feat: add CSP header (#908)

Co-authored-by: Alessandro (Ale) Segala <43508+ItalyPaleAle@users.noreply.github.com>

backend/internal/utils/cache_util.go

@@ -0,0 +1,78 @@
+package utils
+
+import (
+	"context"
+	"sync/atomic"
+	"time"
+
+	"golang.org/x/sync/singleflight"
+)
+
+type CacheEntry[T any] struct {
+	Value     T
+	FetchedAt time.Time
+}
+
+type ErrStale struct {
+	Err error
+}
+
+func (e *ErrStale) Error() string { return "returned stale cache: " + e.Err.Error() }
+func (e *ErrStale) Unwrap() error { return e.Err }
+
+type Cache[T any] struct {
+	ttl   time.Duration
+	entry atomic.Pointer[CacheEntry[T]]
+	sf    singleflight.Group
+}
+
+func New[T any](ttl time.Duration) *Cache[T] {
+	return &Cache[T]{ttl: ttl}
+}
+
+// Get returns the cached value if it's still fresh.
+func (c *Cache[T]) Get() (T, bool) {
+	entry := c.entry.Load()
+	if entry == nil {
+		var zero T
+		return zero, false
+	}
+	if time.Since(entry.FetchedAt) < c.ttl {
+		return entry.Value, true
+	}
+	var zero T
+	return zero, false
+}
+
+// GetOrFetch returns the cached value if it's still fresh, otherwise calls fetch to get a new value.
+func (c *Cache[T]) GetOrFetch(ctx context.Context, fetch func(context.Context) (T, error)) (T, error) {
+	// If fresh, serve immediately
+	if v, ok := c.Get(); ok {
+		return v, nil
+	}
+
+	// Fetch with singleflight to prevent multiple concurrent fetches
+	vAny, err, _ := c.sf.Do("singleton", func() (any, error) {
+		if v2, ok := c.Get(); ok {
+			return v2, nil
+		}
+		val, fetchErr := fetch(ctx)
+		if fetchErr != nil {
+			return nil, fetchErr
+		}
+		c.entry.Store(&CacheEntry[T]{Value: val, FetchedAt: time.Now()})
+		return val, nil
+	})
+
+	if err == nil {
+		return vAny.(T), nil
+	}
+
+	// Fetch failed. Return stale if possible.
+	if e := c.entry.Load(); e != nil {
+		return e.Value, &ErrStale{Err: err}
+	}
+
+	var zero T
+	return zero, err
+}