filter: Copy is_web_public_compatible from server to frontend.

This will be used to check if the narrow being requested by
spectator requires authentication without requesting the server.
Having this check locally, makes this process look snappy to
the user and doesn't result in 404s in the browser log.

frontend_tests/node_tests/filter.js

@@ -1614,3 +1614,39 @@ test("error_cases", ({override}) => {
     const predicate = get_predicate([["pm-with", "Joe@example.com"]]);
     assert.ok(!predicate({type: "private"}));
 });
+
+run_test("is_web_public_compatible", () => {
+    // tests same as test_is_web_public_compatible from test_message_fetch.py
+    assert.ok(Filter.is_web_public_compatible([]));
+    assert.ok(Filter.is_web_public_compatible([{operator: "has", operand: "attachment"}]));
+    assert.ok(Filter.is_web_public_compatible([{operator: "has", operand: "image"}]));
+    assert.ok(Filter.is_web_public_compatible([{operator: "search", operand: "magic"}]));
+    assert.ok(Filter.is_web_public_compatible([{operator: "near", operand: "15"}]));
+    assert.ok(
+        Filter.is_web_public_compatible([
+            {operator: "id", operand: "15"},
+            {operator: "has", operand: "attachment"},
+        ]),
+    );
+    assert.ok(Filter.is_web_public_compatible([{operator: "sender", operand: "hamlet@zulip.com"}]));
+    assert.ok(
+        !Filter.is_web_public_compatible([{operator: "pm-with", operand: "hamlet@zulip.com"}]),
+    );
+    assert.ok(
+        !Filter.is_web_public_compatible([
+            {operator: "group-pm-with", operand: "hamlet@zulip.com"},
+        ]),
+    );
+    assert.ok(Filter.is_web_public_compatible([{operator: "stream", operand: "Denmark"}]));
+    assert.ok(
+        Filter.is_web_public_compatible([
+            {operator: "stream", operand: "Denmark"},
+            {operator: "topic", operand: "logic"},
+        ]),
+    );
+    assert.ok(!Filter.is_web_public_compatible([{operator: "is", operand: "starred"}]));
+    assert.ok(!Filter.is_web_public_compatible([{operator: "is", operand: "private"}]));
+    assert.ok(Filter.is_web_public_compatible([{operator: "streams", operand: "public"}]));
+    // Malformed input not allowed
+    assert.ok(!Filter.is_web_public_compatible([{operator: "has"}]));
+});

static/js/filter.js

@@ -1,6 +1,7 @@
 import Handlebars from "handlebars/runtime";
 import _ from "lodash";
 
+import * as hash_util from "./hash_util";
 import {$t} from "./i18n";
 import * as message_edit from "./message_edit";
 import * as message_parser from "./message_parser";
@@ -1005,4 +1006,16 @@ export class Filter {
     static describe(operators) {
         return Handlebars.Utils.escapeExpression(Filter.describe_unescaped(operators));
     }
+
+    static is_web_public_compatible(ops) {
+        for (const op of ops) {
+            if (op.operand === undefined) {
+                return false;
+            }
+            if (!hash_util.allowed_web_public_narrows.includes(op.operator)) {
+                return false;
+            }
+        }
+        return true;
+    }
 }

static/js/hash_util.js

@@ -283,6 +283,8 @@ export const allowed_web_public_narrows = [
 ];
 
 export function is_web_public_compatible(hash) {
+    // Defines which views are supported for spectators.
+    // This implementation should agree with the similar function in zerver/lib/narrow.py.
     const web_public_allowed_hashes = [
         "",
         "narrow", // full #narrow hash handled in narrow.is_web_public_compatible

zerver/lib/narrow.py

@@ -30,6 +30,7 @@ def check_supported_events_narrow_filter(narrow: Iterable[Sequence[str]]) -> Non
 
 
 def is_web_public_compatible(narrow: Iterable[Dict[str, Any]]) -> bool:
+    # This implementation should agree with the similar function in static/js/hash_utl.js.
     for element in narrow:
         operator = element["operator"]
         if "operand" not in element: