fix(util): skip jinja config files which fail to render by rajibade · Pull Request #7008 · canonical/cloud-init

rajibade · 2026-08-16T13:58:51Z

Proposed Commit Message

fix(util): skip jinja template config files which fail to render

read_conf doesn't handle the None return value 
from render_jinja_payload_from_file() when a
template render fails. This gets passed into
load_yaml which results in an AttributeError on 
NoneType and crashes the running service.

This check was removed from #5350 as unreachable 
due to the render functions not having any return 
annotations. Adding Optional[str] toavoid this being 
caught again and fix the schema to check None return.

Fixes GH-7007

Additional Context

Filed as an issue in [bug] cloud-init-local.service crashes with AttributeError on failed jinja template render #7007
I followed the previous removed code and returned an empty {} as but I don't know if there is another expectation here.

Test Validation

Reproducer in GH issue above.

Test output with current branch

~/cloud-init fix-config-crash ?1 ❯ tox -e py3 -- tests/unittests/test_util.py -k unrendered 

tests/unittests/test_util.py::TestUtil::test_read_conf_with_config_unrendered_template FAILED                                                                                                                  [100%]

====================================================================================================== FAILURES ======================================================================================================
______________________________________________________________________________ TestUtil.test_read_conf_with_config_unrendered_template _______________________________________________________________________________

self = <tests.unittests.test_util.TestUtil object at 0x10537a780>, mocker = <pytest_mock.plugin.MockerFixture object at 0x1053b9a90>, caplog = <_pytest.logging.LogCaptureFixture object at 0x1053b9e80>

>   ???

tests/unittests/test_util.py:569:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
cloudinit/util.py:369: in read_conf
    return load_yaml(config_file, default={})  # pyright: ignore
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
cloudinit/util.py:983: in load_yaml
    blob = decode_binary(blob)
           ^^^^^^^^^^^^^^^^^^^
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

blob = None, encoding = 'utf-8'

    def decode_binary(blob: Union[str, bytes], encoding="utf-8") -> str:
        # Converts a binary type into a text type using given encoding.
>       return blob if isinstance(blob, str) else blob.decode(encoding=encoding)
                                                  ^^^^^^^^^^^
E       AttributeError: 'NoneType' object has no attribute 'decode'

cloudinit/util.py:143: AttributeError
------------------------------------------------------------------------------------------------- Captured log call --------------------------------------------------------------------------------------------------
2026-08-16 13:46:55 WARNING   cloudinit.handlers.jinja_template:jinja_template.py:150 Ignoring jinja template for cfg_path: 'dict object' has no attribute 'c'
2026-08-16 13:46:55 DEBUG     cloudinit.util:util.py:345 Applied instance data in 'vars_path' to configuration loaded from 'cfg_path'

After fix

tests/unittests/test_util.py::TestUtil::test_read_conf_with_config_unrendered_template PASSED    [100%]

Merge type

Squash merge using "Proposed Commit Message"
Rebase and merge unique commits. Requires commit messages per-commit each referencing the pull request number (#<PR_NUM>)

holmanb · 2026-08-17T18:54:42Z

                fname,
                instance_data_file,
            )
+            if rendered is None:


If the function returns None, would it have already logged an error?

Yup here it does.

cloud-init/cloudinit/handlers/jinja_template.py

Line 148 in 34c74ef

LOG.warning("Ignoring jinja template for %s: %s", payload_fn, str(e))

holmanb

Please run tox -e do_format to fix the formatting error.

rajibade · 2026-08-17T21:30:04Z

Please run tox -e do_format to fix the formatting error.

So I did but its another mypy issue that was caught by my changes. Seems the schema also doesn't handle the None return value as well. I'll add a fix in a new commit

rajibade · 2026-08-17T22:04:20Z

hmm not sure the reason for the failing docs check

rajibade · 2026-08-17T22:18:46Z

Oh pylint doesn't play well with the overloading.

holmanb · 2026-08-19T22:21:18Z

            )
+            if rendered is None:
+                error("Failed to render templated user-data.", sys_exit=True)
+                raise SystemExit(1)  # error() exits here


why i'm checking in schema? it didn't handle the None return like the original read_conf function

For the raise, okay so mypy complains because error() doesn't have a no return annotation that can be checked when sys_exit=True happens. If I didn't terminate here, the content = rendered assignment below will get a type checking error as well (after I annotated the jinja templating function to fix another mypy checker).

Would this be a better fit?

Suggested change

raise SystemExit(1) # error() exits here

raise SystemExit(error("Failed to render templated user-data."))

I tried to add No return overloads to the error function as well but pylint kinda lit up and it was probably touching more files than needed. So I dropped it

Trying to render a jinja template file with nested instance-data attributes that isn't provided by the active DataSource fails the init stage and prevent cloud-init from starting up.

How does throwing an exception here fix the reported bug? Even the PR title says "skip jinja config files which fail to render", yet this proposed code throws an exception which crashes the program.

This is not where the bug happens. The bug was fixed in util.read_conf here.

This exception thrown here is a change made as a consequence of fixing the real bug. The cloud-init schema CLI has a tool that also does jinja-template/cloud-config validation. It calls the same jinja render function that util.read_conf does and it also didn't handle the None return from the renderer. I found this out because after annotating render_jinja_payload_from_file with Optional[str], I got a complaint from mypy that there was another call site not properly handling the return.

So this other proposed code doesn't crash the program because it is a CLI command that only validates and isn't part of the running service, but it still needs to cleanly error out and not have a traceback. The service itself will only log on exceptions and continue the program.

