python bindings: Make IFD work by default

flake.nix

@@ -405,6 +405,10 @@
                   final.buildPackages.nix
                 ];
 
+                mesonFlags = [
+                  "-Dnix_binary=${final.lib.getBin final.nix}/bin/nix"
+                ];
+
                 strictDeps = true;
 
                 buildInputs = [
@@ -419,6 +423,16 @@
                 installCheckPhase = ''
                   export TEST_ROOT=$(mktemp -d)
                   export NIX_STATE_DIR=$TEST_ROOT/var/nix
+                  export NIX_STORE_DIR=$TEST_ROOT/store
+                  export NIX_LOCALSTATE_DIR=$TEST_ROOT/var
+                  export NIX_LOG_DIR=$TEST_ROOT/var/log/nix
+
+                  export NIX_CONF_DIR=$TEST_ROOT/etc
+                  mkdir "$NIX_CONF_DIR"
+                  cat > "$NIX_CONF_DIR"/nix.conf <<EOF
+                  sandbox = false
+                  substituters =
+                  EOF
 
                   export PYTHONPATH=$out/${python.sitePackages}
 

python/meson_options.txt

@@ -0,0 +1 @@
+option('nix_binary', type : 'string', description : 'An option')

python/src/eval.cc

@@ -43,6 +43,13 @@ PyObject *eval(PyObject *self, PyObject *args, PyObject *keywds) {
   PyObject *vars = nullptr;
 
   // TODO: Only do this once
+  // By default, Nix sets the build-hook to be "$(readlink /proc/self/exe) __build-remote", expecting the current binary to be Nix itself.
+  // But when we call the Nix library from Python this isn't the case, the current binary is Python then
+  // So we need to change this default, pointing it to the Nix binary instead
+  // Because we can't know the path to the Nix binary from here, we use a preprocessor macro,
+  // which we can then influence from Meson, which we can then influence from the Nix build definition
+  nix::settings.buildHook = NIX_BUILD_HOOK;
+  // And by setting buildHook before calling initNix, we can override the defaults without overriding the user-provided options from the config files
   nix::initNix();
 
   const char *kwlist[] = {"expression", "vars", nullptr};

python/src/meson.build

@@ -9,4 +9,8 @@ python_mod.extension_module('nix', src,
     dependencies : [python_dep, nix_expr_dep, nix_main_dep],
     install: true,
     install_dir: python_mod.sysconfig_path('platlib'),
-    cpp_args: ['-std=c++17', '-fvisibility=hidden'])
+    cpp_args: [
+      '-std=c++17',
+      '-fvisibility=hidden',
+      '-DNIX_BUILD_HOOK="' + get_option('nix_binary') + ' __build-remote"',
+    ])

python/tests.py

@@ -16,5 +16,16 @@ def test_bool(self):
     def test_none(self):
         self.assertEqual(nix.eval("a", vars=dict(a=None)), None)
 
+    def test_ifd(self):
+        expression = """
+        builtins.readFile (derivation {
+          name = "test";
+          args = [ "-c" "echo -n test > $out" ];
+          builder = "/bin/sh";
+          system = builtins.currentSystem;
+        })
+        """
+        self.assertEqual(nix.eval(expression, vars=dict()), "test")
+
 if __name__ == '__main__':
     unittest.main()