Factor out abstract syntax for Store URIs

Need to decouple parsing from actually opening a store for Machine
configs.

Co-authored-by: Théophane Hufschmitt <7226587+thufschmitt@users.noreply.github.com>
Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

src/libstore/store-reference.hh

@@ -0,0 +1,84 @@
+#pragma once
+///@file
+
+#include <variant>
+
+#include "types.hh"
+
+namespace nix {
+
+/**
+ * A parsed Store URI (URI is a slight misnomer...), parsed but not yet
+ * resolved to a specific instance and query parms validated.
+ *
+ * Supported values are:
+ *
+ * - ‘local’: The Nix store in /nix/store and database in
+ *   /nix/var/nix/db, accessed directly.
+ *
+ * - ‘daemon’: The Nix store accessed via a Unix domain socket
+ *   connection to nix-daemon.
+ *
+ * - ‘unix://<path>’: The Nix store accessed via a Unix domain socket
+ *   connection to nix-daemon, with the socket located at <path>.
+ *
+ * - ‘auto’ or ‘’: Equivalent to ‘local’ or ‘daemon’ depending on
+ *   whether the user has write access to the local Nix
+ *   store/database.
+ *
+ * - ‘file://<path>’: A binary cache stored in <path>.
+ *
+ * - ‘https://<path>’: A binary cache accessed via HTTP.
+ *
+ * - ‘s3://<path>’: A writable binary cache stored on Amazon's Simple
+ *   Storage Service.
+ *
+ * - ‘ssh://[user@]<host>’: A remote Nix store accessed by running
+ *   ‘nix-store --serve’ via SSH.
+ *
+ * You can pass parameters to the store type by appending
+ * ‘?key=value&key=value&...’ to the URI.
+ */
+struct StoreReference
+{
+    using Params = std::map<std::string, std::string>;
+
+    /**
+     * Special store reference `""` or `"auto"`
+     */
+    struct Auto
+    {
+        inline bool operator==(const Auto & rhs) const = default;
+        inline auto operator<=>(const Auto & rhs) const = default;
+    };
+
+    /**
+     * General case, a regular `scheme://authority` URL.
+     */
+    struct Specified
+    {
+        std::string scheme;
+        std::string authority;
+
+        bool operator==(const Specified & rhs) const = default;
+        auto operator<=>(const Specified & rhs) const = default;
+    };
+
+    typedef std::variant<Auto, Specified> Variant;
+
+    Variant variant;
+
+    Params params;
+
+    bool operator==(const StoreReference & rhs) const = default;
+    auto operator<=>(const StoreReference & rhs) const = default;
+
+    static StoreReference parse(const std::string & uri, const Params & extraParams = Params{});
+};
+
+/**
+ * Split URI into protocol+hierarchy part and its parameter set.
+ */
+std::pair<std::string, StoreReference::Params> splitUriAndParams(const std::string & uri);
+
+}