Avoid some server fetches for sender:foo queries.

If we find unread messages for a sender, we will
try to render locally narrow for sender searches.

Note that our current implementation brute forces
through all the unread ids.  We can improve this,
although it's not really a bottleneck until we
also support buckets for general filtering.

frontend_tests/node_tests/narrow_unread.js

@@ -71,7 +71,7 @@ function candidate_ids() {
     assert.equal(unread_ids, undefined);
 
     terms = [
-        {operator: 'sender', operand: 'me@example.com'},
+        {operator: 'bogus_operator', operand: 'me@example.com'},
     ];
     set_filter(terms);
     unread_ids = candidate_ids();
@@ -129,6 +129,17 @@ function candidate_ids() {
     unread_ids = candidate_ids();
     assert.deepEqual(unread_ids, [stream_msg.id]);
 
+    terms = [
+        {operator: 'sender', operand: 'me@example.com'},
+    ];
+    set_filter(terms);
+    // note that our candidate ids are just "all" ids now
+    unread_ids = candidate_ids();
+    assert.deepEqual(unread_ids, [stream_msg.id]);
+
+    // this actually does filtering
+    assert_unread_info({flavor: 'not_found'});
+
     terms = [
         {operator: 'pm-with', operand: 'alice@example.com'},
     ];

static/js/narrow_state.js

@@ -256,6 +256,11 @@ exports._possible_unread_message_ids = function () {
         return unread.get_msg_ids_for_starred();
     }
 
+    if (current_filter.can_bucket_by('sender')) {
+        // TODO: see #9352 to make this more efficient
+        return unread.get_all_msg_ids();
+    }
+
     return;
 };