Fixing Flaky Tests: Sourcemap Expansion In Sentry

Aug 15, 2025 by Omar Yusuf 50 views

Flaky Test: Sourcemap Source Expansion in Sentry

Hey everyone,

We've got a flaky test situation on our hands, and it's time to tackle it! This article dives into a specific flaky test within the Sentry project, exploring the details, potential solutions, and the importance of maintaining a stable testing environment. Let's get to it!

What's a Flaky Test?

Before we dive into the specifics, let's clarify what we mean by a "flaky test." In the world of software testing, a flaky test is one that sometimes passes and sometimes fails, even when the code hasn't changed. This inconsistent behavior can be incredibly frustrating, as it makes it difficult to trust the test results and can mask genuine issues within the codebase. Flaky tests can stem from various root causes, such as timing issues, external dependencies, or concurrency problems. Identifying and addressing these tests is crucial for maintaining a reliable and efficient development workflow.

Why are Flaky Tests a Problem?

They erode trust: If tests are failing intermittently, developers start to ignore them, which means real bugs can slip through the cracks.
They slow down development: Debugging flaky tests is time-consuming and frustrating. Developers spend precious time chasing phantom issues instead of building new features.
They can lead to false positives: A passing flaky test might give a false sense of security, while a failing one might indicate a problem where none exists.
They make Continuous Integration/Continuous Deployment (CI/CD) pipelines unreliable: Flaky tests can cause builds to fail randomly, disrupting the automated deployment process.

The Case of `test_sourcemap_source_expansion`

Now, let's focus on the specific flaky test we're dealing with: tests/relay_integration/lang/javascript/test_plugin.py::TestJavascriptIntegration::test_sourcemap_source_expansion. This test is part of Sentry's integration tests, specifically within the JavaScript language support and plugin functionality. It appears to be related to source map handling, which is a crucial aspect of debugging JavaScript applications.

Understanding Source Maps

To grasp the context of this test, it's essential to understand what source maps are. When JavaScript code is minified and bundled for production, it becomes difficult to read and debug. Source maps bridge this gap by mapping the transformed code back to the original source code. This allows developers to debug their applications in the browser using the original, human-readable code, even though the deployed code is minified.

The Role of `test_sourcemap_source_expansion`

The test_sourcemap_source_expansion test likely verifies that Sentry can correctly process source maps and expand the source code information when an error occurs. This ensures that Sentry can accurately pinpoint the location of errors in the original code, making debugging much easier for developers using Sentry to monitor their applications.

Flakiness Statistics

Over the past 30 days, this test has been run 996 times. Out of those runs, it failed 1 time (0.100402%) and was retried 3 times (0.301205%). While the failure rate might seem low, even a small percentage of flakiness can be disruptive, especially in a large codebase with many tests.

Example Flakes

The report provides links to specific instances where the test flaked:

These links are invaluable for debugging the issue. By examining the logs and artifacts from these failed runs, developers can gain insights into the potential causes of the flakiness.

Addressing the Flakiness: What are our Options?

So, what do we do about this flaky test? The report outlines three main options:

Fix the flakiness: This is the ideal solution. It involves diving deep into the test code and the underlying functionality to identify the root cause of the flakiness and implement a fix. This might involve adding retries with appropriate timeouts, mocking external dependencies, or addressing race conditions.
Delete the test: If the test's value is deemed lower than its costs (in terms of flakiness and maintenance), it might be best to remove it. This decision should be made by the test owner, considering the test's purpose and importance.
Reassign the issue: If the current assignee isn't the best person to address the flakiness, the issue can be reassigned to someone with more relevant expertise. This might be a different team member or even a specific team, like the team-devinfra if the flakiness stems from the testing process itself.

