libstore: add unix:// uri protocol argument

Change-Id: I7fc27926aa1d89e2190b1deb0252ad33f2b364a5

lix/libstore/globals.cc

@@ -50,7 +50,6 @@ namespace nix {
    appropriately.  (This wouldn't work on the socket itself since it
    must be deleted and recreated on startup.) */
 #define DEFAULT_SOCKET_DIR "/daemon-socket"
-#define LEGACY_SOCKET "/socket"
 
 Settings settings;
 
@@ -68,14 +67,14 @@ Settings::Settings()
     , nixManDir(canonPath(NIX_MAN_DIR))
 {
     if (auto socketDirFromEnv = getEnvNonEmpty("LIX_DAEMON_SOCKET_DIR")) {
-        nixDaemonSockets_ = {{canonPath(*socketDirFromEnv + LEGACY_SOCKET)}};
+        nixDaemonSockets_ = {{canonPath(*socketDirFromEnv + LEGACY_SOCKET_COMBINED)}};
     } else if (auto socketPathFromEnv = getEnvNonEmpty("NIX_DAEMON_SOCKET_PATH")) {
         nixDaemonSockets_ = {{canonPath(*socketPathFromEnv)}};
     } else {
         auto baseDir = nixStateDir + DEFAULT_SOCKET_DIR;
         // this should always match the list of sockets created by daemonLoop and the socket units
         nixDaemonSockets_ = {
-            {canonPath(baseDir + LEGACY_SOCKET)},
+            {canonPath(baseDir + LEGACY_SOCKET_COMBINED)},
         };
     }
 

lix/libstore/uds-remote-store.cc

@@ -51,7 +51,9 @@ UDSRemoteStore::UDSRemoteStore(
 std::string UDSRemoteStore::getUri()
 {
     if (path) {
-        return std::string("unix://") + *path;
+        return fmt(
+            "unix://%s%s", *path, config().protocol.overridden ? fmt("?protocol=%s", config().protocol) : ""
+        );
     } else {
         return "daemon";
     }
@@ -84,7 +86,17 @@ ref<RemoteStore::Connection> UDSRemoteStore::openConnection()
     std::list<Path> candidates;
 
     if (path) {
-        candidates.emplace_back(*path);
+        if (config().protocol == "any") {
+            candidates.emplace_back(*path + LEGACY_SOCKET_COMBINED);
+        } else {
+            for (const auto & proto : tokenizeString<std::list<std::string>>(config().protocol.get(), " ,")) {
+                if (proto == "legacy-combined") {
+                    candidates.emplace_back(*path);
+                } else {
+                    throw Error("can't connect to %s with unknown daemon protocol %s", *path, proto);
+                }
+            }
+        }
     } else {
         candidates = settings.nixDaemonSockets()
             | std::views::transform([](auto & socket) { return socket.path; })

lix/libstore/uds-remote-store.hh

@@ -8,6 +8,8 @@
 
 namespace nix {
 
+constexpr inline std::string_view LEGACY_SOCKET_COMBINED = "/socket";
+
 struct UDSRemoteStoreConfig : virtual LocalFSStoreConfig, virtual RemoteStoreConfig
 {
     UDSRemoteStoreConfig(const Params & params)
@@ -17,6 +19,24 @@ struct UDSRemoteStoreConfig : virtual LocalFSStoreConfig, virtual RemoteStoreCon
     {
     }
 
+    const Setting<std::string> protocol{
+        this,
+        "legacy-combined",
+        "protocol",
+        R"(
+          Space-or-comma-separated list of protocols to try to connect to, in preference order.
+          Currently supported:
+            - `legacy-combined` (default): legacy wire protocol using a single combined socket.
+              The provided path will be used *unmodified* to locate the combined daemon socket.
+
+          Also supports the special value `any` to try *all* known protocols using the provided
+          path as the *base* directory for sockets. Unlike `legacy-combined` this will append a
+          `/socket` to the given path when trying to connect with the legacy-combined protocol.
+
+          Ignored unless a path is also present.
+        )"
+    };
+
     const std::string name() override { return "Local Daemon Store"; }
 
     std::string doc() override;

lix/libstore/uds-remote-store.md

@@ -4,6 +4,6 @@ R"(
 
 This store type accesses a Nix store by talking to a Nix daemon
 listening on the Unix domain socket *path*. The store pseudo-URL
-`daemon` is equivalent to `unix:///nix/var/nix/daemon-socket/socket`.
+`daemon` is equivalent to `unix:///nix/var/nix/daemon-socket?protocol=any`.
 
 )"