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>

3 files changed, 35 insertions, 2 deletions

diff --git a/docs/changelog/2926.bugfix.rst b/docs/changelog/2926.bugfix.rst
new file mode 100644
index 00000000..bbecda0f
--- /dev/null
+++ b/docs/changelog/2926.bugfix.rst
@@ -0,0 +1,2 @@
+Plugins are now able to access tox.ini config sections using a custom prefix with the same suffix / name as a tox
+``testenv`` - by :user:`masenf`
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,
diff --git a/tests/config/source/test_source_ini.py b/tests/config/source/test_source_ini.py
index 5d61290b..12d4eec2 100644
--- a/tests/config/source/test_source_ini.py
+++ b/tests/config/source/test_source_ini.py
@@ -2,7 +2,9 @@ from __future__ import annotations
 
 from pathlib import Path
 
+from tests.conftest import ToxIniCreator
 from tox.config.loader.section import Section
+from tox.config.sets import ConfigSet
 from tox.config.source.ini import IniSource
 
 
@@ -22,3 +24,27 @@ def test_source_ini_ignore_invalid_factor_filters(tmp_path: Path) -> None:
     loader = IniSource(tmp_path, content="[a]\nb= if c: d")
     res = list(loader.envs({"env_list": []}))  # type: ignore
     assert not res
+
+
+def test_source_ini_custom_non_testenv_sections(tox_ini_conf: ToxIniCreator) -> None:
+    """Validate that a plugin can load section with custom prefix overlapping testenv name."""
+
+    class CustomConfigSet(ConfigSet):
+        def register_config(self) -> None:
+            self.add_config(
+                keys=["a"],
+                of_type=str,
+                default="",
+                desc="d",
+            )
+
+    config = tox_ini_conf("[testenv:foo]\n[custom:foo]\na = b")
+    known_envs = list(config._src.envs(config.core))
+    assert known_envs
+    custom_section = config.get_section_config(
+        section=Section("custom", "foo"),
+        base=[],
+        of_type=CustomConfigSet,
+        for_env=None,
+    )
+    assert custom_section["a"] == "b"