Option 1: Fixing the Flakiness - A Deep Dive

Let's explore the first option, fixing the flakiness, in more detail. This is often the most challenging but also the most rewarding approach. Here's a breakdown of the steps involved:

Reproduce the Flakiness Locally: The first step is to try to reproduce the flakiness on a local development environment. This allows for easier debugging and experimentation. Tools like pytest-rerunfailures can be helpful in automatically rerunning failing tests.
Analyze the Logs and Error Messages: Examine the logs from the failed test runs, both in the CI environment and locally. Look for any error messages, stack traces, or unusual patterns that might provide clues about the root cause.
Inspect the Test Code: Carefully review the test code, paying close attention to areas that might be susceptible to flakiness. Look for:
- Timing issues: Are there any asynchronous operations or external calls that might be taking longer than expected?
- Race conditions: Could multiple threads or processes be interfering with each other?
- External dependencies: Does the test rely on any external services or databases that might be unreliable?
- State management: Is the test properly cleaning up any resources or state after it runs?
Add Logging and Debugging Statements: Add more logging statements to the test code to provide more context during test execution. Use debugging tools to step through the code and observe the behavior of variables and function calls.
Implement Fixes and Test Thoroughly: Once a potential cause is identified, implement a fix and test it thoroughly. This might involve adding retries with appropriate timeouts, mocking external dependencies, or addressing race conditions.
Monitor the Test in CI: After deploying the fix, monitor the test in the CI environment to ensure that the flakiness has been resolved.

Potential Causes for `test_sourcemap_source_expansion`

Given that this test involves source map expansion, here are some potential causes for the flakiness:

Timing issues in source map generation or processing: The source maps might not be fully generated or available when the test tries to use them.
Inconsistencies in the source map format: Different tools or versions of tools might generate slightly different source maps, leading to inconsistencies in the test results.
Problems with the source code being mapped: The source code itself might contain errors or inconsistencies that interfere with the source map expansion process.
Resource limitations: The test might be running in an environment with limited resources, causing timeouts or other issues.

Option 2: Deleting the Test - A Last Resort

Deleting a test should be a last resort, but it's a valid option if the test's value is outweighed by its costs. This might be the case if:

The test is redundant or covers functionality that is already well-tested elsewhere.
The test is overly complex or difficult to maintain.
The test is inherently flaky and cannot be reliably fixed.

Before deleting a test, it's crucial to carefully consider the implications and ensure that the functionality it covers is adequately tested by other means.

Option 3: Reassigning the Issue - Finding the Right Owner

Reassigning the issue is important if the current assignee doesn't have the necessary expertise or context to address the flakiness. If the flakiness is caused by a flaw in the test process itself, the team-devinfra might be the best owner. Clear communication and collaboration are essential to ensure that the issue is assigned to the right person or team.

The Two-Week Deadline

The report mentions that if this issue has no movement for two weeks, a PR proposing to delete the flaky test might be created. This highlights the urgency of addressing flaky tests and the importance of taking action promptly. No pressure, guys!

Conclusion: Let's Squash This Flaky Test!

Flaky tests are a nuisance, but they're also an opportunity to improve the stability and reliability of our codebase. By working together and systematically addressing these issues, we can ensure that our tests provide accurate and trustworthy feedback. Let's squash this test_sourcemap_source_expansion flakiness and keep our tests green!

Let's recap the key takeaways:

Flaky tests are tests that sometimes pass and sometimes fail without any code changes.
They erode trust, slow down development, and can lead to false positives.
test_sourcemap_source_expansion is a flaky test in Sentry related to source map handling.
We have three options: fix the flakiness, delete the test, or reassign the issue.
Fixing the flakiness involves reproducing the issue, analyzing logs, inspecting code, and implementing fixes.
Deleting the test should be a last resort.
Reassigning the issue ensures the right person addresses the problem.
There's a two-week deadline to address the flakiness.
Sentry has a Flaky Test Policy & Responsibilities document for guidance.

So, let's roll up our sleeves, dig into the code, and make our tests more reliable. Happy debugging!