Hdb:go Dev

hdb:go-dev

Develop Go code that compiles and passes tests on the first attempt, using patterns proven in production Go projects.

Usage

/hdb:go-dev <task description>

Description

Implements Go code using a workflow optimized for correctness-on-first-compile. Go's fast compiler makes the edit-compile-test cycle cheap, but avoidable failures still waste time. This skill front-loads the decisions that cause first-attempt failures: interface satisfaction, nil handling, error propagation, concurrency safety, and test isolation. The goal is green on the first go test.

Instructions

When the user invokes /hdb:go-dev <task description>:

Phase 1: Understand the task and codebase

1. Read the project's CLAUDE.md if it exists. It contains project-specific rules that override all defaults in this skill.

2. Identify the project's conventions by reading:

   • go.mod — Go version, module path, existing dependencies
   • Makefile — build targets, build tags, LDFLAGS
   • One representative test file — test style (table-driven, subtests, helpers)
   • One representative handler/command — error handling, logging, context usage

3. Map the change. List:

   • Files to create or modify
   • Interfaces that must be satisfied
   • Functions that will be called from existing code
   • Test files to create or modify

Phase 2: Write code

4. Write types and interfaces first. Define all structs, interfaces, and type aliases before writing any logic. This prevents cascading signature mismatches.

5. Write implementation second. Follow these rules to get it right the first time:

   Error handling — always wrap with context:

   return fmt.Errorf("load config: %w", err)
   

   Never use bare return err. Every error site must add context describing what operation failed. This makes debugging possible without a stack trace.

   Resource cleanup — defer immediately after acquisition:

   f, err := os.Open(path)
   if err != nil {
       return fmt.Errorf("open %s: %w", path, err)
   }
   defer f.Close()
   

   Never separate acquisition from defer. If there's logic between them, a future edit will introduce a leak.

   Nil safety — check interfaces and pointers at boundaries:

   func (s *Server) Start() error {
       if s.db == nil {
           return errors.New("server: database not initialized")
       }
       // ...
   }
   

   Check nil at public API boundaries (exported methods, constructors). Trust nil-safety within a package's private methods.

   Context propagation — accept and pass context everywhere:

   func (s *Store) GetUser(ctx context.Context, id int64) (*User, error) {
       row := s.db.QueryRowContext(ctx, "SELECT ...", id)
       // ...
   }
   

   Every function that does I/O, calls an external service, or could block must accept context.Context as its first parameter.

   Concurrency — protect shared state with the narrowest tool:

   • sync.Mutex for simple shared state
   • sync.RWMutex when reads far outnumber writes
   • atomic.Value / atomic.Int64 for single values
   • Channels for coordination between goroutines
   • Never hold a mutex across I/O or blocking calls

   Goroutine lifecycle — always ensure goroutines can exit:

   go func() {
       for {
           select {
           case <-ctx.Done():
               return
           case job := <-ch:
               process(job)
           }
       }
   }()
   

   Every goroutine must have a termination path via context cancellation or channel close. Goroutine leaks are silent and cumulative.

6. Write tests third. Follow the project's existing test style. When no precedent exists, use these patterns:

   Table-driven tests with subtests:

   func TestParseQuery(t *testing.T) {
       tests := []struct {
           name  string
           input string
           want  Query
       }{
           {name: "empty", input: "", want: Query{}},
           {name: "from operator", input: "from:alice", want: Query{From: "alice"}},
       }
       for _, tt := range tests {
           t.Run(tt.name, func(t *testing.T) {
               got := ParseQuery(tt.input)
               if diff := cmp.Diff(tt.want, got); diff != "" {
                   t.Errorf("ParseQuery(%q) mismatch (-want +got):\n%s", tt.input, diff)
               }
           })
       }
   }
   

   Test isolation with t.TempDir():

   func TestStoreOpen(t *testing.T) {
       dbPath := filepath.Join(t.TempDir(), "test.db")
       s, err := store.Open(dbPath)
       if err != nil {
           t.Fatalf("Open: %v", err)
       }
       defer s.Close()
       // ...
   }
   

   Never use fixed paths or shared temp directories. Every test gets its own t.TempDir().

   HTTP tests with httptest:

   func TestHealthEndpoint(t *testing.T) {
       srv := httptest.NewServer(handler)
       defer srv.Close()
   
       resp, err := http.Get(srv.URL + "/api/health")
       if err != nil {
           t.Fatalf("GET /api/health: %v", err)
       }
       defer resp.Body.Close()
   
       if resp.StatusCode != http.StatusOK {
           t.Errorf("status = %d, want 200", resp.StatusCode)
       }
   }
   

   Mock external binaries for CLI tests:

   func TestAgentInvocation(t *testing.T) {
       testutil.MockBinaryInPath(t, "claude", `#!/bin/sh
   echo '{"result": "ok"}'`)
       // test code that shells out to "claude"
   }
   

   Test helpers get t.Helper():

   func openTestDB(t *testing.T) *Store {
       t.Helper()
       dbPath := filepath.Join(t.TempDir(), "test.db")
       s, err := Open(dbPath)
       if err != nil {
           t.Fatalf("openTestDB: %v", err)
       }
       t.Cleanup(func() { s.Close() })
       return s
   }
   

   Always call t.Helper() first. Always use t.Cleanup() instead of relying on the caller to defer.

