Fetcher cache: Add support for caching facts not related to store paths

src/libfetchers/cache.hh

@@ -10,6 +10,44 @@ struct Cache
 {
     virtual ~Cache() { }
 
+    /* A cache for arbitrary Attrs -> Attrs mappings with a timestamp
+       for expiration. */
+
+    /*
+     * Add a value to the cache. The cache is an arbitrary mapping of
+     * Attrs to Attrs.
+     */
+    virtual void upsert(
+        const Attrs & inAttrs,
+        const Attrs & infoAttrs) = 0;
+
+    /*
+     * Look up a key with infinite TTL.
+     */
+    virtual std::optional<Attrs> lookup(
+        const Attrs & inAttrs) = 0;
+
+    /*
+     * Look up a key. Return nothing if its TTL has exceeded
+     * `settings.tarballTTL`.
+     */
+    virtual std::optional<Attrs> lookupWithTTL(
+        const Attrs & inAttrs) = 0;
+
+    struct Result2
+    {
+        bool expired = false;
+        Attrs infoAttrs;
+    };
+
+    /*
+     * Look up a key. Return a bool denoting whether its TTL has
+     * exceeded `settings.tarballTTL`.
+     */
+    virtual std::optional<Result2> lookupExpired(
+        const Attrs & inAttrs) = 0;
+
+    /* Old cache for things that have a store path. */
     virtual void add(
         ref<Store> store,
         const Attrs & inAttrs,