Mago Analyzer Crashing: Stack Overflow Bug

Nov 30, 2025 by Alex Johnson 43 views

Mago Analyzer Crashing with Stack Overflow: A Deep Dive

Mago analyzer crashed with the message "fatal runtime error: stack overflow, aborting." This issue stems from a stack overflow within the mago analyze process. This is a critical problem, especially since the code being analyzed is valid and passes other checks like code coverage and Psalm. Let's break down the issue, its causes, and potential solutions. The user reports this error when running mago analyze in a PHP project, indicating a problem during the analysis of PHP code.

Understanding the Stack Overflow Error

The "fatal runtime error: stack overflow, aborting" message is a clear indicator that the mago analyzer is exceeding the available stack space during its operation. The stack is a region of memory used to store function calls, local variables, and other data needed by a program during its execution. When a function calls another function, the information about the first function is 'pushed' onto the stack. When the second function finishes, this information is 'popped' off the stack. A stack overflow happens when the stack becomes full, typically due to excessive recursion or deeply nested function calls.

In the context of the Mago analyzer, this means the tool is getting stuck in a cycle of function calls, potentially when analyzing complex type relationships within the PHP code. The provided backtrace from GDB, the debugger, helps pinpoint where the error occurs. It highlights the mago_codex::ttype::combiner::combine and mago_codex::ttype::expander::expand_ functions, suggesting that the type analysis and expansion components are heavily involved in triggering the stack overflow. The repeated calls to expand_union, expand_object and expand_atomic functions are a clear indication of recursive behavior, which, when not properly managed, can exhaust the stack.

Steps to Reproduce and Configuration

The steps to reproduce are straightforward: running mago analyze in the user's project triggers the crash. The user has provided their mago.toml configuration file, which defines the PHP version, source code paths, includes, excludes, and analyzer settings. Notably, the analyzer settings disable some checks (like finding unused definitions and expressions) and enable others (like allowing possibly undefined array keys). While the specific settings might influence the analysis process, they don't seem to be directly related to causing a stack overflow.

The inclusion of vendor directories in the includes section could potentially contribute to the issue if vendor code contains complex type definitions that require deep analysis. However, without access to the actual code, it's hard to definitively say. The user's configuration seems standard for a PHP project using Mago, focusing on analysis within the project's source code directories.

Debugging and Backtrace Analysis

The user’s attempt to debug the issue using GDB is crucial. The backtrace shows a repeated pattern of function calls related to type expansion within the Mago codebase. This recursive nature of the type expansion process appears to be the root cause. Each function call adds data to the stack, and the repeated calls eventually lead to the stack overflow error. The backtrace is very helpful because it provides a snapshot of the execution flow at the moment of the crash.

To further diagnose the problem, developers could analyze the codebase around the lines indicated in the backtrace. It might involve examining how type information is structured, how the expand functions handle different types, and how the recursion is controlled. Adding logging to the expand functions could also provide insights into which types and code paths cause the deepest nesting and most stack usage.

Potential Causes and Solutions

Several factors can contribute to a stack overflow. Complex type definitions, deeply nested structures, or recursive relationships in the PHP code being analyzed can cause a stack overflow. The Mago analyzer might be designed to handle these scenarios, but a specific code pattern could be triggering a pathological case.

Here are the potential causes and solutions:

Infinite Recursion: The most probable cause is an infinite or excessively deep recursion within the type analysis logic. This could occur if the analyzer has trouble resolving complex types, leading to a loop in its internal processes.
- Solution: Review the code around the functions mentioned in the backtrace (combine, expand_union, expand_object, expand_atomic) to identify and fix the recursive loops or inefficiencies. Introduce checks to limit the depth of recursion.
Complex Type Structures: Highly complex type definitions or deeply nested structures in the code may cause the analyzer to consume more stack space.
- Solution: Optimize the type analysis logic to handle complex structures more efficiently. Consider implementing techniques like memoization to cache results and avoid recomputing the same information repeatedly.
Memory Leaks: If the analyzer leaks memory during the analysis of complex code, it may exacerbate stack overflow issues by consuming resources that are used by the stack.
- Solution: Carefully manage memory usage, making sure that resources are properly allocated and deallocated.
Input Data: The specific PHP code being analyzed could contain type patterns that expose a weakness in the analyzer.
- Solution: Provide the developers with a minimal, reproducible test case that triggers the stack overflow. This will help them to isolate the issue and create a fix.

How to Fix the Issue

The primary focus should be on addressing the root cause: the stack overflow. Here’s a detailed approach:

Code Inspection: Examine the code in crates/codex/src/ttype/combiner.rs and crates/codex/src/ttype/expander.rs, as indicated by the backtrace. Look for recursive calls, especially within expand_union, expand_object, and expand_atomic. Identify any potential infinite loops or inefficient recursion patterns.
Add Logging: Insert logging statements within the problematic functions to track the execution flow, the types being processed, and the depth of recursion. This can provide valuable insights into which code paths are causing the issue.
Implement Recursion Limits: Introduce checks to limit the depth of recursion. This can prevent the stack from overflowing. Use a counter to track the recursion depth and abort or return early if a predefined limit is reached.
Optimize Data Structures: Review the data structures used for representing types. Optimize how complex types are stored and processed to reduce the memory footprint and the number of function calls required.
Memoization: Implement memoization to cache the results of expensive computations. This can reduce the number of redundant calculations and potentially mitigate the stack overflow issue.
Create a Test Case: If possible, create a minimal, reproducible test case that triggers the stack overflow. This will help verify that the fix works and prevent regressions in the future.
Profiling: Use a profiler to identify performance bottlenecks within the analyzer. This can help pinpoint areas of the code that are causing the most significant performance overhead and stack usage.
Update Dependencies: Make sure all dependencies are up to date. Sometimes, updates to the underlying libraries can resolve issues that may be contributing to the stack overflow.

Conclusion

The stack overflow error in the Mago analyzer is a significant problem, but it is solvable with the right approach. Debugging, code inspection, and optimization are key to resolving the issue. By focusing on the backtrace, potential causes, and solutions, developers can identify the root cause of the stack overflow and implement effective solutions to improve the stability and reliability of the Mago analyzer.

The steps outlined above provide a structured way to approach the issue, from initial debugging to implementing and testing fixes. By carefully reviewing the code, adding logging, and optimizing the type analysis process, developers can resolve the stack overflow and ensure that the Mago analyzer runs smoothly on a wider range of PHP projects.

For more information on debugging and stack overflow errors, visit Stack Overflow. This platform is a great resource for finding answers to common programming questions and troubleshooting issues. For further assistance or to discuss this problem, you can also consider contributing to the Mago project or seeking help from the community.