ci(buildv2): skip patching the fake_pass build child pipeline

idf-ci emits a skip pipeline containing only a fake_pass job, with no
include of test_child_pipeline.yml, when the modified files match no
buildable apps. patch_buildv2_child_pipeline.py redefined
generate_pytest_child_pipeline unconditionally, assuming the included
template supplied its tags, image and assign_test stage via keyword
merge. With the template absent, the injected job was bare: no tags,
default test stage. It was scheduled on the wrong runner and failed,
turning the allow_failure build_child_pipeline_buildv2 bridge red on
every buildv2 MR whose changes produce an empty app matrix.

Return early when a fake_pass job is present so the child pipeline is
left untouched and skips, matching the default build path. Add a unit
test for the patch script and a host_test job to run it.

tools/ci/dynamic_pipelines/scripts/patch_buildv2_child_pipeline.py

@@ -40,6 +40,13 @@ def patch(path: str) -> None:
     with open(path) as f:
         d = yaml.safe_load(f)
 
+    if 'fake_pass' in d:
+        # idf-ci emitted a skip pipeline (no apps matched the modified files): there
+        # are no build jobs to activate and no included generate_pytest_child_pipeline
+        # to redefine. Leave it untouched so Build system v2 skips like the default path.
+        print('Skip pipeline (fake_pass) detected, leaving it untouched')
+        return
+
     injected = []
     for k, v in d.items():
         if isinstance(v, dict) and 'extends' in v:

tools/ci/dynamic_pipelines/tests/test_patch_buildv2_child_pipeline.py

@@ -0,0 +1,69 @@
+# SPDX-FileCopyrightText: 2026 Espressif Systems (Shanghai) CO LTD
+# SPDX-License-Identifier: Apache-2.0
+"""Unit tests for patch_buildv2_child_pipeline.patch()."""
+
+import importlib.util
+from pathlib import Path
+from typing import Any
+
+import yaml
+
+_SCRIPT = Path(__file__).parent.parent / 'scripts' / 'patch_buildv2_child_pipeline.py'
+_spec = importlib.util.spec_from_file_location('patch_buildv2_child_pipeline', _SCRIPT)
+assert _spec is not None and _spec.loader is not None
+patch_mod: Any = importlib.util.module_from_spec(_spec)
+_spec.loader.exec_module(patch_mod)
+
+
+def _write_and_patch(tmp_path: Path, content: dict[str, Any]) -> dict[str, Any]:
+    p = tmp_path / 'buildv2_child_pipeline.yml'
+    p.write_text(yaml.safe_dump(content, sort_keys=False))
+    patch_mod.patch(str(p))
+    result: dict[str, Any] = yaml.safe_load(p.read_text())
+    return result
+
+
+def test_fake_pass_skip_pipeline_is_left_untouched(tmp_path: Path) -> None:
+    # idf-ci emits a skip pipeline with only a `fake_pass` job when no apps match
+    # the modified files. There is no included generate_pytest_child_pipeline to
+    # redefine, so the patch must leave the pipeline untouched and let it skip.
+    # Mirrors idf_ci.idf_gitlab.pipeline._get_fake_pass_job(): the sole job is
+    # `fake_pass` and no test_child_pipeline.yml is included.
+    skip_pipeline = {
+        'fake_pass': {
+            'stage': 'build',
+            'tags': ['build', 'shiny'],
+            'before_script': [],
+            'after_script': [],
+            'cache': [],
+            'needs': [],
+            'script': ['echo "skip the entire child pipeline"'],
+        },
+        'workflow': {'name': 'build_child_pipeline', 'rules': [{'when': 'always'}]},
+    }
+
+    result = _write_and_patch(tmp_path, dict(skip_pipeline))
+
+    assert 'generate_pytest_child_pipeline' not in result
+    assert result == skip_pipeline
+
+
+def test_build_jobs_get_buildv2_injection(tmp_path: Path) -> None:
+    # A real pipeline (apps matched) carries build jobs that `extends` the dynamic
+    # template. Those must get IDF_BUILD_V2 / PIPELINE_COMMIT_SHA injected and
+    # generate_pytest_child_pipeline redefined with the post-processing step.
+    pipeline = {
+        'build_test_related_apps': {
+            'extends': '.dynamic_build_template',
+            'needs': [{'pipeline': '$PARENT_PIPELINE_ID', 'job': 'generate_build_child_pipeline'}],
+        },
+    }
+
+    result = _write_and_patch(tmp_path, pipeline)
+
+    job = result['build_test_related_apps']
+    assert job['variables']['IDF_BUILD_V2'] == '1'
+    assert job['variables']['PIPELINE_COMMIT_SHA'] == patch_mod.PIPELINE_COMMIT_SHA_V2
+    assert job['needs'][0]['job'] == patch_mod.BUILDV2_GENERATOR_JOB
+    assert 'generate_pytest_child_pipeline' in result
+    assert result['generate_pytest_child_pipeline']['variables']['IDF_BUILD_V2'] == '1'