init: Add flag to avoid loading configuration

src/libmain/shared.cc

@@ -113,7 +113,7 @@ static void sigHandler(int signo) { }
 #endif
 
 
-void initNix()
+void initNix(bool loadConfig)
 {
     /* Turn on buffering for cerr. */
 #if HAVE_PUBSETBUF
@@ -121,7 +121,7 @@ void initNix()
     std::cerr.rdbuf()->pubsetbuf(buf, sizeof(buf));
 #endif
 
-    initLibStore();
+    initLibStore(loadConfig);
 
 #ifndef _WIN32
     unix::startSignalHandlerThread();

src/libmain/shared.hh

@@ -21,8 +21,9 @@ int handleExceptions(const std::string & programName, std::function<void()> fun)
 
 /**
  * Don't forget to call initPlugins() after settings are initialized!
+ * @param loadConfig Whether to load configuration from `nix.conf`, `NIX_CONFIG`, etc. May be disabled for unit tests.
  */
-void initNix();
+void initNix(bool loadConfig = true);
 
 void parseCmdLine(int argc, char * * argv,
     std::function<bool(Strings::iterator & arg, const Strings::iterator & end)> parseArg);

src/libstore-c/nix_api_store.cc

@@ -19,6 +19,16 @@ nix_err nix_libstore_init(nix_c_context * context)
     NIXC_CATCH_ERRS
 }
 
+nix_err nix_libstore_init_no_load_config(nix_c_context * context)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        nix::initLibStore(false);
+    }
+    NIXC_CATCH_ERRS
+}
+
 nix_err nix_init_plugins(nix_c_context * context)
 {
     if (context)

src/libstore-c/nix_api_store.h

@@ -35,6 +35,13 @@ typedef struct StorePath StorePath;
  */
 nix_err nix_libstore_init(nix_c_context * context);
 
+/**
+ * @brief Like nix_libstore_init, but does not load the Nix configuration.
+ *
+ * This is useful when external configuration is not desired, such as when running unit tests.
+ */
+nix_err nix_libstore_init_no_load_config(nix_c_context * context);
+
 /**
  * @brief Loads the plugins specified in Nix's plugin-files setting.
  *

src/libstore/globals.cc

@@ -427,12 +427,13 @@ void assertLibStoreInitialized() {
     };
 }
 
-void initLibStore() {
+void initLibStore(bool loadConfig) {
     if (initLibStoreDone) return;
 
     initLibUtil();
 
-    loadConfFile();
+    if (loadConfig)
+        loadConfFile();
 
     preloadNSS();
 

src/libstore/globals.hh

@@ -1279,9 +1279,10 @@ std::vector<Path> getUserConfigFiles();
 extern const std::string nixVersion;
 
 /**
- * NB: This is not sufficient. You need to call initNix()
+ * @param loadConfig Whether to load configuration from `nix.conf`, `NIX_CONFIG`, etc. May be disabled for unit tests.
+ * @note When using libexpr, and/or libmain, This is not sufficient. See initNix().
  */
-void initLibStore();
+void initLibStore(bool loadConfig = true);
 
 /**
  * It's important to initialize before doing _anything_, which is why we