fix: ensure variants are used on first parsing pass

conda_build/metadata.py

@@ -49,6 +49,7 @@
     find_used_variables_in_shell_script,
     find_used_variables_in_text,
     get_default_variant,
+    get_package_variants,
     get_vars,
     list_of_dicts_to_dict_of_lists,
 )
@@ -1167,7 +1168,21 @@ def __init__(self, path, config=None, variant=None):
         # Therefore, undefined jinja variables are permitted here
         # In the second pass, we'll be more strict. See build.build()
         # Primarily for debugging.  Ensure that metadata is not altered after "finalizing"
-        self.parse_again(permit_undefined_jinja=True, allow_no_other_outputs=True)
+
+        try:
+            # For the first pass, we do a simple read of any variants in the recipe and
+            # use them. These are then discarded. Other operations on the metadata
+            # will restore versions of them as needed. This is done to preserve
+            # old behavior.
+            old_config = self.config
+
+            self.config = get_or_merge_config(config, variant=variant)
+            self.config.variants = get_package_variants(self)
+            self.config.variant = self.config.variants[0]
+            self.parse_again(permit_undefined_jinja=True, allow_no_other_outputs=True)
+        finally:
+            self.config = old_config
+
         self.config.disable_pip = self.disable_pip
         # establish whether this recipe should squish build and host together
 

tests/test-recipes/metadata/inject_jinja2_vars_on_first_pass/meta.yaml

@@ -0,0 +1,16 @@
+package:
+  name: noarch-test
+  version: 0.1
+
+source:
+  path: ../../test-package
+
+build:
+  number: 0
+  noarch: python
+
+requirements:
+  host:
+    - python {{ python_min }}.*
+  run:
+    - python >={{ python_min }}

tests/test_api_build.py

@@ -2078,3 +2078,12 @@ def test_conda_build_script_errors_without_conda_info_handlers(tmp_path, recipe,
         assert "Traceback" in all_output
         assert "CalledProcessError" in all_output
         assert "returned non-zero exit status 1" in all_output
+
+
+def test_api_build_inject_jinja2_vars_on_first_pass(testing_config):
+    recipe_dir = os.path.join(metadata_dir, "inject_jinja2_vars_on_first_pass")
+    with pytest.raises(RuntimeError):
+        api.build(recipe_dir, config=testing_config)
+
+    testing_config.variant = {"python_min": "3.12"}
+    api.build(recipe_dir, config=testing_config)