Custom prefix in .ini source (#2927)

* test_source_ini: custom config can overlap testenv

regression test for plugin behavior in #2926

* IniSource.get_loader: check section.prefix

ensure that loaders returned from .ini source are bound
to the correct section prefix, if specified.

add comment explaining why the code must look up the name
in the _section_mapping

fix #2926

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

1 files changed, 7 insertions, 2 deletions

diff --git a/src/tox/config/source/ini.py b/src/tox/config/source/ini.py
index df6c2876..fb23c0c3 100644
--- a/src/tox/config/source/ini.py
+++ b/src/tox/config/source/ini.py
@@ -40,8 +40,13 @@ class IniSource(Source):
             yield IniSection.from_key(section)
 
     def get_loader(self, section: Section, override_map: OverrideMap) -> IniLoader | None:
-        sections = self._section_mapping.get(section.name)
-        key = sections[0] if sections else section.key
+        # look up requested section name in the generative testenv mapping to find the real config source
+        for key in self._section_mapping.get(section.name) or []:
+            if section.prefix is None or Section.from_key(key).prefix == section.prefix:
+                break
+        else:
+            # if no matching section/prefix is found, use the requested section key as-is (for custom prefixes)
+            key = section.key
         if self._parser.has_section(key):
             return IniLoader(
                 section=section,