Add NAR / Store accessor abstraction

This is primary to allow hydra-queue-runner to extract files like
"nix-support/hydra-build-products" from NARs in binary caches.

src/libstore/local-fs-store.cc

@@ -0,0 +1,71 @@
+#include "fs-accessor.hh"
+#include "store-api.hh"
+
+namespace nix {
+
+struct LocalStoreAccessor : public FSAccessor
+{
+    ref<Store> store;
+
+    LocalStoreAccessor(ref<Store> store) : store(store) { }
+
+    void assertStore(const Path & path)
+    {
+        Path storePath = toStorePath(path);
+        if (!store->isValidPath(storePath))
+            throw Error(format("path ‘%1%’ is not a valid store path") % storePath);
+    }
+
+    FSAccessor::Stat stat(const Path & path) override
+    {
+        assertStore(path);
+
+        struct stat st;
+        if (lstat(path.c_str(), &st)) {
+            if (errno == ENOENT) return {Type::tMissing, 0, false};
+            throw SysError(format("getting status of ‘%1%’") % path);
+        }
+
+        if (!S_ISREG(st.st_mode) && !S_ISDIR(st.st_mode) && !S_ISLNK(st.st_mode))
+            throw Error(format("file ‘%1%’ has unsupported type") % path);
+
+        return {
+            S_ISREG(st.st_mode) ? Type::tRegular :
+            S_ISLNK(st.st_mode) ? Type::tSymlink :
+            Type::tDirectory,
+            S_ISREG(st.st_mode) ? (uint64_t) st.st_size : 0,
+            S_ISREG(st.st_mode) && st.st_mode & S_IXUSR};
+    }
+
+    StringSet readDirectory(const Path & path) override
+    {
+        assertStore(path);
+
+        auto entries = nix::readDirectory(path);
+
+        StringSet res;
+        for (auto & entry : entries)
+            res.insert(entry.name);
+
+        return res;
+    }
+
+    std::string readFile(const Path & path) override
+    {
+        assertStore(path);
+        return nix::readFile(path);
+    }
+
+    std::string readLink(const Path & path) override
+    {
+        assertStore(path);
+        return nix::readLink(path);
+    }
+};
+
+ref<FSAccessor> LocalFSStore::getFSAccessor()
+{
+    return make_ref<LocalStoreAccessor>(ref<Store>(shared_from_this()));
+}
+
+}