Bun Crashes On MacOS Silicon: Memory Leak & Fix

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Bun on macOS Silicon: Crashing with Memory Leaks and How to Fix It

Hey guys! Ever run into a situation where your code just completely shuts down out of the blue? That’s exactly what's been happening with Bun, the super-fast JavaScript runtime, on macOS Silicon. We’re talking about a critical issue: a segmentation fault that leads to crashes and a massive memory leak. This article will dive deep into what’s going on, why it’s happening, and what you can do about it. Buckle up, because we're about to explore the depths of memory management and crash reports!

Understanding the Bun Crash: The Core Problem

Okay, so what exactly is this problem? Imagine trying to pour water into a glass, but the glass has a giant hole. That's essentially what's happening with Bun. Bun, on macOS Silicon, is experiencing a nasty memory leak, which leads to a segmentation fault at address 0x0. A segmentation fault is a fancy term for a critical error that happens when your program tries to access memory it shouldn't be accessing. In this case, it's like the program is trying to read data from a location that doesn't exist – a null pointer dereference, to be exact. The crash report points to the pas_segregated_exclusive_view_create function, part of WebKit's libpas memory allocator. This allocator is crucial because it manages how Bun uses memory.

The Culprit: Memory Leaks and Null Pointers

The root of the problem seems to be the way Bun manages memory on Apple Silicon. Specifically, the libpas allocator within WebKit is struggling. This leads to several issues:

  • Memory Exhaustion: Bun's memory usage balloons rapidly, reaching up to 44GB of RSS (Resident Set Size - the amount of memory actually used by the process) and 63.60GB peak. That’s a lot, especially if your machine has limited RAM. The application continues allocating memory without releasing it, a classic memory leak.
  • Fragmentation: As Bun allocates and frees memory, the available memory becomes fragmented. Think of it like a jigsaw puzzle where the pieces don't fit together anymore. The libpas allocator fails to create new memory views.
  • Null Pointer Dereference: Because of these issues, the program tries to access a memory location (address 0x0) that isn't valid. This null pointer dereference is a common cause of segmentation faults. The error occurs during error handling, which makes it even harder to diagnose and fix.

The Impact: Crashing Code and Frustration

The impact is huge. The Bun process completely crashes, taking down any long-running applications that rely on it. This makes it impossible to use Bun in production environments for tasks that need to run for extended periods, such as background processes or server-side applications. The inability to handle errors gracefully makes debugging a nightmare, and the only solution is to fall back to Node.js, which is less performant.

The Details: Diving into the Technical Aspects

Let’s get a bit more technical, shall we? This isn't just a simple bug; it's a cascade of failures. It's like a chain reaction, where one small problem triggers a series of events that lead to a major crash. This is what you should know:

The Environment: Where the Crash Happens

The crash specifically affects macOS systems running on Apple Silicon (M1/M2/M3 chips). Here are the key details:

  • OS: macOS versions up to v26.0.1.
  • Bun Version: v1.3.1 (the issue was also present in v1.3.0).
  • Architecture: aarch64 (Apple Silicon).
  • Memory Usage: Before the crash, the memory usage is crazy, with 44GB of RSS and 63.60GB peak, far exceeding reasonable limits.
  • Runtime: The crash typically happens after about 930 seconds of runtime.

The Error: Decoding the Crash Report

The crash report is a crucial piece of the puzzle. It tells us exactly where things went wrong. The most important clues are:

  • Error Type: Segmentation fault at address 0x0.
  • Stack Trace: The stack trace shows the crash originates in pas_segregated_exclusive_view_create within WebKit's libpas allocator. This means the memory allocation process is failing.
  • Crash Report URL: A link to the full crash report gives more in-depth information.

The stack trace is a sequence of function calls that lead up to the crash. By analyzing this, developers can pinpoint the precise location of the error and understand how the program got there. Think of it as a roadmap to the crash.

Reproducing the Crash: How to Make it Happen

The crash is reproducible with the following steps:

  1. Start a Claude Code session: Use a system that relies on complex Hook systems (5 concurrent hooks).
  2. Run Extended Analysis: Let the session run for at least 15 minutes.
  3. Memory Monitoring: Watch the memory usage go up and up, exceeding 40GB.
  4. Crash: A segmentation fault occurs at address 0x0 during error handling.

