[go-fan] Go Module Review: sourcegraph/conc

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🐹 Go Fan Report: github.com/sourcegraph/conc

Module Overview

conc is Sourcegraph's "better structured concurrency for go" library that makes concurrent Go code safer, more readable, and easier to maintain. It provides high-level abstractions for common concurrency patterns while preventing goroutine leaks, handling panics gracefully, and reducing boilerplate code.

Key Philosophy: All concurrency should be scoped - goroutines must have an owner that ensures proper cleanup.

Current Usage in gh-aw

• Files: 1 file (pkg/cli/logs_orchestrator.go)
• Import Count: 1 import
• Key APIs Used:
  • pool.NewWithResults[DownloadResult]() - For collecting results from concurrent tasks
  • .WithMaxGoroutines(MaxConcurrentDownloads) - Bounded concurrency

Usage Pattern

The project uses conc for concurrent artifact downloading in the logs orchestrator:

// Line 481 in logs_orchestrator.go
p := pool.NewWithResults[DownloadResult]().WithMaxGoroutines(MaxConcurrentDownloads)

for _, run := range actualRuns {
    run := run // capture loop variable
    p.Go(func() DownloadResult {
        // Download artifacts and process logs
        return result
    })
}

results := p.Wait() // Blocks until all downloads complete

This pattern:

• ✅ Prevents goroutine leaks (all goroutines scoped to pool)
• ✅ Handles panics automatically with stack traces
• ✅ Limits concurrency to prevent resource exhaustion
• ✅ Collects results in a type-safe manner
• ✅ Properly captures loop variables

Research Findings

Repository Stats

• Version: v0.3.0 (February 2023)
• Repository: https://github.com/sourcegraph/conc
• Stars: 8.8k+ ⭐
• Maturity: Pre-1.0 (targeting 1.0 for March 2023)

Key Features Available

1. WaitGroup: Safer sync.WaitGroup with automatic panic recovery
2. Pool Types:
   • Pool - Basic concurrent task runner
   • ResultPool - Collects results (used in gh-aw) ✓
   • ErrorPool - Aggregates errors from tasks
   • ContextPool - Context-aware with cancellation on failure
3. Stream: Process ordered streams with parallel execution
4. Iter: Concurrent Map() and ForEach() for slices
5. Panics: Explicit panic catching and recovery

Recent Updates (v0.3.0)

• Pool types now panic if configured after use (safety improvement)
• ContextPool always cancels context when Wait() returns
• ContextPool cancels context when tasks panic
• Improved multi-error handling (uses stdlib in Go 1.20+)
• Better panic recovery API (panics.Recovered)

Best Practices from Maintainers

1. Scoped Concurrency: Always wait for goroutines before going out of scope
2. Panic Propagation: Let the pool handle panics - they'll be re-raised with stack traces
3. Configuration Before Use: Configure pools before calling .Go()
4. Context Integration: Use ContextPool for cancellation support

Improvement Opportunities

🏃 Quick Wins

1. Add Configuration Comment

   • Document why MaxConcurrentDownloads was chosen as the concurrency limit
   • Helps future maintainers understand the performance/resource trade-off
   • Location: pkg/cli/logs_orchestrator.go:481

2. Simplify Progress Tracking

   • Current code uses manual atomic counter: atomic.AddInt64(&completedCount, 1)
   • While it works, consider documenting this pattern or using a callback approach
   • Benefit: Clearer code intent

3. Document Panic Behavior

   • Add comment explaining that panics in download tasks are automatically recovered
   • Location: Around the p.Go() call
   • Benefit: Clarity for code reviewers

✨ Feature Opportunities

1. Context Cancellation Support ⭐ HIGH VALUE

   • Current: No way to cancel in-flight downloads on Ctrl+C
   • Enhancement: Use pool.ContextPool for graceful shutdown

   ctx, cancel := context.WithCancel(context.Background())
   defer cancel()
   
   p := pool.NewWithResults[DownloadResult]().
       WithContext(ctx).
       WithMaxGoroutines(MaxConcurrentDownloads)

