feat: add cache, outbound HTTP, request protection, tasks and module scaffolding

cache/cache.go

@@ -0,0 +1,135 @@
+// Package cache provides replaceable byte-oriented caches and bounded local
+// request coalescing. Storage errors are never silently treated as misses.
+package cache
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"sync"
+	"time"
+)
+
+var (
+	ErrMiss     = errors.New("cache: key not found")
+	ErrClosed   = errors.New("cache: closed")
+	ErrTooLarge = errors.New("cache: entry exceeds capacity")
+	ErrBusy     = errors.New("cache: too many concurrent loads")
+)
+
+// Store implementations copy values and accept ttl=0 for no expiration.
+// Negative TTLs are invalid. Delete is successful for missing keys.
+type Store interface {
+	Get(context.Context, string) ([]byte, error)
+	Set(context.Context, string, []byte, time.Duration) error
+	Delete(context.Context, ...string) error
+}
+
+func Check(ctx context.Context, key string, ttl time.Duration) error {
+	if ctx == nil {
+		return errors.New("cache: nil context")
+	}
+	if err := ctx.Err(); err != nil {
+		return err
+	}
+	if key == "" || len(key) > 1024 {
+		return errors.New("cache: key must contain 1..1024 bytes")
+	}
+	if ttl < 0 {
+		return errors.New("cache: negative TTL")
+	}
+	return nil
+}
+
+type LoaderConfig struct {
+	MaxLoads int           `json:"maxLoads"`
+	Timeout  time.Duration `json:"timeout"`
+}
+
+type flight struct {
+	done  chan struct{}
+	value []byte
+	err   error
+}
+
+// Loader coalesces concurrent misses for the same key within this process.
+// Caller cancellation stops waiting without canceling other callers' load.
+// Load functions must observe their context, which has a bounded deadline.
+type Loader struct {
+	store   Store
+	config  LoaderConfig
+	mu      sync.Mutex
+	flights map[string]*flight
+}
+
+func NewLoader(store Store, cfg LoaderConfig) (*Loader, error) {
+	if store == nil || cfg.MaxLoads < 0 || cfg.Timeout < 0 {
+		return nil, errors.New("cache: invalid loader configuration")
+	}
+	if cfg.MaxLoads == 0 {
+		cfg.MaxLoads = 128
+	}
+	if cfg.Timeout == 0 {
+		cfg.Timeout = 5 * time.Second
+	}
+	return &Loader{store: store, config: cfg, flights: make(map[string]*flight)}, nil
+}
+
+func (l *Loader) GetOrLoad(ctx context.Context, key string, ttl time.Duration, load func(context.Context) ([]byte, error)) ([]byte, error) {
+	if err := Check(ctx, key, ttl); err != nil {
+		return nil, err
+	}
+	if load == nil {
+		return nil, errors.New("cache: nil loader")
+	}
+	value, err := l.store.Get(ctx, key)
+	if !errors.Is(err, ErrMiss) {
+		return value, err
+	}
+	l.mu.Lock()
+	f := l.flights[key]
+	if f == nil {
+		if len(l.flights) >= l.config.MaxLoads {
+			l.mu.Unlock()
+			return nil, ErrBusy
+		}
+		f = &flight{done: make(chan struct{})}
+		l.flights[key] = f
+		go l.run(context.WithoutCancel(ctx), key, ttl, load, f)
+	}
+	l.mu.Unlock()
+	select {
+	case <-ctx.Done():
+		return nil, ctx.Err()
+	case <-f.done:
+		return append([]byte(nil), f.value...), f.err
+	}
+}
+
+func (l *Loader) run(parent context.Context, key string, ttl time.Duration, load func(context.Context) ([]byte, error), f *flight) {
+	ctx, cancel := context.WithTimeout(parent, l.config.Timeout)
+	defer cancel()
+	defer func() {
+		if value := recover(); value != nil {
+			f.err = fmt.Errorf("cache: loader panic: %v", value)
+		}
+		l.mu.Lock()
+		delete(l.flights, key)
+		close(f.done)
+		l.mu.Unlock()
+	}()
+	// Another request may have populated the key between the first Get and
+	// flight acquisition. Recheck inside the coalesced load.
+	f.value, f.err = l.store.Get(ctx, key)
+	if !errors.Is(f.err, ErrMiss) {
+		return
+	}
+	f.value, f.err = load(ctx)
+	if f.err == nil {
+		f.err = ctx.Err()
+	}
+	if f.err == nil {
+		f.err = l.store.Set(ctx, key, f.value, ttl)
+	}
+	f.value = append([]byte(nil), f.value...)
+}