The fact that it's reproducible means developers can repeatedly test and confirm their fixes.

Fixing the Problem: Possible Solutions and Mitigation

Alright, so what can be done to fix this? This is where the rubber meets the road. Several solutions are being considered, and some mitigation attempts have been tried. Here’s a breakdown:

Potential Solutions: Long-Term Fixes

The core issue lies within the libpas memory allocator and how it interacts with the Apple Silicon architecture. Possible solutions include:

  1. Fixing libpas: The most direct solution is to address the memory management issues within libpas itself. This could involve optimizing memory allocation, improving garbage collection, and preventing fragmentation.
  2. Null Pointer Checks: Implementing robust null pointer checks throughout the code, especially in the pas_segregated_exclusive_view_create function, can help prevent the segfault.
  3. Automatic Memory Pressure Handling: This involves implementing mechanisms to automatically manage memory pressure and trigger garbage collection when memory usage gets too high. This is a proactive approach to prevent the leak from growing out of control.
  4. Memory Pool Management: Providing memory pool management for long-running processes to improve memory allocation efficiency.
  5. Alternative Allocator: Exploring the possibility of using an alternative memory allocator that is better suited for the Apple Silicon architecture.

Mitigation Attempts: What's Been Tried

While a full fix is in the works, developers have tried several mitigation strategies:

  1. Upgrading Bun: The initial thought was that upgrading Bun versions would solve the problem, but it didn't. The issue persists even in the latest versions.
  2. Reducing Hook Timeouts: Reducing the hook timeouts from 5000ms to 3000ms has been attempted, but the problem still occurs under extended usage.
  3. Implementing Memory Monitoring: Memory monitoring tools have been implemented to track memory usage and provide alerts when it exceeds certain thresholds. This helps in early detection of the leak.
  4. Disabling Memory-Intensive Operations: Some memory-intensive operations have been disabled to reduce the overall memory footprint.

Priority: Why This Needs Immediate Attention

This is a critical issue. It prevents the use of Bun for production, long-running applications, which is a major concern. The Bun team needs to prioritize this, and hopefully, they will have a fix soon.

The Technical Deep Dive: Dissecting the Code

Let’s get our hands dirty and dive deeper into the code. This is a bit of a detective story, piecing together clues to understand what’s going wrong.

WebKit's libpas: The Memory Manager

libpas is part of WebKit, the engine that powers Safari and other web browsers. It's responsible for managing memory. It uses a sophisticated allocator that tries to efficiently allocate and free memory, reducing fragmentation and improving performance. But on macOS Silicon, it seems to have some trouble keeping up.

JavaScriptCore Heap: Where Things Go Wrong

Bun uses JavaScriptCore (JSC) to execute JavaScript code. The JSC heap is where the objects and data of your JavaScript programs live. The memory leak appears to be happening within the JSC heap, causing the libpas allocator to fail. Think of the heap as a container for all the variables, objects, and data your code uses. As you run your code, the heap expands and contracts as memory is allocated and freed. The memory leak means the heap keeps growing without shrinking.

Error Stack Trace: The Crash’s Blueprint

The error stack trace is a crucial piece of the puzzle. It shows the series of function calls leading to the crash. In this case, the stack trace indicates that the crash originates in pas_segregated_exclusive_view_create, a function within libpas. This function is responsible for creating memory views, and its failure suggests memory exhaustion or fragmentation. The stack trace is like a roadmap. It shows the steps the program takes that eventually cause it to crash. By analyzing the stack trace, developers can figure out what went wrong and how to fix it.

Conclusion: The Path Forward

This is a complex problem, but the good news is that the Bun team is aware of it and working on a fix. In the meantime, there are some things you can do to mitigate the issue. Keep an eye on your memory usage and consider using Node.js as a fallback for long-running processes until a permanent solution is found. We'll be updating this article as we learn more about the fix. Stay tuned, and happy coding!

This memory leak issue is a serious one, but the fact that it's been identified and is being actively worked on is a positive sign. By understanding the root causes and potential solutions, we can better navigate the challenges and look forward to a more stable Bun experience on macOS Silicon.

Thanks for reading, and let me know if you have any questions in the comments below! Let’s hope the fix comes soon, so we can all get back to enjoying the speed and efficiency of Bun without the fear of sudden crashes.