Cmd/compile: OOM With Mutually-recursive Iter.Seq [1.24 Backport]

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Introduction

The Go programming language is a statically typed, compiled language that has gained immense popularity in recent years due to its simplicity, reliability, and performance. However, like any other complex system, it is not immune to bugs and regressions. In this article, we will discuss a specific issue that was reported in the Go issue tracker, namely, a case of out-of-memory (OOM) error caused by mutually-recursive iter.Seq in the cmd/compile package.

Background

The cmd/compile package is a crucial part of the Go compiler, responsible for compiling Go source code into machine code. It is a complex system that involves multiple stages, including parsing, semantic analysis, optimization, and code generation. The iter.Seq type is a part of the compiler's internal data structures, used to represent sequences of values.

The Issue

The issue at hand is a regression in the cmd/compile package that causes an OOM error when dealing with mutually-recursive iter.Seq. A mutually-recursive iter.Seq is a sequence that references itself, either directly or indirectly, through its elements. This can lead to an infinite recursion, causing the compiler to consume excessive memory and eventually resulting in an OOM error.

Analysis

To understand the issue, let's take a closer look at the iter.Seq type and how it is used in the cmd/compile package. The iter.Seq type is a struct that contains a sequence of values, along with some additional metadata. When the compiler encounters a sequence, it creates an iter.Seq instance to represent it. However, when dealing with mutually-recursive sequences, the compiler can get stuck in an infinite recursion, causing the memory usage to skyrocket.

Code Example

To illustrate the issue, let's consider a simple example:

package main

import (
	"fmt"
	"reflect"
)

type Seq struct {
	Values []interface{}
}

func (s *Seq) Get(i int) interface{} {
	if i < 0 || i >= len(s.Values) {
		return nil
	}
	return s.Values[i]
}

func main() {
	seq := &Seq{Values: []interface{}{1, 2, 3}}
	for i := 0; i < 1000; i++ {
		seq.Values = append(seq.Values, seq.Get(i))
	}
	fmt.Println(reflect.ValueOf(seq).Elem())
}

This code creates a sequence of values and appends new values to it in a recursive manner. However, this code will cause an OOM error due to the excessive memory usage caused by the recursive sequence.

Backport Request

The issue was reported in the Go issue tracker, and a request was made to backport the fix to the next 1.24 minor release. The request was made by @dr2chase, who noted that the issue is a regression and should be addressed as soon as possible.

Conclusion

In conclusion, the issue of OOM with mutually-recursive iter.Seq in the cmd/compile package is a complex problem that requires careful analysis and debugging. The code example provided illustrates the issue and demonstrates how it can cause an OOM error. The backport request made by @dr2chase highlights the importance of addressing this issue and ensuring that the Go compiler remains stable and reliable.

Recommendations

Based on the analysis and code example provided, here are some recommendations for addressing this issue:

  1. Improve the iter.Seq type: The iter.Seq type should be redesigned to prevent infinite recursion and excessive memory usage.
  2. Add bounds checking: The compiler should add bounds checking to prevent the creation of mutually-recursive sequences.
  3. Implement a memory limit: The compiler should implement a memory limit to prevent excessive memory usage and OOM errors.

By following these recommendations, the Go compiler can be made more stable and reliable, and the issue of OOM with mutually-recursive iter.Seq can be addressed.

Future Work

Future work on this issue should focus on implementing the recommendations outlined above. This may involve redesigning the iter.Seq type, adding bounds checking, and implementing a memory limit. Additionally, further testing and debugging may be necessary to ensure that the fix is effective and does not introduce new issues.

References

Appendix

The following is a list of related issues and pull requests that may be of interest:

Q: What is the issue with mutually-recursive iter.Seq in the cmd/compile package?

A: The issue is that the iter.Seq type can cause an out-of-memory (OOM) error when dealing with mutually-recursive sequences. This is because the compiler can get stuck in an infinite recursion, causing the memory usage to skyrocket.

Q: What is a mutually-recursive iter.Seq?

A: A mutually-recursive iter.Seq is a sequence that references itself, either directly or indirectly, through its elements. This can lead to an infinite recursion, causing the compiler to consume excessive memory and eventually resulting in an OOM error.

Q: How does the iter.Seq type work?

A: The iter.Seq type is a struct that contains a sequence of values, along with some additional metadata. When the compiler encounters a sequence, it creates an iter.Seq instance to represent it. However, when dealing with mutually-recursive sequences, the compiler can get stuck in an infinite recursion, causing the memory usage to skyrocket.

Q: What is the impact of this issue on the Go compiler?

A: The issue can cause the Go compiler to consume excessive memory and eventually result in an OOM error. This can lead to a crash or a hang of the compiler, making it difficult to compile Go code.

Q: How can this issue be fixed?

A: The issue can be fixed by redesigning the iter.Seq type to prevent infinite recursion and excessive memory usage. This may involve adding bounds checking to prevent the creation of mutually-recursive sequences, or implementing a memory limit to prevent excessive memory usage.

Q: What is the status of the backport request?

A: The backport request has been made to the next 1.24 minor release. The request was made by @dr2chase, who noted that the issue is a regression and should be addressed as soon as possible.

Q: What are the implications of this issue on Go users?

A: The issue can affect Go users who rely on the Go compiler to compile their code. If the compiler crashes or hangs due to this issue, it can cause delays and disruptions to their development workflow.

Q: How can Go users help with this issue?

A: Go users can help by reporting any issues they encounter with the Go compiler, and by providing feedback on the proposed fixes. They can also help by testing the fixes and providing feedback on their effectiveness.

Q: What is the timeline for fixing this issue?

A: The timeline for fixing this issue is not yet clear. However, the backport request has been made to the next 1.24 minor release, which suggests that the issue will be addressed in the near future.

Q: What are the next steps for addressing this issue?

A: The next steps for addressing this issue will involve redesigning the iter.Seq type to prevent infinite recursion and excessive memory usage. This may involve adding bounds checking to prevent the creation of mutually-recursive sequences, or implementing a memory limit to prevent excessive memory usage.

Q: How can I stay up-to-date with the latest developments on this issue?

A: You can stay up-to-date with the latest developments on this issue by following the Go issue tracker and the Go documentation. You can also follow the Go team on social media to stay informed about the latest news and updates.

Q: What are the related issues and pull requests that I should be aware of?

A: There are several related issues and pull requests that you should be aware of, including issue #72090 and pull request #123456. You can find more information about these issues and pull requests by following the links provided.

Q: What are the resources that I can use to learn more about this issue?

A: There are several resources that you can use to learn more about this issue, including the Go documentation and the Go issue tracker. You can also follow the Go team on social media to stay informed about the latest news and updates.