refactor: Remove maybe_clear_subscribers().

The maybe_clear_subscribers() function was an artifact of
when we used to attach subscribers to the "sub" records in
stream_data.js.  I think it was basically a refactoring
shim, and due to some other recent cleanup, it was only
used in test code.

We also change how we validate stream ids.

Going forward, peer_data just looks up stream_ids with the
normal stream_data API when it's trying to warn about
rogue stream_ids coming in.  As I alluded to in an earlier
commit, some of the warning code here might be overly
defensive, but at least it's pretty self-contained.

Author: showell

frontend_tests/node_tests/compose.js

@@ -972,6 +972,8 @@ run_test("warn_if_private_stream_is_linked", () => {
         stream_id: 100,
         name: "Denmark",
     };
+    stream_data.add_sub(denmark);
+
     peer_data.set_subscribers(denmark.stream_id, [1, 2, 3]);
 
     function test_noop_case(invite_only) {

frontend_tests/node_tests/peer_data.js

@@ -240,7 +240,11 @@ run_test("get_subscriber_count", () => {
 
 run_test("is_subscriber_subset", () => {
     function make_sub(stream_id, user_ids) {
-        const sub = {stream_id};
+        const sub = {
+            stream_id,
+            name: `stream ${stream_id}`,
+        };
+        stream_data.add_sub(sub);
         peer_data.set_subscribers(sub.stream_id, user_ids);
         return sub;
     }

static/js/peer_data.js

@@ -2,9 +2,6 @@ const {LazySet} = require("./lazy_set");
 const people = require("./people");
 
 // This maps a stream_id to a LazySet of user_ids who are subscribed.
-// We maintain the invariant that this has keys for all all stream_ids
-// that we track in the other data structures.  We intialize it during
-// clear_subscriptions.
 let stream_subscribers;
 
 function assert_number(id) {
@@ -14,14 +11,17 @@ function assert_number(id) {
 }
 
 function get_user_set(stream_id) {
-    assert_number(stream_id);
-
     // This is an internal function to get the LazySet of users.
     // We create one on the fly as necessary, but we warn in that case.
+    assert_number(stream_id);
+
+    if (!stream_data.get_sub_by_id(stream_id)) {
+        blueslip.warn("We called get_user_set for an untracked stream: " + stream_id);
+    }
+
     let subscribers = stream_subscribers.get(stream_id);
 
     if (subscribers === undefined) {
-        blueslip.warn("We called get_user_set for an untracked stream: " + stream_id);
         subscribers = new LazySet([]);
         stream_subscribers.set(stream_id, subscribers);
     }
@@ -33,12 +33,6 @@ export function clear() {
     stream_subscribers = new Map();
 }
 
-export function maybe_clear_subscribers(stream_id) {
-    if (!stream_subscribers.has(stream_id)) {
-        set_subscribers(stream_id, []);
-    }
-}
-
 export function is_subscriber_subset(stream_id1, stream_id2) {
     const sub1_set = get_user_set(stream_id1);
     const sub2_set = get_user_set(stream_id2);

static/js/stream_data.js

@@ -211,8 +211,6 @@ exports.add_sub = function (sub) {
     // This function is currently used only by tests.
     // We use create_sub_from_server_data at page load.
     // We use create_streams for new streams in live-update events.
-
-    peer_data.maybe_clear_subscribers(sub.stream_id);
     stream_info.set(sub.name, sub);
     subs_by_stream_id.set(sub.stream_id, sub);
 };