   • Benefits:
     • Immediate cancellation on user interrupt
     • Better user experience for long-running operations
     • Integrates with existing timeout logic
   • Effort: Low (simple API change)

2. Error Aggregation

   • Current: Per-task error handling in results
   • Enhancement: Use pool's built-in error aggregation

   p := pool.NewWithResults[DownloadResult]().
       WithErrors().
       WithMaxGoroutines(MaxConcurrentDownloads)
   
   results, err := p.Wait() // err is aggregated from all tasks

   • Benefit: Centralized error reporting

3. Iterator Pattern for Simple Cases

   • Consider using iter.Map() for simpler concurrent operations:

   results := iter.Map(runs, func(run *WorkflowRun) DownloadResult {
       // download logic
   })

   • When to use: Single-pass transformations without complex control flow

📐 Best Practice Alignment

1. Explicit Resource Cleanup

   • Current code handles cleanup well, but could benefit from explicit documentation
   • Add comments explaining resource cleanup guarantees

2. Testing Coverage

   • Add tests for:
     • Panic recovery behavior
     • Result ordering
     • Concurrent download limits
     • Resource cleanup on errors

3. Configuration Tuning

   • Make MaxConcurrentDownloads configurable based on:
     • System resources
     • Network bandwidth
     • GitHub API rate limits

🔧 General Improvements

1. Observability

   • Add metrics for:
     • Pool utilization (active goroutines)
     • Download throughput
     • Error rates
   • Tool: Consider using pool callbacks or wrappers

2. Backpressure Handling

   • For very large batches, implement backpressure to avoid:
     • Overwhelming GitHub API
     • Filling up filesystem
     • Memory exhaustion

3. Dynamic Concurrency

   • Adjust concurrency based on:
     • Available bandwidth
     • API response times
     • Error rates

Recommendations

Priority 1: Add Context Cancellation ⭐

Why: Significantly improves user experience for long-running downloads
Effort: Low
Impact: High

Enable users to gracefully cancel downloads with Ctrl+C:

ctx, cancel := context.WithTimeout(context.Background(), timeout)
defer cancel()

p := pool.NewWithResults[DownloadResult]().
    WithContext(ctx).
    WithMaxGoroutines(MaxConcurrentDownloads)

Priority 2: Document Concurrency Decisions

Why: Helps maintainability
Effort: Very Low
Impact: Medium

Add comments explaining:

• Why MaxConcurrentDownloads was chosen
• Panic recovery behavior
• Resource cleanup guarantees

Priority 3: Add Test Coverage

Why: Prevent regressions in concurrent behavior
Effort: Medium
Impact: High

Test scenarios:

• Multiple downloads complete successfully
• Panic in one task doesn't break others
• Results maintain order
• Proper cleanup on errors

Future Consideration: Error Aggregation

Why: Centralized error handling
Effort: Low
Impact: Medium

Could simplify error reporting by using pool's built-in error aggregation.

Next Steps

1. ✅ Immediate: Add documentation comments for concurrency configuration
2. 🎯 Short-term: Implement context cancellation support
3. 📝 Medium-term: Add test coverage for concurrent download behavior
4. 🔍 Long-term: Consider making concurrency limits configurable based on system resources

Conclusion

The current usage of conc in gh-aw is well-implemented and idiomatic. The code correctly:

• Prevents goroutine leaks through scoped concurrency
• Handles panics automatically
• Limits concurrency to prevent resource exhaustion
• Collects results in a type-safe manner

The main enhancement opportunity is adding context cancellation support for better user experience during long-running operations. This is a low-effort, high-impact improvement that aligns with conc's design philosophy.

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Module summary saved to: specs/mods/sourcegraph-conc.md

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