From T267945#6875458
Neither ApiClient nor ApiClientLogin override ApiBase::isWriteMode(), so they return the default false, which causes database performance warnings (about 35k per day at the time of writing). For example:
Expectation (writes <= 0) by ApiMain::setRequestExpectations not met (actual: 1): query-m: INSERT INTO cu_changes [...]
In this example, the login attempt is logged to the cu_changes table. Given that writes like this legitimately occur on login, isWriteMode should return true.