feat(Subscription): idempotent add and remove of teardowns

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);