Make the NIX_PATH environment variable take precedence over config files

src/libexpr/eval-settings.cc

@@ -5,50 +5,6 @@
 
 namespace nix {
 
-/* Very hacky way to parse $NIX_PATH, which is colon-separated, but
-   can contain URLs (e.g. "nixpkgs=https://bla...:foo=https://"). */
-static Strings parseNixPath(const std::string & s)
-{
-    Strings res;
-
-    auto p = s.begin();
-
-    while (p != s.end()) {
-        auto start = p;
-        auto start2 = p;
-
-        while (p != s.end() && *p != ':') {
-            if (*p == '=') start2 = p + 1;
-            ++p;
-        }
-
-        if (p == s.end()) {
-            if (p != start) res.push_back(std::string(start, p));
-            break;
-        }
-
-        if (*p == ':') {
-            auto prefix = std::string(start2, s.end());
-            if (EvalSettings::isPseudoUrl(prefix) || hasPrefix(prefix, "flake:")) {
-                ++p;
-                while (p != s.end() && *p != ':') ++p;
-            }
-            res.push_back(std::string(start, p));
-            if (p == s.end()) break;
-        }
-
-        ++p;
-    }
-
-    return res;
-}
-
-EvalSettings::EvalSettings()
-{
-    auto var = getEnv("NIX_PATH");
-    if (var) nixPath = parseNixPath(*var);
-}
-
 Strings EvalSettings::getDefaultNixPath()
 {
     Strings res;

src/libexpr/eval-settings.hh

@@ -5,8 +5,6 @@ namespace nix {
 
 struct EvalSettings : Config
 {
-    EvalSettings();
-
     static Strings getDefaultNixPath();
 
     static bool isPseudoUrl(std::string_view s);

src/libexpr/eval.cc

@@ -420,6 +420,43 @@ void initGC()
     gcInitialised = true;
 }
 
+/* Very hacky way to parse $NIX_PATH, which is colon-separated, but
+   can contain URLs (e.g. "nixpkgs=https://bla...:foo=https://"). */
+static Strings parseNixPath(const std::string & s)
+{
+    Strings res;
+
+    auto p = s.begin();
+
+    while (p != s.end()) {
+        auto start = p;
+        auto start2 = p;
+
+        while (p != s.end() && *p != ':') {
+            if (*p == '=') start2 = p + 1;
+            ++p;
+        }
+
+        if (p == s.end()) {
+            if (p != start) res.push_back(std::string(start, p));
+            break;
+        }
+
+        if (*p == ':') {
+            auto prefix = std::string(start2, s.end());
+            if (EvalSettings::isPseudoUrl(prefix) || hasPrefix(prefix, "flake:")) {
+                ++p;
+                while (p != s.end() && *p != ':') ++p;
+            }
+            res.push_back(std::string(start, p));
+            if (p == s.end()) break;
+        }
+
+        ++p;
+    }
+
+    return res;
+}
 
 ErrorBuilder & ErrorBuilder::atPos(PosIdx pos)
 {
@@ -526,8 +563,15 @@ EvalState::EvalState(
 
     /* Initialise the Nix expression search path. */
     if (!evalSettings.pureEval) {
+        // Command line specification
         for (auto & i : _searchPath.elements)
             searchPath.elements.emplace_back(SearchPath::Elem {i});
+        // Environment variable
+        auto environment = getEnv("NIX_PATH");
+        if (environment)
+            for (auto & i : parseNixPath(*environment))
+                searchPath.elements.emplace_back(SearchPath::Elem::parse(i));
+        // Config files and defaults
         for (auto & i : evalSettings.nixPath.get())
             searchPath.elements.emplace_back(SearchPath::Elem::parse(i));
     }

tests/nix_path.sh

@@ -7,6 +7,15 @@ export NIX_PATH=non-existent=/non-existent/but-unused-anyways:by-absolute-path=$
 nix-instantiate --eval -E '<by-absolute-path/simple.nix>' --restrict-eval
 nix-instantiate --eval -E '<by-relative-path/simple.nix>' --restrict-eval
 
+tmpfile=$(mktemp)
+echo "nix-path = " > "$tmpfile"
+export NIX_USER_CONF_FILES="$tmpfile"
+
+# This should still work because the NIX_PATH environment variable will take
+# precedence over the empty nix path set by the configuration file
+nix-instantiate --eval -E '<by-relative-path/simple.nix>' --restrict-eval
+
+
 # Should ideally also test this, but there’s no pure way to do it, so just trust me that it works
 # nix-instantiate --eval -E '<nixpkgs>' -I nixpkgs=channel:nixos-unstable --restrict-eval