Fix concurrent events in PollingEventSubscriber

src.ts/providers/subscriber-polling.ts

@@ -7,6 +7,10 @@ function copy(obj: any): any {
     return JSON.parse(JSON.stringify(obj));
 }
 
+function stall(duration: number): Promise<void> {
+    return new Promise((resolve) => { setTimeout(resolve, duration); });
+}
+
 /**
  *  Return the polling subscriber for common events.
  *
@@ -219,6 +223,7 @@ export class PollingEventSubscriber implements Subscriber {
     // The most recent block we have scanned for events. The value -2
     // indicates we still need to fetch an initial block number
     #blockNumber: number;
+    #concurrentBlockNumber: number;
 
     /**
      *  Create a new **PollingTransactionSubscriber** attached to
@@ -230,18 +235,31 @@ export class PollingEventSubscriber implements Subscriber {
         this.#poller = this.#poll.bind(this);
         this.#running = false;
         this.#blockNumber = -2;
+        this.#concurrentBlockNumber = -2;
     }
 
     async #poll(blockNumber: number): Promise<void> {
         // The initial block hasn't been determined yet
         if (this.#blockNumber === -2) { return; }
 
+        // Debounce concurrent calls to #poll within a 16ms time window
+        if (blockNumber > this.#concurrentBlockNumber) {
+            this.#concurrentBlockNumber = blockNumber;
+        }
+        await stall(16);
+        if (blockNumber !== this.#concurrentBlockNumber) {
+            return;
+        }
+
         const filter = copy(this.#filter);
         filter.fromBlock = this.#blockNumber + 1;
         filter.toBlock = blockNumber;
 
         const logs = await this.#provider.getLogs(filter);
 
+        // Undo debounce state
+        this.#concurrentBlockNumber = -2;
+
         // No logs could just mean the node has not indexed them yet,
         // so we keep a sliding window of 60 blocks to keep scanning
         if (logs.length === 0) {