message_fetch: Raise fatal error in an impossible case.

When fetching older/new messages, we used to resort to the pointer
to act as anchor when message list was empty.

This appears to be an impossible case, as
`fetch_status.can_load_newer_messages`
should be false in this case and user cannot be scrolling an
empty message_list in the first case.

Hence, we raise a fatal error to inform user of the same.

frontend_tests/node_tests/message_fetch.js

@@ -16,9 +16,8 @@ zrequire('people');
 set_global('recent_topics', {
     process_messages: noop,
 });
-set_global('page_params', {
+// Still required for page_params.initial_pointer
-    pointer: 444,
+set_global('page_params', {});
-});
 set_global('ui_report', {
     hide_error: noop,
 });
@@ -311,6 +310,23 @@ run_test('loading_newer', () => {
             can_call_again: true,
         });
 
+        // The msg_list is empty and we are calling frontfill, which should
+        // raise fatal error.
+        if (opts.empty_msg_list) {
+            assert.throws(
+                () => {
+                    message_fetch.maybe_load_newer_messages({
+                        msg_list: msg_list,
+                        show_loading: noop,
+                        hide_loading: noop,
+                    });
+                },
+                {
+                    name: "Error",
+                    message: "There are no message available to frontfill.",
+                }
+            );
+        } else {
             message_fetch.maybe_load_newer_messages({
                 msg_list: msg_list,
                 show_loading: noop,
@@ -324,6 +340,7 @@ run_test('loading_newer', () => {
                 response: data.resp,
             });
         }
+    }
 
     (function test_narrow() {
         const msg_list = simulate_narrow();
@@ -345,6 +362,18 @@ run_test('loading_newer', () => {
         test_happy_path({
             msg_list: msg_list,
             data: data,
+            empty_msg_list: true,
+        });
+
+        msg_list.append_to_view = () => {};
+        // Instead of using 444 as page_param.pointer, we
+        // should have a message with that id in the message_list.
+        msg_list.append(message_range(444, 445), false);
+
+        test_happy_path({
+            msg_list: msg_list,
+            data: data,
+            empty_msg_list: false,
         });
 
         assert.equal(msg_list.data.fetch_status.can_load_newer_messages(), true);
@@ -402,6 +431,16 @@ run_test('loading_newer', () => {
         test_happy_path({
             msg_list: msg_list,
             data: data[0],
+            empty_msg_list: true,
+        });
+
+        message_list.all.append_to_view = () => {};
+        message_list.all.append(message_range(444, 445), false);
+
+        test_happy_path({
+            msg_list: msg_list,
+            data: data[0],
+            empty_msg_list: false,
         });
 
         assert.equal(msg_list.data.fetch_status.can_load_newer_messages(), true);

static/js/message_fetch.js

@@ -252,18 +252,19 @@ exports.load_messages_for_narrow = function (opts) {
 };
 
 exports.get_backfill_anchor = function (msg_list) {
-    let oldest_message_id;
-
     if (msg_list === home_msg_list) {
         msg_list = message_list.all;
     }
 
-    if (msg_list.first() === undefined) {
+    const oldest_msg = msg_list.first();
-        oldest_message_id = page_params.pointer;
+    if (oldest_msg) {
-    } else {
+        return oldest_msg.id;
-        oldest_message_id = msg_list.first().id;
     }
-    return oldest_message_id;
+
+    // msg_list is empty, which is an impossible
+    // case, raise a fatal error.
+    blueslip.fatal("There are no message available to backfill.");
+    return;
 };
 
 exports.get_frontfill_anchor = function (msg_list) {
@@ -277,7 +278,13 @@ exports.get_frontfill_anchor = function (msg_list) {
         return last_msg.id;
     }
 
-    return page_params.pointer;
+    // Although it is impossible that we reach here since we
+    // are already checking `msg_list.fetch_status.can_load_newer_messages`
+    // and user cannot be scrolling down on an empty message_list to
+    // fetch more data, and if user is, then the available data is wrong
+    // and we raise a fatal error.
+    blueslip.fatal("There are no message available to frontfill.");
+    return;
 };
 
 exports.maybe_load_older_messages = function (opts) {
@@ -315,7 +322,7 @@ exports.do_backfill = function (opts) {
 };
 
 exports.maybe_load_newer_messages = function (opts) {
-    // This function gets called when you scroll to the top
+    // This function gets called when you scroll to the bottom
     // of your window, and you want to get messages newer
     // than what the browsers originally fetched.
     const msg_list = opts.msg_list;