Intermittent `test-sheet-10` Failure On MacOS Intel

This article addresses an intermittent failure observed in the test-sheet-10 test specifically on macOS Intel runners within the Continuous Integration (CI) environment of the liquidaty/zsv project. This issue manifests as a discrepancy between the expected output and the actual output generated during the test execution. This document details the problem, the observed behavior, and potential causes, and future steps to be taken.

Understanding the test-sheet-10 Failure

The test-sheet-10 test is a crucial component of the liquidaty/zsv project's testing suite. It evaluates the functionality related to data filtering and indexing within the zsv library, a high-performance CSV processing tool. The test involves processing a large dataset, filtering it based on specific criteria, and then comparing the resulting output with a pre-defined expected output. Any mismatch between the generated and expected outputs indicates a failure in the filtering or indexing logic or a problem with the test setup itself.

The Specific Failure Scenario

The failure in question occurs intermittently on macOS Intel runners. This means that the test passes successfully in most runs but fails sporadically. The error logs indicate that the generated output differs from the expected output, specifically in the row ordering or content of the filtered data. The error message highlights the differing lines and characters, providing a starting point for debugging. It's important to note that rerunning the CI workflow often results in a successful test execution, suggesting a non-deterministic factor at play.

Analyzing the Error Logs

The provided error logs offer valuable clues into the nature of the failure. The logs show a comparison between two output files: tmp/test-sheet-10.out (the generated output) and expected/test-sheet-10.out (the expected output). The cmp command reveals a difference starting at character 86 on line 2. This discrepancy indicates that the outputs diverge relatively early in the test execution, potentially due to an issue with the initial filtering or indexing steps. Furthermore, the logs highlight specific rows that are present in one output but missing or incorrectly ordered in the other. These rows provide concrete examples of the data inconsistencies causing the failure.

Possible Causes and Initial Hypotheses

Given the intermittent nature of the failure and the specific error patterns observed, several potential causes come to mind:

Race Conditions and Concurrency Issues

One possibility is a race condition within the test or the underlying zsv library. Race conditions occur when multiple threads or processes access and modify shared resources concurrently, leading to unpredictable outcomes. In the context of test-sheet-10, a race condition could manifest as incorrect data ordering or missing rows in the filtered output. This is particularly plausible given the test involves filtering a large dataset, which may involve parallel processing or multi-threaded operations. The intermittent nature of the failure aligns with the characteristics of race conditions, as their occurrence depends on the timing and interleaving of thread executions.

Timing-Related Issues

Another potential cause is a timing-related issue, particularly one related to screen buffer updates within timeout. The initial assessment in the issue description suggests that the failure might stem from a problem with how the test interacts with the screen buffer or a timeout mechanism. If the test relies on specific timing assumptions related to screen updates, variations in system load or resource availability on the macOS Intel runners could lead to missed updates or premature timeouts. This hypothesis is supported by the fact that rerunning the workflow often succeeds, suggesting that the timing issue is sensitive to environmental factors.

System Resource Constraints

macOS Intel runners in the CI environment may have resource constraints that contribute to the failure. Limited memory, CPU availability, or disk I/O bandwidth could impact the performance and stability of the test execution. If test-sheet-10 is resource-intensive, particularly in terms of memory usage or disk I/O, it may be more susceptible to failures on runners with constrained resources. The intermittent nature of the failure could be explained by variations in resource availability on the runners over time. Investigating the resource utilization of the test during execution could provide insights into this possibility.

Flaky Test Logic or Dependencies

A less likely, but still possible, cause is a flaw in the test logic itself or in one of its dependencies. The test may contain subtle errors or assumptions that lead to incorrect results under certain circumstances. Alternatively, a dependency used by the test may exhibit flaky behavior, causing intermittent failures. To rule out this possibility, a thorough review of the test code and its dependencies is necessary.

Steps Taken and Future Investigation

Documentation and Issue Creation

The first step in addressing this issue was to document the observed behavior and create an issue to track the investigation. This ensures that the problem is not overlooked and that progress is recorded. The initial issue description provides a valuable starting point for further analysis.

Rerunning the CI Workflow

Rerunning the CI workflow has been a useful approach for confirming the intermittent nature of the failure. While rerunning doesn't fix the underlying problem, it helps to establish the frequency and patterns of the failures. This information can be valuable in identifying potential causes and prioritizing investigation efforts.

Future Investigation Plan

To gain a deeper understanding of the test-sheet-10 failure, the following steps are planned:

1. In-depth Log Analysis: A more detailed analysis of the error logs will be conducted to identify specific patterns and correlations. This includes examining the differences between successful and failed test runs to pinpoint the exact conditions that trigger the failure.
2. Resource Utilization Monitoring: Monitoring the resource utilization (CPU, memory, disk I/O) of the test execution on the macOS Intel runners will help determine if resource constraints are contributing to the problem. Tools and techniques for resource monitoring within the CI environment will be employed.
3. Code Review: A thorough review of the test-sheet-10 code and the relevant parts of the zsv library will be conducted to identify potential race conditions, timing issues, or other logical errors. This includes examining the filtering and indexing logic, as well as the code that interacts with the screen buffer or timeout mechanisms.
4. Test Environment Isolation: Attempts will be made to isolate the test environment to minimize external factors that could be contributing to the failure. This may involve running the test in a controlled environment with specific resource allocations and configurations.
5. Debugging and Instrumentation: Debugging tools and instrumentation techniques will be used to trace the execution of the test and identify the point at which the failure occurs. This may involve adding logging statements, breakpoints, or other debugging aids to the code.
6. Concurrency Testing: If race conditions are suspected, specialized concurrency testing tools and techniques will be used to detect and reproduce the issue. This may involve simulating concurrent access to shared resources and analyzing the resulting behavior.

Conclusion

The intermittent failure of test-sheet-10 on macOS Intel runners presents a challenge to the stability and reliability of the liquidaty/zsv project's CI pipeline. By systematically investigating the potential causes, gathering data, and employing appropriate debugging techniques, it is expected that the root cause of the failure can be identified and addressed. The investigation plan outlined above provides a roadmap for this process, and progress will be tracked and documented as it unfolds.

For more information on Continuous Integration and testing best practices, you can visit the Continuous Integration Wikipedia page. This external resource offers a comprehensive overview of CI concepts, methodologies, and benefits.