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backendcritical PriorityID: golang-sync-errgroup-context

Go errgroup Context-Aware Parallel Fan-Out & First-Error Abort Standard

High-concurrency Go standard for fan-out task execution. Mandates using golang.org/x/sync/errgroup with context propagation to coordinate parallel goroutines, cancel sibling tasks immediately on first error, and eliminate goroutine leaks caused by unbuffered channels.

Prohibited Anti-Pattern: AI spawns raw goroutines using sync.WaitGroup without error propagation, leaving sibling goroutines running and leaking memory when one worker fails.

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---
description: Go errgroup Context-Aware Parallel Fan-Out & First-Error Abort Standard - High-concurrency Go standard for fan-out task execution. Mandates using golang.org/x/sync/errgroup with context propagation to coordinate parallel goroutines, cancel sibling tasks immediately on first error, and eliminate goroutine leaks caused by unbuffered channels.
globs: **/*.go
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/golang-sync-errgroup-context"
---

# Go errgroup Context-Aware Parallel Fan-Out & First-Error Abort Standard

> **Severity**: CRITICAL | **Category**: backend
> **Anti-Pattern Warning**: AI spawns raw goroutines using sync.WaitGroup without error propagation, leaving sibling goroutines running and leaking memory when one worker fails.

## System Constraints
You are a Principal Go Systems Architect. When executing parallel concurrent operations (such as multi-service fan-outs or batch validations), DO NOT use primitive sync.WaitGroup with manual error channels. Use errgroup.WithContext(ctx) to propagate cancellation to sibling tasks upon first error.

## Strict Guidelines
- MUST initialize errgroup with g, ctx := errgroup.WithContext(parentCtx).
- MUST respect ctx.Done() inside all spawned worker functions to abort work when a sibling encounters an error.
- MUST cap concurrent goroutine spawning using g.SetLimit(maxWorkers) to prevent unbounded thread/memory explosion.
- MUST NOT read or write shared maps or slice indexes concurrently without synchronization inside errgroup functions.
- MUST verify that g.Wait() error is returned and checked before consuming aggregated result datasets.

## Bad vs Good Implementations
### Coordinated Parallel Fan-Out with errgroup in Go

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Raw WaitGroup lacks error returns and context cancellation. When one operation fails, the other goroutines continue executing, wasting CPU, network, and database resources.
```go
// FATAL: sync.WaitGroup ignores errors and leaks sibling goroutines
func FetchUserData(userID string) (*UserData, error) {
	var wg sync.WaitGroup
	wg.Add(2)
	go func() {
		defer wg.Done()
		// If fetchProfile fails, fetchOrders continues running pointlessly!
		fetchProfile(userID)
	}()
	go func() {
		defer wg.Done()
		fetchOrders(userID)
	}()
	wg.Wait()
	return nil, nil // Errors swallowed entirely!
}
```

#### Production Standard (Correct)
> **Standard**: Uses errgroup.WithContext. If either call returns an error, the derived context is cancelled immediately, aborting the in-flight sibling request and freeing resources.
```go
package service

import (
	"context"
	"golang.org/x/sync/errgroup"
)

func FetchUserData(ctx context.Context, userID string) (*UserData, error) {
	g, ctx := errgroup.WithContext(ctx)
	g.SetLimit(4) // Concurrency throttle

	var profile *Profile
	var orders []*Order

	g.Go(func() error {
		p, err := fetchProfile(ctx, userID)
		if err != nil {
			return err // Cancels sibling fetchOrders immediately!
		}
		profile = p
		return nil
	})

	g.Go(func() error {
		o, err := fetchOrders(ctx, userID)
		if err != nil {
			return err // Cancels sibling fetchProfile immediately!
		}
		orders = o
		return nil
	})

	if err := g.Wait(); err != nil {
		return nil, err
	}
	return &UserData{Profile: profile, Orders: orders}, nil
}
```

Recommended MCP Servers (mcp-serverlist.com)

Equip agent with system tools to verify code for this stack.

Go Concurrency & Goroutine Leak MCP

Inspects Go runtime goroutine stack traces to flag leaked goroutines and un-aborted contexts.

Recommended Agent Skills (askills.dev)

Ready-to-use executable skill functions tailored to this stack.

askills: Go errgroup Scaffolder

Generates context-aware Go errgroup task fan-outs with worker limits and abort semantics.

