golang

name	golang
description	Go development: conventions, architecture, concurrency, performance, and code review. Use when working with .go files, go.mod, or user asks about goroutines, channels, error handling, interfaces.
compatibility	Requires Go compiler. Optional: golangci-lint.
allowed-tools	["mcp__acp__Read","mcp__acp__Edit","mcp__acp__Write","mcp__acp__Bash"]

ABOUTME: Complete Go development guide - code, design, concurrency, performance, review

ABOUTME: Conventions, error layering, concurrency rules, modern stdlib preferences

Go Development

Quick Reference

gofmt -w . && goimports -w . && go fix ./... && go vet ./...
go test ./... && go test -race ./... && go test -cover ./...
govulncheck ./...
go build -pgo=cpu.pprof -o bin/app ./cmd/app
golangci-lint run

See also: _AST_GREP.md, _PATTERNS.md, source-control

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Version (determine, don't assume)

See ../_LANG_COMMON.md. Fetch the truth:

go version                                      # project toolchain (for existing repos: also check go.mod)
curl -s https://go.dev/VERSION?m=text | head -1 # latest upstream stable (for new projects)

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Pre-Commit Verification (MANDATORY)

make check && make test-e2e must pass (enforced by the pre-commit-gate hook; see ../_LANG_COMMON.md). What make check expands to for Go:

gofmt -w .                    # Fix formatting FIRST (sqlc/codegen can misalign)
go fix ./... && go fix ./...  # Run twice for synergistic fixes
go vet ./...                  # Static analysis
go build ./...                # Compilation check
go test -race -count=1 ./...  # Tests with race detector
govulncheck ./...             # Reachable vulnerability scan
golangci-lint run             # Lint

Why gofmt before build: Code generators (sqlc, protoc) may produce code gofmt disagrees with. Always run gofmt -w after regeneration and before commit.

go fix modernizers are version-gated by go.mod. Preview with go fix -diff ./.... List with go tool fix help.

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Code Conventions

Formatting: gofmt/goimports: NON-NEGOTIABLE.

Naming: Short vars in funcs (i, c), descriptive at pkg level (ErrNotFound). Receivers 1-2 letter (c *Client). Initialisms all-caps or all-lower (ServeHTTP, appID). Packages lowercase singular.

Errors: Always handle (never _). Wrap: fmt.Errorf("decompress %v: %w", name, err). Lowercase, no punctuation, guard clauses. Never wrap io.EOF (callers use ==).

Error layering: Repo wraps infra errors with context. Service translates to domain sentinels (ErrUserNotFound, ErrInsufficientFunds). Handler maps sentinels to HTTP/gRPC codes. Log errors only at system boundaries (handlers, consumers, workers), not at every layer.

// Domain sentinels
var ErrUserNotFound = errors.New("user not found")

// Service: translate infra → domain
if errors.Is(err, sql.ErrNoRows) { return nil, ErrUserNotFound }

// Handler: map domain → HTTP
if errors.Is(err, ErrUserNotFound) { http.Error(w, "not found", 404); return }

Structured errors (APIs only): For HTTP/gRPC APIs needing machine-readable error codes in responses, define an AppError type with Code/Message/wrapped cause. Not needed for CLIs, workers, or internal packages: use sentinels + %w wrapping.

Testing: Table-driven with t.Run(), t.Helper() in helpers, t.Context() for cancellation.

Function literals: Extract complex callbacks into named vars. Nested literals around slices/maps/iterators hurt readability fast.

Build tags for simulation: //go:build simulation in driver_sim.go, //go:build !simulation in driver_real.go. Same type, different impl. Use for hardware, external APIs, infra deps.

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Architecture & Design

Project structure:

cmd/api-server/main.go    # Entry points
internal/domain/          # Business entities
internal/service/         # Use cases
internal/repository/      # Data access

Organize by feature/domain, not technical layer. Avoid /src, /utils, /common, /helpers.

Functional Options: preferred for optional configuration (WithX(...) Option + NewServer(opts ...Option)).

Constructor Injection: Accept interfaces, return structs. No global mutable state: pass deps explicitly.

Interfaces: Small (1-3 methods), accept interfaces, return structs.

Useful Zero Values: Uninitialized struct = safe to use or obviously invalid. Stdlib examples: sync.Mutex, bytes.Buffer.

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Concurrency

Golden Rules:

• Always know WHEN and HOW a goroutine terminates
• Libraries are synchronous: never launch goroutines from lib code unless concurrency IS the feature

errgroup (preferred over WaitGroup), context always first param, bounded pools for load, sender closes channels.

Cancellation traps (review these explicitly, they pass tests and hang/panic in prod):

• for x := range ch over an externally-produced channel (network stream, LLM provider) has no ctx.Done() injection point. If the producer hangs, the consumer hangs forever. Use select { case x, ok := <-ch: ...; case <-ctx.Done(): return }. Bare range is fine only when you own the channel's lifecycle and it is guaranteed to close.
• nil passed as a context.Context argument: <-nil in a select never fires, so cancellation silently does nothing (and ParseSSE(nil, ...)-style calls panic on first cancel in prod). Grep for nil context args.
• Unbuffered hub/notify channels that block forever if the receiving goroutine has already exited (shutdown deadlock). Pair every blocking send with a select on ctx.Done().
• A goroutine writing a shared buffer (bytes.Buffer) that another path reads: data race in tests, corruption in prod.

For detailed concurrency patterns, performance optimization, profiling, and code review checklists, see references/golang-patterns.md.

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Resources

Effective Go | Code Review Comments | Release Notes | goperf.dev | fgprof