daemon: use per-store state directory for socket path

Author: amaanq

src/libstore/include/nix/store/local-fs-store.hh

@@ -64,6 +64,12 @@ public:
 
     PathSetting realStoreDir{
         this, rootDir.get() ? *rootDir.get() + "/nix/store" : storeDir, "real", "Physical path of the Nix store."};
+
+    /**
+     * The default daemon socket path for this store, based on its state
+     * directory.
+     */
+    std::filesystem::path getDefaultDaemonSocketPath() const;
 };
 
 struct alignas(8) /* Work around ASAN failures on i686-linux. */

src/libstore/include/nix/store/uds-remote-store.hh

@@ -33,8 +33,8 @@ struct UDSRemoteStoreConfig : std::enable_shared_from_this<UDSRemoteStoreConfig>
     /**
      * The path to the unix domain socket.
      *
-     * The default is `settings.nixDaemonSocketFile`, but we don't write
-     * that below, instead putting in the constructor.
+     * The default is `stateDir / "daemon-socket/socket"`, overridden by
+     * `NIX_DAEMON_SOCKET_PATH` if set.
      */
     Path path;
 

src/libstore/local-fs-store.cc

@@ -18,6 +18,11 @@ Path LocalFSStoreConfig::getDefaultLogDir()
     return settings.getLogFileSettings().nixLogDir.string();
 }
 
+std::filesystem::path LocalFSStoreConfig::getDefaultDaemonSocketPath() const
+{
+    return std::filesystem::path{stateDir.get()} / "daemon-socket" / "socket";
+}
+
 LocalFSStoreConfig::LocalFSStoreConfig(PathView rootDir, const Params & params)
     : StoreConfig(params)
     /* Default `?root` from `rootDir` if non set

src/libstore/uds-remote-store.cc

@@ -24,7 +24,9 @@ UDSRemoteStoreConfig::UDSRemoteStoreConfig(
     : Store::Config{params}
     , LocalFSStore::Config{params}
     , RemoteStore::Config{params}
-    , path{authority.empty() ? settings.nixDaemonSocketFile.string() : authority}
+    , path{
+          !authority.empty() ? std::string{authority}
+                             : getEnvNonEmpty("NIX_DAEMON_SOCKET_PATH").value_or(getDefaultDaemonSocketPath().string())}
 {
     if (uriSchemes().count(scheme) == 0) {
         throw UsageError("Scheme must be 'unix'");
@@ -38,10 +40,8 @@ std::string UDSRemoteStoreConfig::doc()
         ;
 }
 
-// A bit gross that we now pass empty string but this is knowing that
-// empty string will later default to the same nixDaemonSocketFile. Why
-// don't we just wire it all through? I believe there are cases where it
-// will live reload so we want to continue to account for that.
+// Empty authority will default to the per-store socket path based on
+// stateDir.
 UDSRemoteStoreConfig::UDSRemoteStoreConfig(const Params & params)
     : UDSRemoteStoreConfig(*uriSchemes().begin(), "", params)
 {
@@ -60,7 +60,7 @@ StoreReference UDSRemoteStoreConfig::getReference() const
     /* We specifically return "daemon" here instead of "unix://" or "unix://${path}"
      * to be more compatible with older versions of nix. Some tooling out there
      * tries hard to parse store references and it might not be able to handle "unix://". */
-    if (path == settings.nixDaemonSocketFile)
+    if (path == getEnvNonEmpty("NIX_DAEMON_SOCKET_PATH").value_or(getDefaultDaemonSocketPath().string()))
         return {
             .variant = StoreReference::Daemon{},
             .params = getQueryParams(),

src/nix/unix/daemon.cc

@@ -267,10 +267,18 @@ static void daemonLoop(ref<StoreConfig> storeConfig, std::optional<TrustedFlag>
     }
 #endif
 
+    auto socketPath = [&]() -> std::filesystem::path {
+        if (auto envPath = getEnvNonEmpty("NIX_DAEMON_SOCKET_PATH"))
+            return *envPath;
+        if (auto * localFSConfig = dynamic_cast<LocalFSStoreConfig *>(&*storeConfig))
+            return localFSConfig->getDefaultDaemonSocketPath();
+        return settings.nixDaemonSocketFile;
+    }();
+
     try {
         unix::serveUnixSocket(
             {
-                .socketPath = settings.nixDaemonSocketFile,
+                .socketPath = socketPath,
                 .socketMode = 0666,
             },
             [&](AutoCloseFD remote, std::function<void()> closeListeners) {