Architectural Specification & Invariant Rationale

Architectural Invariant Design: Go errgroup Context-Aware Parallel Fan-Out & First-Error Abort Standard

Go errgroup Context-Aware Parallel Fan-Out & First-Error Abort Standard defines an industrial-grade behavioral standard for backend engineering. High-concurrency Go standard for fan-out task execution. Mandates using golang.org/x/sync/errgroup with context propagation to coordinate parallel goroutines, cancel sibling tasks immediately on first error, and eliminate goroutine leaks caused by unbuffered channels. Modern software architectures require strict adherence to deterministic patterns to guarantee high availability, maintainability, and zero security vulnerabilities.

Framework Paradigm Shift & Drift MechanicsLLMs are trained on millions of open-source repositories containing outdated code, legacy idioms, and unpatched antipatterns. When coding assistants generate implementations for this stack, they default to common statistical tokens: AI spawns raw goroutines using sync.WaitGroup without error propagation, leaving sibling goroutines running and leaking memory when one worker fails. Without an explicit system instruction file, developers spend hours debugging silent failures.

Failure Modes & Production Blast Radius

Hazard Analysis: What Breaks Without This Rule

The following failure modes have been cataloged from hundreds of hours of production code reviews where AI assistants produced non-compliant implementations:

Failure Mode	AI Symptom (The Anti-Pattern)	Severity	Production Impact & Remedy
AI Hallucination & Deprecation Drift	AI spawns raw goroutines using sync.WaitGroup without error propagation, leaving sibling goroutines running and leaking memory when one worker fails.	Critical	Violating this industrial standard introduces regression bugs, security risks, and unexpected runtime failures in production. Enforce explicit behavioral system directives across all AI coding assistants in your repository.
Silent Architecture Inconsistency	AI assistants produce mixed paradigms across files, creating fragmented codebase conventions.	Medium	Increases cognitive load during code reviews and leads to maintenance debt across engineering teams. Pin project conventions in .cursor/rules, CLAUDE.md, and AGENTS.md so all team agents follow identical guardrails.

Architectural Directives in Detail

Step-by-Step Production Directives

Each rule constraint below is injected into the AI agent's system prompt to establish deterministic boundaries:

1

MUST initialize errgroup with g, ctx

MUST initialize errgroup with g, ctx := errgroup.WithContext(parentCtx).

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.

Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.

2

MUST respect ctx.Done() inside all spawned worker functions to abort work when a sibling encounters an error.

MUST respect ctx.Done() inside all spawned worker functions to abort work when a sibling encounters an error.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.

Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.

3

MUST cap concurrent goroutine spawning using g.SetLimit(maxWorkers) to prevent unbounded thread/memory explosion.

MUST cap concurrent goroutine spawning using g.SetLimit(maxWorkers) to prevent unbounded thread/memory explosion.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.

Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.

4

MUST NOT read or write shared maps or slice indexes concurrently without synchronization inside errgroup functions.

MUST NOT read or write shared maps or slice indexes concurrently without synchronization inside errgroup functions.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.

Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.

5

MUST verify that g.Wait() error is returned and checked before consuming aggregated result datasets.

MUST verify that g.Wait() error is returned and checked before consuming aggregated result datasets.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.

Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.

Technical Questions & Answers

Frequently Asked Technical Questions

Common questions developers face when configuring AI coding assistants for this stack:

Q:Why is Go errgroup Context-Aware Parallel Fan-Out & First-Error Abort Standard critical for AI coding assistants?

High-concurrency Go standard for fan-out task execution. Mandates using golang.org/x/sync/errgroup with context propagation to coordinate parallel goroutines, cancel sibling tasks immediately on first error, and eliminate goroutine leaks caused by unbuffered channels. Without explicit behavioral boundaries, AI coding assistants (Cursor, Claude Code, Windsurf, Codex) rely on training data averages, which frequently produce deprecated, inefficient, or vulnerable code patterns.

Q:How does adopting this rule prevent AI hallucinations?

This rule explicitly declares what the AI must avoid (AI spawns raw goroutines using sync.WaitGroup without error propagation, leaving sibling goroutines running and leaking memory when one worker fails.) and prescribes verified positive examples. Grounding the LLM with negative and positive pairs eliminates guesswork.

Q:Which AI coding environments are supported?

This specification is multi-platform compatible: Cursor (.cursor/rules/*.mdc & .cursorrules), Anthropic Claude Code (CLAUDE.md), Windsurf (.windsurfrules), Codex / Cline (AGENTS.md), and GitHub Copilot (.github/copilot-instructions.md).

Q:Can I combine this rule with other technology stacks?

Yes. Use the Interactive Stack Builder or download an Agent Pack to merge this rule with other frontend, backend, database, security, and DevOps rules without syntax collision.

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