- Make each event byte cost the same as a storage byte [#359]
+
 ### Fixed
 
 - Fix incomplete removal of economic protocol functionality

piecrust/src/config.rs

@@ -0,0 +1,8 @@
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+//
+// Copyright (c) DUSK NETWORK. All rights reserved.
+
+// The gas cost for each byte
+pub const BYTE_STORE_COST: i64 = 4;

piecrust/src/imports.rs

@@ -18,6 +18,7 @@ use piecrust_uplink::{
     ContractError, ContractId, ARGBUF_LEN, CONTRACT_ID_BYTES,
 };
 
+use crate::config::BYTE_STORE_COST;
 use crate::instance::{Env, WrappedInstance};
 use crate::Error;
 
@@ -324,6 +325,16 @@ pub(crate) fn emit(
     check_ptr(instance, topic_ofs, topic_len)?;
     check_arg(instance, arg_len)?;
 
+    // charge for each byte emitted in an event
+    let gas_remaining = instance.get_remaining_gas();
+    let gas_cost = BYTE_STORE_COST as u64 * (topic_len as u64 + arg_len as u64);
+
+    if gas_cost > gas_remaining {
+        instance.set_remaining_gas(0);
+        Err(Error::OutOfGas)?;
+    }
+    instance.set_remaining_gas(gas_remaining - gas_cost);
+
     let data = instance.with_arg_buf(|buf| {
         let arg_len = arg_len as usize;
         Vec::from(&buf[..arg_len])

piecrust/src/lib.rs

@@ -118,6 +118,7 @@
 #[macro_use]
 mod bytecode_macro;
 mod call_tree;
+mod config;
 mod contract;
 mod error;
 mod imports;

piecrust/src/vm.rs

@@ -18,6 +18,7 @@ use dusk_wasmtime::{
 };
 use tempfile::tempdir;
 
+use crate::config::BYTE_STORE_COST;
 use crate::session::{Session, SessionData};
 use crate::store::ContractStore;
 use crate::Error::{self, PersistenceError};
@@ -58,7 +59,6 @@ fn config() -> Config {
     // Support 64-bit memories
     config.wasm_memory64(true);
 
-    const BYTE_STORE_COST: i64 = 4;
     const BYTE4_STORE_COST: i64 = 4 * BYTE_STORE_COST;
     const BYTE8_STORE_COST: i64 = 8 * BYTE_STORE_COST;
     const BYTE16_STORE_COST: i64 = 16 * BYTE_STORE_COST;

piecrust/tests/eventer.rs

@@ -38,3 +38,55 @@ pub fn vm_center_events() -> Result<(), Error> {
 
     Ok(())
 }
+
+#[test]
+pub fn event_costs() -> Result<(), Error> {
+    let vm = VM::ephemeral()?;
+
+    let mut session = vm.session(SessionData::builder())?;
+
+    let eventer_id = session.deploy(
+        contract_bytecode!("eventer"),
+        ContractData::builder().owner(OWNER),
+        LIMIT,
+    )?;
+
+    // This call is to "prime" the contract
+    let _ = session.call::<_, (u64, u64)>(
+        eventer_id,
+        "emit_input",
+        &vec![1u8; 100],
+        LIMIT,
+    )?;
+
+    let mut costs = vec![];
+
+    for size in (4..=40).step_by(4) {
+        let input = vec![1u8; size];
+        let (spent_before, spent_after) = session
+            .call::<_, (u64, u64)>(eventer_id, "emit_input", &input, LIMIT)?
+            .data;
+        let cost = spent_after - spent_before;
+        print!("{cost} ");
+        costs.push(cost);
+    }
+
+    // cost grows linearly with the amount of bytes processed, at a predictable
+    // rate.
+    //
+    // NOTE: it is not possible to directly test emission costs, unless this is
+    //       externally configurable
+    let mut cost_diffs = Vec::with_capacity(costs.len() - 1);
+    for i in 0..costs.len() - 1 {
+        cost_diffs.push(costs[i + 1] - costs[i]);
+    }
+    let (ref_cost_diff, cost_diffs) = cost_diffs.split_first().unwrap();
+    for cost_diff in cost_diffs {
+        assert_eq!(
+            cost_diff, ref_cost_diff,
+            "cost should grow at a linear rate"
+        );
+    }
+
+    Ok(())
+}