Fall back to AJAX on a Moodle course-context validation error

src/py_moodle/transport/webservice.py

@@ -14,10 +14,22 @@
 from ..http import MoodleHttpError, MoodleWebserviceError, request_webservice
 from . import TransportError, TransportUnavailableError
 
-#: Substring (case-insensitive) that identifies an invalid/expired token
-#: Moodle exception, distinguishing "try another transport" from a genuine
-#: webservice-level failure.
-_INVALID_TOKEN_MARKER = "invalid token"
+#: Substrings (case-insensitive) identifying a Moodle webservice failure
+#: that means "this call path isn't usable right now, try another
+#: transport" rather than a genuine webservice-level failure worth
+#: surfacing immediately:
+#:   - an invalid/expired token;
+#:   - a webservice-side context-validation failure (observed in practice
+#:     as "You cannot execute functions in the course context (course
+#:     id:N). ... Context does not exist"), e.g. when the target course's
+#:     context is momentarily stale relative to the calling user's cached
+#:     session state. The AJAX transport re-derives context from the
+#:     current page/session instead of a cached webservice-side lookup,
+#:     so it is not subject to the same failure.
+_TRANSPORT_UNAVAILABLE_MARKERS = (
+    "invalid token",
+    "cannot execute functions in the course context",
+)
 
 
 def call(
@@ -41,15 +53,17 @@ def call(
 
     Raises:
         TransportUnavailableError: If Moodle reports an invalid/expired
-            token, indicating the caller should fall back to another
-            transport.
+            token, or a context-validation failure, indicating the caller
+            should fall back to another transport (see
+            :data:`_TRANSPORT_UNAVAILABLE_MARKERS`).
         TransportError: If the call fails for any other reason (network
-            error, non-token Moodle exception, invalid JSON).
+            error, non-token/context Moodle exception, invalid JSON).
     """
     try:
         return request_webservice(session, base_url, wsfunction, params, token=token)
     except MoodleWebserviceError as exc:
-        if _INVALID_TOKEN_MARKER in str(exc).lower():
+        message = str(exc).lower()
+        if any(marker in message for marker in _TRANSPORT_UNAVAILABLE_MARKERS):
             raise TransportUnavailableError(str(exc)) from exc
         raise TransportError(str(exc)) from exc
     except MoodleHttpError as exc:

tests/unit/test_transport.py

@@ -83,6 +83,37 @@ def test_webservice_call_raises_transport_unavailable_on_invalid_token():
         )
 
 
+def test_webservice_call_raises_transport_unavailable_on_context_error():
+    """A 'cannot execute functions in the course context' error signals fallback.
+
+    Regression test for a Moodle webservice quirk observed in CI: a call to
+    a generic, non-course-scoped function like ``core_course_get_courses``
+    can still fail with a stale/invalid course-context validation error
+    unrelated to the token itself. This must be treated the same as an
+    invalid token -- fall back to another transport -- rather than
+    surfacing immediately, since the AJAX transport re-derives context from
+    the current session instead of a cached webservice-side lookup.
+    """
+    session = StubSession(
+        post_result=StubResponse(
+            json_data={
+                "exception": "invalid_parameter_exception",
+                "errorcode": "invalidparameter",
+                "message": (
+                    "You cannot execute functions in the course context "
+                    "(course id:8). The context error message was: Invalid "
+                    "parameter value detected (Context does not exist)"
+                ),
+            }
+        )
+    )
+
+    with pytest.raises(TransportUnavailableError):
+        webservice_transport.call(
+            session, BASE_URL, "core_course_get_courses", FAKE_TOKEN
+        )
+
+
 def test_webservice_call_raises_transport_error_on_other_moodle_exception():
     """A non-token Moodle exception raises a plain TransportError."""
     session = StubSession(
@@ -177,6 +208,37 @@ def test_list_courses_falls_back_to_ajax_on_invalid_token():
     assert result == [{"id": 1}, {"id": 2}]
 
 
+def test_list_courses_falls_back_to_ajax_on_context_error():
+    """list_courses() falls back to AJAX on a course-context validation error.
+
+    Same regression as test_webservice_call_raises_transport_unavailable_on_context_error,
+    exercised through the public list_courses() entry point.
+    """
+    session = StubSession(
+        get_result=StubResponse(text="<html></html>"),
+        post_dispatch={
+            f"{BASE_URL}/webservice/rest/server.php": StubResponse(
+                json_data={
+                    "exception": "invalid_parameter_exception",
+                    "errorcode": "invalidparameter",
+                    "message": (
+                        "You cannot execute functions in the course context "
+                        "(course id:8). The context error message was: Invalid "
+                        "parameter value detected (Context does not exist)"
+                    ),
+                }
+            ),
+            f"{BASE_URL}/lib/ajax/service.php": StubResponse(
+                json_data=[{"error": False, "data": [{"id": 2}, {"id": 1}]}]
+            ),
+        },
+    )
+
+    result = list_courses(session, BASE_URL, token=FAKE_TOKEN, sesskey=FAKE_SESSKEY)
+
+    assert result == [{"id": 1}, {"id": 2}]
+
+
 def test_list_courses_raises_when_neither_transport_usable():
     """list_courses() raises a clear error with no token and no sesskey."""
     session = StubSession()