feat(Subscription): idempotent add and remove of teardowns (#6401)

* feat(Subscription): idempotent add and remove of teardowns

- Adding the same instance of a teardown more than once is the same as adding it once
- Removing a teardown with `Subscription.prototype.remove` will remove it much faster

BREAKING CHANGE: Adding the same function instance to a subscription as a teardown multiple times will now result in that function being executed only once on teardown. This brings us inline with the behavior of EventTarget, and also makes removing teardowns faster. The workaround is to make sure you are adding a new function instance to the Subscription each time if you need the same effect.

Resolves #6400

* refactor: use optional chaining

spec/operators/delay-spec.ts

@@ -245,7 +245,7 @@ describe('delay', () => {
         tap({
           next() {
             const [[subscriber]] = subscribeSpy.args;
-            counts.push(subscriber._teardowns.length);
+            counts.push(subscriber._teardowns.size);
           },
           complete() {
             expect(counts).to.deep.equal([1, 1]);

spec/operators/switchAll-spec.ts

@@ -318,7 +318,7 @@ describe('switchAll', () => {
     });
 
     // Expect one child of switchAll(): The oStream
-    expect((sub as any)._teardowns?.[0]._teardowns?.length).to.equal(1);
+    expect([...(sub as any)._teardowns?.values()][0]._teardowns?.size).to.equal(1);
     sub.unsubscribe();
   });
 
@@ -333,10 +333,10 @@ describe('switchAll', () => {
       oStreamControl.next(n); // creates inner
     });
     // Expect one child of switchAll(): The oStream
-    expect((sub as any)._teardowns?.[0]._teardowns?.length).to.equal(1);
+    expect([...((sub as any)._teardowns?.values() ?? [])][0]._teardowns?.size).to.equal(1);
     // Expect two children of subscribe(): The destination and the first inner
     // See #4106 - inner subscriptions are now added to destinations
-    expect((sub as any)._teardowns?.length).to.equal(2);
+    expect((sub as any)._teardowns?.size).to.equal(2);
     sub.unsubscribe();
   });
 

src/internal/Subscription.ts

@@ -34,7 +34,7 @@ export class Subscription implements SubscriptionLike {
    * The list of registered teardowns to execute upon unsubscription. Adding and removing from this
    * list occurs in the {@link #add} and {@link #remove} methods.
    */
-  private _teardowns: Exclude<TeardownLogic, void>[] | null = null;
+  private _teardowns: Set<Exclude<TeardownLogic, void>> | null = null;
 
   /**
    * @param initialTeardown A function executed first as part of the teardown
@@ -134,7 +134,7 @@ export class Subscription implements SubscriptionLike {
           }
           teardown._addParent(this);
         }
-        (this._teardowns = this._teardowns ?? []).push(teardown);
+        (this._teardowns = this._teardowns ?? new Set()).add(teardown);
       }
     }
   }
@@ -189,8 +189,7 @@ export class Subscription implements SubscriptionLike {
    * @param teardown The teardown to remove from this subscription
    */
   remove(teardown: Exclude<TeardownLogic, void>): void {
-    const { _teardowns } = this;
-    _teardowns && arrRemove(_teardowns, teardown);
+    this._teardowns?.delete(teardown);
 
     if (teardown instanceof Subscription) {
       teardown._removeParent(this);