Fixes #2071 When `metadata.yaml` is empty, packing a charm now says the file is empty instead of claiming it breaks the Juju specification. **What this code is for** `add_metadata` in `charmcraft/preprocess.py` runs when Charmcraft loads a project, before `charmcraft pack` and other commands. If a `metadata.yaml` sits next to `charmcraft.yaml`, it reads that file with `util.safe_yaml_load` and copies its fields into the project data. **Before this PR** A charm author has a valid `charmcraft.yaml` and a leftover `metadata.yaml` that holds only a license comment, then runs `charmcraft pack`. 1. `add_metadata` finds `metadata.yaml` and loads it with `util.safe_yaml_load`. 2. A file with no content, or only comments, loads as `None`. 3. `None` is not a `dict`, so the `if not isinstance(metadata_yaml, dict)` check raises. 4. The author sees `Invalid file: 'metadata.yaml'` with `Ensure metadata.yaml meets the juju metadata.yaml specification.` 5. The author goes looking for a schema mistake in a file that has nothing in it. **After this PR** Same author, same command. 1. Same as above through step 2. 2. A new `if metadata_yaml is None` check runs first and raises `Empty file: 'metadata.yaml'`. 3. The author sees `Add the charm metadata to metadata.yaml or delete the file.` and knows what to do. 4. A `metadata.yaml` that holds a list or a plain string still gets `Invalid file: 'metadata.yaml'`, as before. **The diagram** ```mermaid flowchart LR classDef box fill:#eef1ff,stroke:#4c5bd4,color:#111827 classDef bad fill:#ffd6d6,stroke:#cc0000,color:#111827,stroke-width:3px classDef good fill:#c8f0c8,stroke:#008800,color:#111827,stroke-width:3px subgraph BEFORE["BEFORE: empty file reported as invalid"] direction TB B1["charmcraft pack"] --> B2["add_metadata"] B2 --> B3["util.safe_yaml_load returns None"] B3 --> B4{"not isinstance dict"} B4 -->|"None is not a dict"| B5["Invalid file: 'metadata.yaml'"] B4 -->|"mapping"| B6["fields copied into project"] end subgraph AFTER["AFTER: empty file reported as empty"] direction TB A1["charmcraft pack"] --> A2["add_metadata"] A2 --> A3["util.safe_yaml_load returns None"] A3 --> A4{"THE CHANGE: metadata_yaml is None"} A4 -->|"empty or only comments"| A5["Empty file: 'metadata.yaml'"] A4 -->|"mapping"| A6["fields copied into project"] end B1 ~~~ A1 class B1,B2,B3,B6 box class A1,A2,A3,A6 box class B4,B5 bad class A4,A5 good linkStyle 3 stroke:#cc0000,stroke-width:5px linkStyle 8 stroke:#008800,stroke-width:5px style BEFORE fill:#fff5f5,stroke:#cc0000,color:#111827 style AFTER fill:#f3fff3,stroke:#008800,color:#111827 ``` **The change** `add_metadata` gets an `if metadata_yaml is None` check before the existing `isinstance` check. It raises `CraftValidationError` with the message `Empty file: 'metadata.yaml'`, a resolution telling the author to fill or delete the file, the same docs link, and the same exit code 65. `tests/unit/test_preprocess.py` now expects the new message for an empty file and a comment only file, and keeps a case for a list that still gets `Invalid file`. The 4.6 release notes list the fix. **What changes for a user** - An empty or comment only `metadata.yaml` now gives a message that names the real problem. - A `metadata.yaml` with the wrong shape still gets `Invalid file: 'metadata.yaml'`. - A valid `metadata.yaml` is read exactly as before. - The exit code stays 65 in both error cases. --- - [x] I've followed the [contribution guidelines](https://github.com/canonical/charmcraft/blob/main/CONTRIBUTING.md). - [ ] I've signed the [CLA](http://www.ubuntu.com/legal/contributors/). - [ ] I've successfully run `make lint && make test`. - [x] I've added or updated any relevant documentation. - [x] In documents I changed, I [added a meta description](https://canonical-starflow.readthedocs-hosted.com/how-to/add-a-page-meta-description/) if one was missing. - [x] I've updated the relevant release notes.