Phase 3: Verify

7. Run the verification sequence. Execute these in order, fixing issues between each step:

   go build ./...          # Catch compilation errors
   go vet ./...            # Catch common mistakes (printf args, unreachable code, etc.)
   go test ./...           # Run all tests
   

   If the project uses build tags (e.g., fts5), include them:

   go test -tags fts5 ./...
   

8. Run the formatter. Format is non-negotiable in Go:

   go fmt ./...
   

   Stage any formatting changes. Formatting-only changes are still changes.

9. Run the linter if golangci-lint is available:

   golangci-lint run ./...
   

Project Infrastructure Setup

When starting a new Go project or adding infrastructure to an existing one, apply these patterns:

Module initialization

go mod init github.com/user/project

Use the full GitHub path even for private projects. It prevents import conflicts if the module is ever referenced externally.

Directory layout

project/
├── cmd/projectname/       # CLI entry point
│   ├── main.go            # Cobra root command, signal handling
│   └── cmd/               # Subcommands (one file per command)
├── internal/              # All application packages
│   ├── config/            # TOML config loading
│   ├── store/             # Database access (SQLite, Postgres)
│   ├── testutil/          # Shared test helpers, builders, fixtures
│   └── ...                # Domain packages
├── Makefile               # Build, test, lint, install targets
├── go.mod
├── CLAUDE.md              # Project-specific AI development rules
└── .githooks/pre-commit   # Format + lint check

Rules:

• Everything except cmd/ and main.go goes in internal/. This prevents external imports of unstable code.
• One primary type per file. broadcaster.go contains EventBroadcaster and its helpers, not unrelated types.
• Test files live next to the code they test (store.go + store_test.go).
• testutil/ is a shared package for test helpers, builders, and fixtures used across multiple packages.

Makefile

VERSION ?= $(shell git describe --tags --always --dirty 2>/dev/null || echo "dev")
COMMIT  ?= $(shell git rev-parse --short HEAD 2>/dev/null || echo "unknown")
DATE    ?= $(shell date -u +%Y-%m-%dT%H:%M:%SZ)
LDFLAGS  = -ldflags "-X main.version=$(VERSION) -X main.commit=$(COMMIT) -X main.date=$(DATE)"

.PHONY: build install test lint fmt vet clean

build:
	go build $(LDFLAGS) -o bin/projectname ./cmd/projectname

install:
	go install $(LDFLAGS) ./cmd/projectname

test:
	go test ./...

lint:
	golangci-lint run ./...

fmt:
	go fmt ./...

vet:
	go vet ./...

clean:
	rm -rf bin/

Pre-commit hook

#!/bin/sh
# .githooks/pre-commit

# Check formatting
UNFORMATTED=$(gofmt -l .)
if [ -n "$UNFORMATTED" ]; then
    echo "Files need formatting:"
    echo "$UNFORMATTED"
    exit 1
fi

# Run vet
go vet ./...

Enable with:

git config core.hooksPath .githooks

Database schema embedding

import "embed"

//go:embed schema.sql
var schemaSQL string

func (s *Store) initSchema() error {
    _, err := s.db.Exec(schemaSQL)
    return err
}

Embed SQL schemas in the binary. Never load schema files from the filesystem at runtime — it breaks when the binary runs from a different directory.

Configuration pattern

type Config struct {
    DataDir    string `toml:"data_dir"`
    ServerAddr string `toml:"server_addr"`
    MaxWorkers int    `toml:"max_workers"`
}

func DefaultConfig() *Config {
    return &Config{
        ServerAddr: "127.0.0.1:8080",
        MaxWorkers: 4,
    }
}

func Load(path string) (*Config, error) {
    cfg := DefaultConfig()
    if _, err := toml.DecodeFile(path, cfg); err != nil {
        return nil, fmt.Errorf("load config %s: %w", path, err)
    }
    return cfg, nil
}

Always provide DefaultConfig() so the application works without a config file. Load overlays the file on top of defaults.

CLI entry point

func main() {
    ctx, cancel := signal.NotifyContext(context.Background(), os.Interrupt, syscall.SIGTERM)
    defer cancel()

    if err := rootCmd.ExecuteContext(ctx); err != nil {
        os.Exit(1)
    }
}

Trap signals at the top. Pass context through Cobra into all subcommands. Never call os.Exit() from inside a subcommand — return an error and let main() exit.

Go Problem Areas to Avoid

These are the patterns that most frequently cause first-attempt failures or production bugs:

1. Interface pollution

Wrong: Define an interface with 10 methods that mirrors the concrete type. Right: Define the smallest interface the caller needs.

