add NIX_USER_CONF_FILES

Motivation: maintain project-level configuration files.

Document the whole situation a bit better so that it corresponds to the
implementation, and add NIX_USER_CONF_FILES that allows overriding
which user files Nix will load during startup.

src/libmain/shared.cc

@@ -260,7 +260,10 @@ void printVersion(const string & programName)
         cfg.push_back("signed-caches");
 #endif
         std::cout << "Features: " << concatStringsSep(", ", cfg) << "\n";
-        std::cout << "Configuration file: " << settings.nixConfDir + "/nix.conf" << "\n";
+        std::cout << "System configuration file: " << settings.nixConfDir + "/nix.conf" << "\n";
+        std::cout << "User configuration files: " <<
+            concatStringsSep(":", settings.nixUserConfFiles)
+            << "\n";
         std::cout << "Store directory: " << settings.nixStore << "\n";
         std::cout << "State directory: " << settings.nixStateDir << "\n";
     }

src/libstore/globals.cc

@@ -31,6 +31,7 @@ Settings::Settings()
     , nixLogDir(canonPath(getEnv("NIX_LOG_DIR").value_or(NIX_LOG_DIR)))
     , nixStateDir(canonPath(getEnv("NIX_STATE_DIR").value_or(NIX_STATE_DIR)))
     , nixConfDir(canonPath(getEnv("NIX_CONF_DIR").value_or(NIX_CONF_DIR)))
+    , nixUserConfFiles(getUserConfigFiles())
     , nixLibexecDir(canonPath(getEnv("NIX_LIBEXEC_DIR").value_or(NIX_LIBEXEC_DIR)))
     , nixBinDir(canonPath(getEnv("NIX_BIN_DIR").value_or(NIX_BIN_DIR)))
     , nixManDir(canonPath(NIX_MAN_DIR))
@@ -77,13 +78,29 @@ void loadConfFile()
        ~/.nix/nix.conf or the command line. */
     globalConfig.resetOverriden();
 
-    auto dirs = getConfigDirs();
-    // Iterate over them in reverse so that the ones appearing first in the path take priority
-    for (auto dir = dirs.rbegin(); dir != dirs.rend(); dir++) {
-        globalConfig.applyConfigFile(*dir + "/nix/nix.conf");
+    auto files = settings.nixUserConfFiles;
+    for (auto file = files.rbegin(); file != files.rend(); file++) {
+        globalConfig.applyConfigFile(*file);
     }
 }
 
+std::vector<Path> getUserConfigFiles()
+{
+    // Use the paths specified in NIX_USER_CONF_FILES if it has been defined
+    auto nixConfFiles = getEnv("NIX_USER_CONF_FILES");
+    if (nixConfFiles.has_value()) {
+        return tokenizeString<std::vector<string>>(nixConfFiles.value(), ":");
+    }
+
+    // Use the paths specified by the XDG spec
+    std::vector<Path> files;
+    auto dirs = getConfigDirs();
+    for (auto & dir : dirs) {
+        files.insert(files.end(), dir + "/nix/nix.conf");
+    }
+    return files;
+}
+
 unsigned int Settings::getDefaultCores()
 {
     return std::max(1U, std::thread::hardware_concurrency());

src/libstore/globals.hh

@@ -53,9 +53,12 @@ public:
     /* The directory where state is stored. */
     Path nixStateDir;
 
-    /* The directory where configuration files are stored. */
+    /* The directory where system configuration files are stored. */
     Path nixConfDir;
 
+    /* A list of user configuration files to load. */
+    std::vector<Path> nixUserConfFiles;
+
     /* The directory where internal helper programs are stored. */
     Path nixLibexecDir;
 
@@ -378,6 +381,9 @@ void initPlugins();
 
 void loadConfFile();
 
+// Used by the Settings constructor
+std::vector<Path> getUserConfigFiles();
+
 extern const string nixVersion;
 
 }

src/libutil/util.hh

@@ -16,6 +16,7 @@
 #include <sstream>
 #include <optional>
 #include <future>
+#include <iterator>
 
 #ifndef HAVE_STRUCT_DIRENT_D_TYPE
 #define DT_UNKNOWN 0