How to Test Race Conditions in a Browser Extension an AI Agent Modifies
Short answer: define what must stay true when actions overlap, control their completion order in the test, and add a real browser scenario. A test in which two calls run one after another does not prove the code handles overlap. Define the invariant before writing the test A browser extension connects local code to state that changes outside it. A tab can close, a group can change, and a…
To test race conditions in a browser extension, it is crucial to define the invariant that must remain constant during overlapping actions. This invariant should be established before writing the test. A browser extension connects local code to state that can change outside of it, so the test should focus on preserving this state between the start and completion of an action.
In the example provided, two requests for a group's name on the same tab could result in two groups being created. The fix for this issue involved ordering actions per tab and re-reading the tab state inside a lock. The test should create a scenario where two requests see a tab with no group, allowing one to prepare to create a group while the other runs concurrently.
The first call's promise should be held open, and the second started before completing the first. Afterward, the number of times group creation ran and the tab's final state should be checked. If the requirement is one group, then two successful results are insufficient. The test should represent the real interface, not an artificial one that only accommodates the code's convenience.
In cases where multiple resources are shared across the extension, a different approach is needed. For instance, PR #72 introduces a single queue for acquiring a debugger connection to prevent two overlapping acquisitions. It is essential to define which actions share state and determine which can run in parallel. When a guard is released, the action must ensure that the next action can proceed without blocking the queue, even if there is an error within the action.
Connection ownership should be separated from external signals. A failed debugger connection rejects the promise, and an onDetach event occurs when the browser ends the session. A probe can distinguish a connection the extension still holds after a worker restart from a connection failure. Tests should be conducted for both paths, against the behavior the code actually implements.
A test at the Chrome boundary is necessary to ensure that permissions, browser messages, and close events behave as expected. This test should include scenarios such as two overlapping requests, a tab closing mid-action, and DevTools opening during a session. The extension must have the debugger permission in the manifest, and using the tabGroups API requires the tabGroups permission.
The report should separate the layers tested, such as the mock, the browser, the event order, and the version against which the test was run. It is unnecessary to claim coverage of every possible race condition, but rather to provide evidence for the tested scenario and establish clear boundaries for what still needs testing.
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