Cache eth_calls with in-memory cache (#410)

* Revert "Revert "Cache eth_calls in our InstrumentedEVMProvider (#406)" (#407)"

This reverts commit 0dcd42e.

* Cache RPC Responses with metrics

* linter

Author: mikeki

lib/handlers/evm/provider/InstrumentedEVMProvider.ts

@@ -25,11 +25,50 @@ export type InstrumentedEVMProviderProps = {
 export class InstrumentedEVMProvider extends ethers.providers.StaticJsonRpcProvider {
   private readonly name: ProviderName
   private readonly metricPrefix: string
+  private readonly blockCache: Map<string, Promise<any>>
 
   constructor({ url, network, name }: InstrumentedEVMProviderProps) {
     super(url, network)
     this.name = name
     this.metricPrefix = `RPC_${this.name}_${this.network.chainId}`
+    this.blockCache = new Map()
+    // Set an event listener to clear the cache on every new block.
+    this.on('block', () => {
+      metric.putMetric('RPCProviderCacheClear', 1, MetricLoggerUnit.Count)
+      this.blockCache.clear()
+    })
+  }
+
+  // Adds caching functionality to the RPC provider
+  override send(method: string, params: Array<any>): Promise<any> {
+    // Only cache eth_call's.
+    if (method !== 'eth_call') return super.send(method, params)
+    let key: string | undefined = undefined
+
+    try {
+      key = `call:${JSON.stringify(params)}`
+    } catch (e) {
+      metric.putMetric('RPCProviderCacheKeyError', 1, MetricLoggerUnit.Count)
+    }
+
+    if (key) {
+      const cached = this.blockCache.get(key)
+      if (cached) {
+        metric.putMetric('RPCProviderCacheHit', 1, MetricLoggerUnit.Count)
+        return cached
+      } else {
+        metric.putMetric('RPCProviderCacheMiss', 1, MetricLoggerUnit.Count)
+      }
+    }
+
+    const result = super.send(method, params)
+
+    if (key) {
+      metric.putMetric('RPCProviderCacheInsert', 1, MetricLoggerUnit.Count)
+      this.blockCache.set(key, result)
+    }
+
+    return result
   }
 
   override call(transaction: Deferrable<TransactionRequest>, blockTag?: BlockTag | Promise<BlockTag>): Promise<string> {