Move /src to /subprojects

This will facilitate breaking up Nix into multiple packages for each
component with Meson.

subprojects/libfetchers/cache.hh

@@ -0,0 +1,96 @@
+#pragma once
+///@file
+
+#include "fetchers.hh"
+#include "path.hh"
+
+namespace nix::fetchers {
+
+/**
+ * A cache for arbitrary `Attrs` -> `Attrs` mappings with a timestamp
+ * for expiration.
+ */
+struct Cache
+{
+    virtual ~Cache() { }
+
+    /**
+     * A domain is a partition of the key/value cache for a particular
+     * purpose, e.g. git revision to revcount.
+     */
+    using Domain = std::string_view;
+
+    /**
+     * A cache key is a domain and an arbitrary set of attributes.
+     */
+    using Key = std::pair<Domain, Attrs>;
+
+    /**
+     * Add a key/value pair to the cache.
+     */
+    virtual void upsert(
+        const Key & key,
+        const Attrs & value) = 0;
+
+    /**
+     * Look up a key with infinite TTL.
+     */
+    virtual std::optional<Attrs> lookup(
+        const Key & key) = 0;
+
+    /**
+     * Look up a key. Return nothing if its TTL has exceeded
+     * `settings.tarballTTL`.
+     */
+    virtual std::optional<Attrs> lookupWithTTL(
+        const Key & key) = 0;
+
+    struct Result
+    {
+        bool expired = false;
+        Attrs value;
+    };
+
+    /**
+     * Look up a key. Return a bool denoting whether its TTL has
+     * exceeded `settings.tarballTTL`.
+     */
+    virtual std::optional<Result> lookupExpired(
+        const Key & key) = 0;
+
+    /**
+     * Insert a cache entry that has a store path associated with
+     * it. Such cache entries are always considered stale if the
+     * associated store path is invalid.
+     */
+    virtual void upsert(
+        Key key,
+        Store & store,
+        Attrs value,
+        const StorePath & storePath) = 0;
+
+    struct ResultWithStorePath : Result
+    {
+        StorePath storePath;
+    };
+
+    /**
+     * Look up a store path in the cache. The returned store path will
+     * be valid, but it may be expired.
+     */
+    virtual std::optional<ResultWithStorePath> lookupStorePath(
+        Key key,
+        Store & store) = 0;
+
+    /**
+     * Look up a store path in the cache. Return nothing if its TTL
+     * has exceeded `settings.tarballTTL`.
+     */
+    virtual std::optional<ResultWithStorePath> lookupStorePathWithTTL(
+        Key key,
+        Store & store) = 0;
+};
+
+ref<Cache> getCache();
+
+}