[utils] enhance YAML emission with recursive handling

[levkropp]

fixes #4176

This pull request refactors the emit_yaml function in src/utils/yaml_node_utils.cpp to handle YAML nodes recursively, allowing for the addition of special handling, which is needed to properly handle scalar strings that resemble octal numbers.

Enhancements to YAML node handling:

• Refactored std::string mp::utils::emit_yaml(const YAML::Node& node) to introduce a recursive helper function (emit_node) for processing YAML nodes. This allows the function to handle Map, Sequence, and scalar nodes more robustly.
• Added special handling for scalar strings that look like octal numbers (e.g., "0755") by tagging them with YAML::LocalTag("str") to preserve their string representation in the emitted YAML.

These changes seem necessary due to how yaml-cpp handles re-emission of YAML input. If the YAML input uses quotes ("0755"), the YAML spec says this is a string and should remain a string, however it appears that yaml-cpp is seeing a scalar "0755", and then writing it out in a way that causes it to be emitted as a number. These changes should not affect anything with regards to YAML parsing other than this particular case where a cloud-init file has a write_files section with a permissions string defined.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.33%. Comparing base (47f57a0) to head (35809ad).
⚠️ Report is 495 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4177      +/-   ##
==========================================
+ Coverage   89.31%   89.33%   +0.01%     
==========================================
  Files         259      259              
  Lines       15684    15711      +27     
==========================================
+ Hits        14008    14035      +27     
  Misses       1676     1676              

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[levkropp]

I have confirmed that:

• Our personal fork's changes to yaml-cpp are not responsible for these issues with processing permissions octals
• The latest upstream of yaml-cpp still has these issues with octal scalars
• We can implement our existing necessary fixes to colons directly into emit_yaml and no longer need to use our own fork of yaml_cpp (our fork's changes only is necessary for strings with spaces after colons (e.g. key: value) which we can indeed handle in emit_yaml)

HOWEVER: I am getting issues with timeouts, even though the cloud-init configuration completes successfully. For some reason on the latest yaml-cpp there are issues on the SSH or gRPC level regarding communication. The multipass exec command we run to check if a cloud-init config succeeded works when run manually, but it never gets called

[levkropp]

Because of the issues I have been seeing with updating to the latest upstream of yaml-cpp, I have reverted the PR back to the single commit which implements the fix for #4176 , and have created a new branch to deal with investigations into what is necessary to make everything work without the fork's changes. I will create a separate PR to deal with updating yaml-cpp to upstream.

~/yaml-cloudinit-octal-parsing/build$ ./bin/multipass launch --name cdo --cpus 4 --memory 8G --disk 50G --cloud-init https://raw.githubusercontent.com/canonical/multipass/refs/heads/main/data/cloud-init-yaml/cloud-init-charm-dev.yaml
Launched: cdo
~/yaml-cloudinit-octal-parsing/build$ ./bin/multipass shell cdo
Welcome to Ubuntu 24.04.2 LTS (GNU/Linux 6.8.0-60-generic x86_64)

 * Documentation:  https://help.ubuntu.com
 * Management:     https://landscape.canonical.com
 * Support:        https://ubuntu.com/pro

 System information as of Mon Jul  7 11:31:25 EDT 2025

  System load:  0.22              Processes:             133
  Usage of /:   3.8% of 47.39GB   Users logged in:       0
  Memory usage: 3%                IPv4 address for ens3: 10.44.19.6
  Swap usage:   0%


Expanded Security Maintenance for Applications is not enabled.

85 updates can be applied immediately.
65 of these updates are standard security updates.
To see these additional updates run: apt list --upgradable

Enable ESM Apps to receive additional future security updates.
See https://ubuntu.com/esm or run: sudo pro status


To run a command as administrator (user "root"), use "sudo <command>".
See "man sudo_root" for details.

ubuntu@cdo:~$ sudo cloud-init schema --system
Found cloud-config data types: user-data, vendor-data, network-config

1. user-data at /var/lib/cloud/instances/cdo/cloud-config.txt:
  Valid schema user-data

2. vendor-data at /var/lib/cloud/instances/cdo/vendor-cloud-config.txt:
Cloud config schema deprecations: system_info:  Deprecated in version 24.2. System and/or distro specific settings. This is not intended to be overridden by user data or vendor data.
  Valid schema vendor-data

3. network-config at /var/lib/cloud/instances/cdo/network-config.json:
  Valid schema network-config
ubuntu@cdo:~$ 

[xmkg]

Suggested change

	std::string value = n.Scalar();
	const std::string value = n.Scalar();

[xmkg]

Suggested change

	else
	{
	emitter << n;
	}

[xmkg]

Suggested change

	else
	{
	emitter << n;
	}
	emitter << n;

[xmkg]

LGTM 👍

[Sploder12]

LGTM!