feat: Optimise sandbox startup time by only initialising the BB solve…

…r once. (#2240)

This PR ensures that we only perform BB solver initialisation once
within the acir simulator.

# Checklist:
Remove the checklist to signal you've completed it. Enable auto-merge if
the PR is ready to merge.
- [ ] If the pull request requires a cryptography review (e.g.
cryptographic algorithm implementations) I have added the 'crypto' tag.
- [ ] I have reviewed my diff in github, line by line and removed
unexpected formatting changes, testing logs, or commented-out code.
- [ ] Every change is related to the PR description.
- [ ] I have
[linked](https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue)
this pull request to relevant issues (if any exist).

yarn-project/acir-simulator/src/client/simulator.ts

@@ -22,7 +22,7 @@ import { UnconstrainedFunctionExecution } from './unconstrained_execution.js';
  * The ACIR simulator.
  */
 export class AcirSimulator {
-  private static solver: WasmBlackBoxFunctionSolver; // ACVM's backend
+  private static solver: Promise<WasmBlackBoxFunctionSolver>; // ACVM's backend
   private log: DebugLogger;
 
   constructor(private db: DBOracle) {
@@ -42,8 +42,8 @@ export class AcirSimulator {
    *
    * @returns ACVM WasmBlackBoxFunctionSolver
    */
-  public static async getSolver(): Promise<WasmBlackBoxFunctionSolver> {
-    if (!this.solver) this.solver = await createBlackBoxSolver();
+  public static getSolver(): Promise<WasmBlackBoxFunctionSolver> {
+    if (!this.solver) this.solver = createBlackBoxSolver();
     return this.solver;
   }
 

yarn-project/sequencer-client/src/sequencer/sequencer.ts

@@ -129,10 +129,11 @@ export class Sequencer {
         return;
       }
 
-      this.log(`Processing ${validTxs.length} txs...`);
+      const blockNumber = (await this.l2BlockSource.getBlockNumber()) + 1;
+
+      this.log.info(`Building block ${blockNumber} with ${validTxs.length} transactions...`);
       this.state = SequencerState.CREATING_BLOCK;
 
-      const blockNumber = (await this.l2BlockSource.getBlockNumber()) + 1;
       const newGlobalVariables = await this.globalsBuilder.buildGlobalVariables(new Fr(blockNumber));
       const prevGlobalVariables = (await this.l2BlockSource.getL2Block(-1))?.globalVariables ?? GlobalVariables.empty();