feat: add isPubSub method to detect PubSub implementations (#2707)

Adds a type guard function to signal to tsc that an object
implements the PubSub interface.

packages/interface-compliance-tests/src/pubsub/api.ts

@@ -1,4 +1,4 @@
-import { isStartable, start, stop } from '@libp2p/interface'
+import { isPubSub, isStartable, start, stop } from '@libp2p/interface'
 import { expect } from 'aegir/chai'
 import delay from 'delay'
 import pDefer from 'p-defer'
@@ -39,6 +39,10 @@ export default (common: TestSetup<PubSub, PubSubArgs>): void => {
       mockNetwork.reset()
     })
 
+    it('is a PubSub implementation', () => {
+      expect(isPubSub(pubsub)).to.be.true()
+    })
+
     it('can start correctly', async () => {
       if (!isStartable(pubsub)) {
         return

packages/interface/src/pubsub/index.ts

@@ -268,3 +268,16 @@ export interface PeerStreamEvents {
   'stream:outbound': CustomEvent<never>
   'close': CustomEvent<never>
 }
+
+/**
+ * All Pubsub implementations must use this symbol as the name of a property
+ * with a boolean `true` value
+ */
+export const pubSubSymbol = Symbol.for('@libp2p/pubsub')
+
+/**
+ * Returns true if the passed argument is a PubSub implementation
+ */
+export function isPubSub (obj?: any): obj is PubSub {
+  return Boolean(obj?.[pubSubSymbol])
+}

packages/pubsub-floodsub/src/index.ts

@@ -32,7 +32,7 @@
  * ```
  */
 
-import { serviceDependencies } from '@libp2p/interface'
+import { pubSubSymbol, serviceDependencies } from '@libp2p/interface'
 import { PubSubBaseProtocol, type PubSubComponents } from '@libp2p/pubsub'
 import { toString } from 'uint8arrays/to-string'
 import { SimpleTimeCache } from './cache.js'
@@ -78,6 +78,8 @@ export class FloodSub extends PubSubBaseProtocol {
     })
   }
 
+  readonly [pubSubSymbol] = true
+
   readonly [Symbol.toStringTag] = '@libp2p/floodsub'
 
   readonly [serviceDependencies]: string[] = [