fix: sync should only happen when both sides are enabled (#764)

This change:

- Fixes core unreplication, closing issue [#762].

- Adds "fuzz testing" for sync, which tries a bunch of random actions
  and makes sure things work as expected.

[#762]: #762

Co-Authored-By: Gregor MacLennan <[email protected]>

src/lib/hypercore-helpers.js

@@ -0,0 +1,18 @@
+import { assert } from '../utils.js'
+
+/**
+ * @param {import('hypercore')<'binary', any>} core Core to unreplicate. Must be ready.
+ * @param {import('protomux')} protomux
+ */
+export function unreplicate(core, protomux) {
+  assert(core.discoveryKey, 'Core should have a discovery key')
+  protomux.unpair({
+    protocol: 'hypercore/alpha',
+    id: core.discoveryKey,
+  })
+  for (const channel of protomux) {
+    if (channel.protocol !== 'hypercore/alpha') continue
+    if (!channel.id.equals(core.discoveryKey)) continue
+    channel.close()
+  }
+}

src/sync/peer-sync-controller.js

@@ -2,6 +2,7 @@ import mapObject from 'map-obj'
 import { NAMESPACES, PRESYNC_NAMESPACES } from '../constants.js'
 import { Logger } from '../logger.js'
 import { ExhaustivenessError, createMap } from '../utils.js'
+import { unreplicate } from '../lib/hypercore-helpers.js'
 /** @import { CoreRecord } from '../core-manager/index.js' */
 /** @import { Role } from '../roles.js' */
 /** @import { SyncEnabledState } from './sync-api.js' */
@@ -255,16 +256,22 @@ export class PeerSyncController {
 
   /**
    * @param {import('hypercore')<'binary', any>} core
+   * @returns {Promise<void>}
    */
-  #unreplicateCore(core) {
+  async #unreplicateCore(core) {
     if (core === this.#coreManager.creatorCore) return
-    const peerToUnreplicate = core.peers.find(
-      (peer) => peer.protomux === this.#protomux
-    )
-    if (!peerToUnreplicate) return
-    this.#log('unreplicating core %k', core.key)
-    peerToUnreplicate.channel.close()
+
     this.#replicatingCores.delete(core)
+
+    const isCoreReady = Boolean(core.discoveryKey)
+    if (!isCoreReady) {
+      await core.ready()
+      const wasReEnabledWhileWaiting = this.#replicatingCores.has(core)
+      if (wasReEnabledWhileWaiting) return
+    }
+
+    unreplicate(core, this.#protomux)
+    this.#log('unreplicated core %k', core.key)
   }
 
   /**