drafts: Rename undefined draft topics to empty string.

This fixes an error state that came out of #22094.
The code causing the error was fixed in #23238 but some
drafts still have undefined topics which has some strange
side effects. One day we'll have typescript help catch
this kind of thing!

More details on CZO: https://chat.zulip.org/#narrow/stream/9-issues/topic/live.20update.20issue.20with.20drafts/near/1457913

frontend_tests/node_tests/drafts.js

@@ -382,9 +382,7 @@ test("rename_stream_recipient", ({override_rewire}) => {
 // There were some buggy drafts that had their topics
 // renamed to `undefined` in #23238.
 // TODO/compatibility: The next two tests can be deleted
-// in 2023 since all relevant drafts will have either
-// been run through this code or else been deleted after
-// 30 days.
+// when we get to delete drafts.fix_drafts_with_undefined_topics.
 test("catch_buggy_draft_error", () => {
     const stream_A = {
         subscribed: false,
@@ -428,6 +426,30 @@ test("catch_buggy_draft_error", () => {
     assert.equal(draft.topic, undefined);
 });
 
+test("fix_buggy_draft", ({override_rewire}) => {
+    override_rewire(drafts, "set_count", noop);
+
+    const buggy_draft = {
+        stream: "stream name",
+        stream_id: 1,
+        // This is the bug: topic never be undefined for a stream
+        // message draft.
+        topic: undefined,
+        type: "stream",
+        content: "Test stream message",
+        updatedAt: Date.now(),
+    };
+    const data = {id1: buggy_draft};
+    const ls = localstorage();
+    ls.set("drafts", data);
+    const draft_model = drafts.draft_model;
+
+    drafts.fix_drafts_with_undefined_topics();
+    const draft = draft_model.getDraft("id1");
+    assert.equal(draft.stream, "stream name");
+    assert.equal(draft.topic, "");
+});
+
 test("delete_all_drafts", () => {
     const draft_model = drafts.draft_model;
     const ls = localstorage();

static/js/drafts.js

@@ -106,6 +106,32 @@ export const draft_model = (function () {
     return exports;
 })();
 
+// A one-time fix for buggy drafts that had their topics renamed to
+// `undefined` when the topic was moved to another stream without
+// changing the topic. The bug was introduced in
+// 4c8079c49a81b08b29871f9f1625c6149f48b579 and fixed in
+// aebdf6af8c6675fbd2792888d701d582c4a1110a; but servers running
+// intermediate versions may have generated some bugged drafts with
+// this invalid topic value.
+//
+// TODO/compatibility: This can be deleted once servers can no longer
+// directly upgrade from Zulip 6.0beta1 and earlier development branch where the bug was present,
+// since we expect bugged drafts will have either been run through
+// this code or else been deleted after 30 (DRAFT_LIFETIME) days.
+let fixed_buggy_drafts = false;
+export function fix_drafts_with_undefined_topics() {
+    const data = draft_model.get();
+    for (const draft_id of Object.keys(data)) {
+        const draft = data[draft_id];
+        if (draft.type === "stream" && draft.topic === undefined) {
+            const draft = data[draft_id];
+            draft.topic = "";
+            draft_model.editDraft(draft_id, draft, false);
+        }
+    }
+    fixed_buggy_drafts = true;
+}
+
 export function sync_count() {
     const drafts = draft_model.get();
     set_count(Object.keys(drafts).length);
@@ -773,6 +799,10 @@ export function set_initial_element(drafts) {
 export function initialize() {
     remove_old_drafts();
 
+    if (!fixed_buggy_drafts) {
+        fix_drafts_with_undefined_topics();
+    }
+
     window.addEventListener("beforeunload", () => {
         update_draft();
     });