cache: Add a "disabled" cache

README.md

@@ -104,7 +104,7 @@ alertmanager {
 # When the alert-mode is set to single, ntfy-alertmanager will cache each single alert
 # to avoid sending recurrences.
 cache {
-    # The type of cache that will be used (either memory or redis; default is memory).
+    # The type of cache that will be used (either disabled, memory or redis; default is memory).
     type memory
     # How long messages stay in the cache for
     duration 24h

cache/cache.go

@@ -22,6 +22,8 @@ func NewCache(cfg config.CacheConfig) (Cache, error) {
 		return NewMemoryCache(cfg.Duration), nil
 	case "redis":
 		return NewRedisCache(cfg.RedisURL, cfg.Duration)
+	case "disabled":
+		return NewDisabledCache()
 	default:
 		return nil, fmt.Errorf("illegal type %q", cfg.Type)
 	}

cache/disabled.go

@@ -0,0 +1,23 @@
+package cache
+
+// DisabledCache is the disabled cache.
+type DisabledCache struct{}
+
+// NewDisabledCache creates a new disabled cache.
+func NewDisabledCache() (Cache, error) {
+	c := new(DisabledCache)
+	return c, nil
+}
+
+// Set is an empty function to implement the interface.
+func (c *DisabledCache) Set(_ string, _ string) error {
+	return nil
+}
+
+// Contains is an empty function to implement the interface.
+func (c *DisabledCache) Contains(_ string, _ string) (bool, error) {
+	return false, nil
+}
+
+// Cleanup is an empty function to implement the interface.
+func (c *DisabledCache) Cleanup() {}