go-api/cache.go

// SPDX-License-Identifier: EUPL-1.2

package api

import (
	"bytes"
	"net/http"
	"sync"
	"time"

	"github.com/gin-gonic/gin"
)

// cacheEntry holds a cached response body, status code, headers, and expiry.
type cacheEntry struct {
	status  int
	headers http.Header
	body    []byte
	expires time.Time
}

// cacheStore is a simple thread-safe in-memory cache keyed by request URL.
type cacheStore struct {
	mu      sync.RWMutex
	entries map[string]*cacheEntry
}

// newCacheStore creates an empty cache store.
func newCacheStore() *cacheStore {
	return &cacheStore{
		entries: make(map[string]*cacheEntry),
	}
}

// get retrieves a non-expired entry for the given key.
// Returns nil if the key is missing or expired.
func (s *cacheStore) get(key string) *cacheEntry {
	s.mu.RLock()
	entry, ok := s.entries[key]
	s.mu.RUnlock()

	if !ok {
		return nil
	}
	if time.Now().After(entry.expires) {
		s.mu.Lock()
		delete(s.entries, key)
		s.mu.Unlock()
		return nil
	}
	return entry
}

// set stores a cache entry with the given TTL.
func (s *cacheStore) set(key string, entry *cacheEntry) {
	s.mu.Lock()
	s.entries[key] = entry
	s.mu.Unlock()
}

// cacheWriter intercepts writes to capture the response body and status.
type cacheWriter struct {
	gin.ResponseWriter
	body *bytes.Buffer
}

func (w *cacheWriter) Write(data []byte) (int, error) {
	w.body.Write(data)
	return w.ResponseWriter.Write(data)
}

func (w *cacheWriter) WriteString(s string) (int, error) {
	w.body.WriteString(s)
	return w.ResponseWriter.WriteString(s)
}

// cacheMiddleware returns Gin middleware that caches GET responses in memory.
// Only successful responses (2xx) are cached. Non-GET methods pass through.
func cacheMiddleware(store *cacheStore, ttl time.Duration) gin.HandlerFunc {
	return func(c *gin.Context) {
		// Only cache GET requests.
		if c.Request.Method != http.MethodGet {
			c.Next()
			return
		}

		key := c.Request.URL.RequestURI()

		// Serve from cache if a valid entry exists.
		if entry := store.get(key); entry != nil {
			for k, vals := range entry.headers {
				for _, v := range vals {
					c.Writer.Header().Set(k, v)
				}
			}
			c.Writer.Header().Set("X-Cache", "HIT")
			c.Writer.WriteHeader(entry.status)
			_, _ = c.Writer.Write(entry.body)
			c.Abort()
			return
		}

		// Wrap the writer to capture the response.
		cw := &cacheWriter{
			ResponseWriter: c.Writer,
			body:           &bytes.Buffer{},
		}
		c.Writer = cw

		c.Next()

		// Only cache successful responses.
		status := cw.ResponseWriter.Status()
		if status >= 200 && status < 300 {
			headers := make(http.Header)
			for k, vals := range cw.ResponseWriter.Header() {
				headers[k] = vals
			}
			store.set(key, &cacheEntry{
				status:  status,
				headers: headers,
				body:    cw.body.Bytes(),
				expires: time.Now().Add(ttl),
			})
		}
	}
}