Without the schema fix

root@localhost ~]# sudo cloud-init schema --config-file /etc/cloud/cloud.cfg.d/10_test.cfg 2026-08-20 19:47:46,407 - jinja_template.py[WARNING]: Ignoring jinja template for /etc/cloud/cloud.cfg.d/10_test.cfg: 'dict object' has no attribute 'placement' 2026-08-20 19:47:46,407 - util.py[WARNING]: Skipping jinja config file '/etc/cloud/cloud.cfg.d/10_test.cfg'. Failed to render template. 2026-08-20 19:47:46,417 - jinja_template.py[WARNING]: Ignoring jinja template for /etc/cloud/cloud.cfg.d/10_test.cfg: 'dict object' has no attribute 'placement' 2026-08-20 19:47:46,417 - util.py[WARNING]: Skipping jinja config file '/etc/cloud/cloud.cfg.d/10_test.cfg'. Failed to render template. 2026-08-20 19:47:46,421 - jinja_template.py[WARNING]: Ignoring jinja template for /etc/cloud/cloud.cfg.d/10_test.cfg: 'dict object' has no attribute 'placement' Traceback (most recent call last): File "/usr/sbin/cloud-init", line 9, in <module> sys.exit(main.main()) ~~~~~~~~~^^ File "/usr/lib/python3.14/site-packages/cloudinit/cmd/main.py", line 1310, in main return sub_main(args, parser) File "/usr/lib/python3.14/site-packages/cloudinit/cmd/main.py", line 1448, in sub_main retval = functor(name, args) File "/usr/lib/python3.14/site-packages/cloudinit/config/schema.py", line 1478, in handle_schema_args performed_schema_validation = validate_cloudconfig_file( cfg_part.config_path, ...<3 lines>... instance_data_path, ) File "/usr/lib/python3.14/site-packages/cloudinit/config/schema.py", line 1123, in validate_cloudconfig_file decoded_config = _get_config_type_and_rendered_userdata( config_path, decoded_content, instance_data_path ) File "/usr/lib/python3.14/site-packages/cloudinit/config/schema.py", line 1065, in _get_config_type_and_rendered_userdata user_data_type = type_from_starts_with(content) File "/usr/lib/python3.14/site-packages/cloudinit/handlers/__init__.py", line 293, in type_from_starts_with payload_lc = util.decode_binary(payload).lower() ~~~~~~~~~~~~~~~~~~^^^^^^^^^ File "/usr/lib/python3.14/site-packages/cloudinit/util.py", line 143, in decode_binary return blob if isinstance(blob, str) else blob.decode(encoding=encoding) ^^^^^^^^^^^ AttributeError: 'NoneType' object has no attribute 'decode' [root@localhost ~]#

With the schema fix

[root@localhost ~]# sudo cloud-init schema --config-file /etc/cloud/cloud.cfg.d/10_test.cfg 2026-08-20 19:44:04,811 - jinja_template.py[WARNING]: Ignoring jinja template for /etc/cloud/cloud.cfg.d/10_test.cfg: 'dict object' has no attribute 'placement' 2026-08-20 19:44:04,811 - util.py[WARNING]: Skipping jinja config file '/etc/cloud/cloud.cfg.d/10_test.cfg. Failed to render template. 2026-08-20 19:44:04,821 - jinja_template.py[WARNING]: Ignoring jinja template for /etc/cloud/cloud.cfg.d/10_test.cfg: 'dict object' has no attribute 'placement' 2026-08-20 19:44:04,821 - util.py[WARNING]: Skipping jinja config file '/etc/cloud/cloud.cfg.d/10_test.cfg'. Failed to render template. 2026-08-20 19:44:04,825 - jinja_template.py[WARNING]: Ignoring jinja template for /etc/cloud/cloud.cfg.d/10_test.cfg: 'dict object' has no attribute 'placement' Error: Failed to render templated user-data.

I can make updates to the PR description to make this more clearer

In any case, throwing a SystemExit in runtime code is still not what we want - this isn't a pattern used anywhere else.

You can satisfy mypy without throwing the exception by conditionally assigning only when it has a value.

Thanks. That makes sense. i'll update it

updated @holmanb

read_conf doesn't handle the None return value from render_jinja_payload_from_file() when a template render fails. This gets passed into load_yaml which results in an AttributeError on NoneType and crashes the running service. This check was removed from canonical#5350 as unreachable due to the render functions not having any return annotations. Adding Optional[str] to avoid this being caught again and fix the schema to check None return. Fixes canonicalGH-7007

rajibade · 2026-08-26T22:24:47Z

All good on here @holmanb?

holmanb reviewed Aug 17, 2026

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holmanb requested changes Aug 17, 2026

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rajibade force-pushed the fix-config-crash branch from 42ebe05 to 0ba1652 Compare August 17, 2026 21:34

rajibade requested a review from holmanb August 17, 2026 21:36

rajibade force-pushed the fix-config-crash branch from 0ba1652 to 5f1c6d6 Compare August 17, 2026 22:29

holmanb self-assigned this Aug 18, 2026

holmanb reviewed Aug 19, 2026

View reviewed changes

rajibade requested a review from holmanb August 19, 2026 22:41

rajibade force-pushed the fix-config-crash branch from 5f1c6d6 to 3552582 Compare August 21, 2026 23:30

	raise SystemExit(1) # error() exits here
	raise SystemExit(error("Failed to render templated user-data."))

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rajibade commented Aug 26, 2026

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