python: Fix for IFD

python/src/python-module.cc

@@ -27,6 +27,11 @@ static struct PyModuleDef nixmodule = {
     NixMethods};
 
 extern "C" _public_ PyObject *PyInit_nix(void) {
+  // 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
+  nix::settings.buildHook = nix::settings.nixBinDir + "/nix __build-remote";
+  // And by setting buildHook before calling initNix, we can override the defaults without overriding the user-provided options from the config files
   nix::initNix();
   nix::initGC();
 

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" "printf \\"%s\\" test > $out" ];
+          builder = "/bin/sh";
+          system = builtins.currentSystem;
+        })
+        """
+        self.assertEqual(nix.eval(expression, vars=dict()), "test")
+
 if __name__ == '__main__':
     unittest.main()