api: Add limit of total messages available per request of 5000.

Tweaked by tabbott to use a declared constant rather than just use
5000 in multiple places; this also means we can change the count
without updating translations.

Fixes #10446.

templates/zerver/api/get-messages.md

@@ -14,8 +14,7 @@ this endpoint to fetch the messages matching any search query that is
 supported by Zulip's powerful full-text search backend.
 
 When a narrow is not specified, it can be used to fetch a user's
-entire message history (though we recommend paginating to 1000
-messages at a time).
+message history (We recommend paginating to 1000 messages at a time).
 
 In either case, you specify an `anchor` message (or ask the server to
 calculate the first unread message for you and use that as the
@@ -26,7 +25,9 @@ whether there are more messages matching the query that were not
 returned due to the `num_before` and `num_after` limits.
 
 We recommend using `num_before <= 1000` and `num_after <= 1000` to
-avoid generating very large HTTP responses.
+avoid generating very large HTTP responses. A maximum of 5000 messages
+can be obtained per request; attempting to exceed this will result in an
+error.
 
 ## Usage examples
 

zerver/tests/test_narrow.py

@@ -1705,6 +1705,19 @@ class GetOldMessagesTest(ZulipTestCase):
             self.assert_json_error(result,
                                    "Missing '%s' argument" % (required_args[i][0],))
 
+    def test_get_messages_limits(self) -> None:
+        """
+        A call to GET /json/messages requesting more than
+        MAX_MESSAGES_PER_FETCH messages returns an error message.
+        """
+        self.login(self.example_email("hamlet"))
+        result = self.client_get("/json/messages", dict(anchor=1, num_before=3000, num_after=3000))
+        self.assert_json_error(result, "Too many messages requested (maximum 5000).")
+        result = self.client_get("/json/messages", dict(anchor=1, num_before=6000, num_after=0))
+        self.assert_json_error(result, "Too many messages requested (maximum 5000).")
+        result = self.client_get("/json/messages", dict(anchor=1, num_before=0, num_after=6000))
+        self.assert_json_error(result, "Too many messages requested (maximum 5000).")
+
     def test_bad_int_params(self) -> None:
         """
         num_before, num_after, and narrow must all be non-negative

zerver/views/messages.py

@@ -52,6 +52,7 @@ import ujson
 import datetime
 
 LARGER_THAN_MAX_MESSAGE_ID = 10000000000000000
+MAX_MESSAGES_PER_FETCH = 5000
 
 class BadNarrowOperator(JsonableError):
     code = ErrorCode.BAD_NARROW
@@ -694,6 +695,9 @@ def get_messages_backend(request: HttpRequest, user_profile: UserProfile,
                          apply_markdown: bool=REQ(validator=check_bool, default=True)) -> HttpResponse:
     if anchor is None and not use_first_unread_anchor:
         return json_error(_("Missing 'anchor' argument (or set 'use_first_unread_anchor'=True)."))
+    if num_before + num_after > MAX_MESSAGES_PER_FETCH:
+        return json_error(_("Too many messages requested (maximum %s)."
+                            % (MAX_MESSAGES_PER_FETCH,)))
     include_history = ok_to_include_history(narrow, user_profile)
 
     if include_history: