DOM: Implement finally() Observable operator

See WICG/observable#153. This CL implements the
`finally()` operator. It registers a callback which runs on both
producer-initiated and consumer-initiated unsubscription. Its
implementation is relatively straightforward, just utilizing the
relevant subscriber's teardown list.

This CL used to be really complicated, as was the spec PR. This has all
been simplified since the factors that made it complicated (and broken)
have been landed. See:
 - WICG/observable#151 (comment)
 - WICG/observable#154
 - https://crrev.com/c/5676226

Now that the simplifying factors have been landed, this CL can proceed
with a simple implementation, which passes all of the tests.

[email protected]

Bug: 40282760
Change-Id: Ifff220ec094abd2187f35020eae85c1bca502f89
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5654720
Reviewed-by: Di Zhang <[email protected]>
Commit-Queue: Dominic Farolino <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1423349}

dom/observable/tentative/observable-finally.any.js

@@ -0,0 +1,265 @@
+// Because we test that the global error handler is called at various times.
+setup({allow_uncaught_exception: true});
+
+test(() => {
+  const source = new Observable((subscriber) => {
+    subscriber.next(1);
+    subscriber.next(2);
+    subscriber.next(3);
+    subscriber.complete();
+  });
+
+  const results = [];
+
+  source
+    .finally(() => {
+      results.push("finally called");
+    })
+    .subscribe({
+      next: (value) => results.push(value),
+      error: (e) => results.push(e.message),
+      complete: () => results.push("complete"),
+    });
+
+  assert_array_equals(results, [1, 2, 3, "finally called", "complete"],
+      "finally is called with teardown timing, before complete() is forwarded");
+}, "finally(): Mirrors all values and completions from source");
+
+test(() => {
+  const source = new Observable((subscriber) => {
+    subscriber.next(1);
+    subscriber.next(2);
+    subscriber.next(3);
+    subscriber.error(new Error("error from source"));
+  });
+
+  const results = [];
+
+  source
+    .finally(() => {
+      results.push("finally called");
+    })
+    .subscribe({
+      next: (value) => results.push(value),
+      error: (e) => results.push(e.message),
+      complete: () => results.push("complete"),
+    });
+
+  assert_array_equals(results, [1, 2, 3, "finally called", "error from source"],
+      "finally is called with teardown timing, before complete() is forwarded");
+}, "finally(): Mirrors all values and errors from the source");
+
+test(() => {
+  const results = [];
+
+  const source = new Observable((subscriber) => {
+    results.push("source subscribe");
+    subscriber.addTeardown(() => results.push("source teardown"));
+    results.push("source send complete");
+    subscriber.complete();
+  });
+
+  const result = source.finally(() => {
+    results.push("finally handler");
+  });
+
+  result.subscribe({
+    complete: () => results.push("result complete"),
+  });
+
+  assert_array_equals(results, [
+    "source subscribe",
+    "source send complete",
+    "source teardown",
+    "finally handler",
+    "result complete",
+  ]);
+}, "finally(): Callback handler fires BEFORE the source observable completes");
+
+test(() => {
+  const results = [];
+
+  const source = new Observable((subscriber) => {
+    results.push("source subscribe");
+    subscriber.addTeardown(() => results.push("source teardown"));
+    results.push("source send error");
+    subscriber.error(new Error("error from source"));
+  });
+
+  const result = source.finally(() => {
+    results.push("finally handler");
+  });
+
+  result.subscribe({
+    error: (e) => results.push(e.message),
+  });
+
+  assert_array_equals(results, [
+    "source subscribe",
+    "source send error",
+    "source teardown",
+    "finally handler",
+    "error from source",
+  ]);
+}, "finally(): Callback handler fires BEFORE the source observable errors");
+
+test(() => {
+  const results = [];
+
+  const source = new Observable((subscriber) => {
+    subscriber.complete();
+  });
+
+  const result = source