// Wrong: mirrors the concrete Store
type Database interface {
    Open() error
    Close() error
    GetUser(ctx context.Context, id int64) (*User, error)
    ListUsers(ctx context.Context) ([]User, error)
    InsertUser(ctx context.Context, u *User) error
    DeleteUser(ctx context.Context, id int64) error
    // ... 15 more methods
}

// Right: the handler only needs two methods
type UserGetter interface {
    GetUser(ctx context.Context, id int64) (*User, error)
}

Define interfaces where they're consumed (in the caller's package), not where they're implemented. This prevents import cycles and keeps interfaces small.

2. Goroutine leaks

Wrong:

go func() {
    for msg := range ch {
        process(msg)
    }
}()
// ch is never closed, goroutine lives forever

Right:

go func() {
    for {
        select {
        case <-ctx.Done():
            return
        case msg, ok := <-ch:
            if !ok {
                return
            }
            process(msg)
        }
    }
}()

Every goroutine must exit when its parent context cancels. Test this by calling cancel() in tests and verifying the goroutine returns.

3. Race conditions in tests

Wrong: Tests mutate package-level variables without synchronization. Right:

• Use t.Setenv() for environment variables (automatically marks test as non-parallel)
• Use t.TempDir() for file-based isolation
• Avoid t.Parallel() when tests share mutable state
• Run CI with -race to catch data races: go test -race ./...

4. Nil map/slice panics

Wrong:

type Config struct {
    Labels map[string]string
}
// c.Labels["key"] = "value"  // panic if Labels is nil

Right:

func NewConfig() *Config {
    return &Config{
        Labels: make(map[string]string),
    }
}

Always initialize maps in constructors. Nil slices are safe to append to, but nil maps panic on write.

5. Forgetting to check rows.Err()

Wrong:

rows, _ := db.Query("SELECT ...")
for rows.Next() {
    // ...
}
// Missing: rows.Err() check

Right:

rows, err := db.Query("SELECT ...")
if err != nil {
    return fmt.Errorf("query users: %w", err)
}
defer rows.Close()
for rows.Next() {
    if err := rows.Scan(&u.ID, &u.Name); err != nil {
        return fmt.Errorf("scan user: %w", err)
    }
    users = append(users, u)
}
if err := rows.Err(); err != nil {
    return fmt.Errorf("iterate users: %w", err)
}

rows.Next() can stop due to an error, not just end of results. Always check rows.Err() after the loop.

6. Import cycles

Go forbids circular imports. When package A needs a type from package B and vice versa:

• Extract the shared type into a third package (often internal/types or the consuming package defines an interface)
• Define an interface in the consuming package that the other package's concrete type satisfies
• Never restructure to use interface{} or any to dodge the cycle

7. Closing HTTP response bodies

Wrong:

resp, err := http.Get(url)
if err != nil {
    return err
}
// forgot resp.Body.Close()

Right:

resp, err := http.Get(url)
if err != nil {
    return fmt.Errorf("GET %s: %w", url, err)
}
defer resp.Body.Close()

Always defer resp.Body.Close() immediately after checking the error. Leaking HTTP response bodies exhausts connection pools.

8. SQL NULL handling

Wrong:

var name string
row.Scan(&name)  // panics or empty string if NULL

Right:

var name sql.NullString
row.Scan(&name)
if name.Valid {
    user.Name = name.String
}

Use sql.NullString, sql.NullInt64, etc. for nullable columns. Or use pointer types (*string) with appropriate scan targets.

Dependency Preferences

When the project has no precedent for a choice, prefer these well-tested libraries:

Avoid adding dependencies for:

• HTTP clients (use net/http)
• JSON handling (use encoding/json)
• String manipulation (use strings, strconv)
• File I/O (use os, io, path/filepath)
• Regex (use regexp)
• Time (use time)

The Go stdlib is unusually capable. Every external dependency adds compile time, supply chain risk, and upgrade burden.

Guidelines

• Green on first go test. Front-load type correctness and interface satisfaction. The Go compiler is fast — use it as a feedback tool, not a crutch.
• Read before writing. Read every file that will be modified and every interface that must be satisfied. Misunderstanding an existing signature wastes a full edit-compile-fix cycle.
• Error messages are for humans. "query users: %w" tells you what failed. "%w" alone tells you nothing. Every fmt.Errorf must add context.
• Tests are isolated. t.TempDir(), t.Setenv(), t.Cleanup(). No shared state between tests. No test depends on another test running first.
• Interfaces are small. 1-3 methods. Defined where consumed, not where implemented. If an interface has more than 5 methods, it's probably a concrete type in disguise.
• Concurrency is explicit. Every goroutine has a cancellation path. Every shared variable has a documented synchronization mechanism. Prefer channels for coordination, mutexes for protection.
• Dependencies are earned. Check if the stdlib solves the problem before adding a dependency. net/http, encoding/json, log/slog, database/sql cover most needs.
• Format before commit. go fmt ./... then go vet ./.... Always. Stage formatting changes alongside logic changes.
• Respect CLAUDE.md. The project's instructions override everything in this skill. Read it first.