Cache path info lookups in SQLite

This re-implements the binary cache database in C++, allowing it to be
used by other Store backends, in particular the S3 backend.

src/libstore/nar-info-disk-cache.hh

@@ -0,0 +1,28 @@
+#pragma once
+
+#include "ref.hh"
+#include "nar-info.hh"
+
+namespace nix {
+
+class NarInfoDiskCache
+{
+public:
+    typedef enum { oValid, oInvalid, oUnknown } Outcome;
+
+    virtual void createCache(const std::string & uri) = 0;
+
+    virtual bool cacheExists(const std::string & uri) = 0;
+
+    virtual std::pair<Outcome, std::shared_ptr<NarInfo>> lookupNarInfo(
+        const std::string & uri, const Path & storePath) = 0;
+
+    virtual void upsertNarInfo(
+        const std::string & uri, std::shared_ptr<ValidPathInfo> narInfo) = 0;
+};
+
+/* Return a singleton cache object that can be used concurrently by
+   multiple threads. */
+ref<NarInfoDiskCache> getNarInfoDiskCache();
+
+}