+    .finally(() => {
+      results.push("finally handler 1");
+    })
+    .finally(() => {
+      results.push("finally handler 2");
+    });
+
+  result.subscribe({ complete: () => results.push("result complete") });
+
+  assert_array_equals(results,
+    ["finally handler 1", "finally handler 2", "result complete"]);
+}, "finally(): Handlers run in composition order");
+
+test(() => {
+  const source = new Observable(subscriber => {
+    subscriber.error("producer error");
+  });
+
+  const results = [];
+
+  self.addEventListener('error', e => results.push(e.error.message), {once: true});
+
+  source
+    .finally(() => {
+      throw new Error("error from finally");
+    })
+    .subscribe({
+      next: () => results.push("next"),
+      error: (e) => results.push(e),
+      complete: () => results.push("complete"),
+    });
+
+  assert_array_equals(results, ["error from finally", "producer error"]);
+}, "finally(): Errors thrown in the finally handler " +
+   "(during Subscriber#error()) are reported to the global immediately");
+
+test(() => {
+  const source = new Observable((subscriber) => {
+    subscriber.complete();
+  });
+
+  const results = [];
+
+  self.addEventListener('error', e => results.push(e.error.message), {once: true});
+
+  source
+    .finally(() => {
+      throw new Error("error from finally");
+    })
+    .subscribe({
+      next: () => results.push("next"),
+      error: (e) => results.push("unreached"),
+      complete: () => results.push("complete"),
+    });
+
+  assert_array_equals(results, ["error from finally", "complete"]);
+}, "finally(): Errors thrown in the finally handler " +
+   "(during Subscriber#complete()) are reported to the global immediately");
+
+test(() => {
+  const results = [];
+
+  const source = new Observable((subscriber) => {
+    subscriber.addTeardown(() => results.push("source teardown"));
+  });
+
+  const controller = new AbortController();
+
+  source
+    .finally(() => results.push("downstream finally handler"))
+    .subscribe({}, { signal: controller.signal });
+
+  controller.abort();
+
+  assert_array_equals(results, ["source teardown", "downstream finally handler"]);
+}, "finally(): Callback is run if consumer aborts the subscription");
+
+test(() => {
+  const results = [];
+  const result = new Observable((subscriber) => {
+    subscriber.next(1);
+    subscriber.next(2);
+    subscriber.complete();
+  }).flatMap((value) => {
+    results.push(`flatMap ${value}`);
+    return new Observable((subscriber) => {
+      subscriber.next(value);
+      subscriber.next(value);
+      subscriber.next(value);
+      subscriber.complete();
+    }).finally(() => {
+      results.push(`finally ${value}`);
+    });
+  });
+
+  result.subscribe({
+    next: (value) => results.push(`result ${value}`),
+    complete: () => results.push("result complete"),
+  });
+
+  assert_array_equals(results, [
+    "flatMap 1",
+    "result 1",
+    "result 1",
+    "result 1",
+    "finally 1",
+    "flatMap 2",
+    "result 2",
+    "result 2",
+    "result 2",
+    "finally 2",
+    "result complete",
+  ]);
+}, "finally(): Callback is run before next inner subscription in flatMap()");
+
+test(() => {
+  const results = [];
+  const result = new Observable((subscriber) => {
+    subscriber.next(1);
+    subscriber.next(2);
+    subscriber.complete();
+  }).switchMap((value) => {
+    results.push(`switchMap ${value}`);
+    return new Observable((subscriber) => {
+      subscriber.next(value);
+      subscriber.next(value);
+      subscriber.next(value);
+      subscriber.complete();
+    }).finally(() => {
+      results.push(`finally ${value}`);
+    });
+  });
+
+  result.subscribe({
+    next: (value) => results.push(`result ${value}`),
+    complete: () => results.push("result complete"),
+  });
+
+  assert_array_equals(results, [
+    "switchMap 1",
+    "result 1",
+    "result 1",
+    "result 1",
+    "finally 1",
+    "switchMap 2",
+    "result 2",
+    "result 2",
+    "result 2",
+    "finally 2",
+    "result complete",
+  ]);
+}, "finally(): Callback is run before next inner subscription in switchMap()");