### Bug Description Desktop files can have a missing Exec key and still be valid: - applications can be dbus activated - applications can be started by a systemd user unit. Under those conditions, we still want them to have a desktop file for the Shell to match a desktop entry with the application GUI. The typical use case is our prompting-client service, spawing a GUI when a snapd prompt is requested. This GUI has an associated icon and entry shown, but shouldn’t be started directly (it has NoDisplay=true). The Shell accepts it without Exec line. `desktop-file-validate` also raises no errors, but snapcraft rejects it due to this line: https://github.com/canonical/snapcraft/blob/2f1cc3677b329921f8a16a66615cf7b7de495515/snapcraft_legacy/internal/meta/desktop.py#L56 I think snapcraft should validate the desktop file using desktop-file-validate directly to not encode this kind of knowledge. ### To Reproduce * Ship a desktop file in `meta/gui` that don’t have a `Exec` entry. * Build the snap with `snapcraft` ### Environment * Snapcraft using LXD * Ubuntu questing (development version) ### snapcraft.yaml ```yaml Irrelevant for this bug report. ``` ### Relevant log output ```shell $ snapcraft Running snapcraft without a command will not be possible in future releases. Use 'snapcraft pack' instead. Pulling prompting-clienterror: rustup could not choose a version of rustc to run, because one wasn't specified explicitly, and no default is configured. help: run 'rustup default stable' to download the latest stable release of Rust and set it as your default toolchain. Failed to generate desktop file 'usr/share/applications/prompting-client-daemon.desktop': missing 'Exec' key Failed to execute pack in instance. Recommended resolution: Run the same command again with --debug to shell into the environment if you wish to introspect this failure. $ desktop-file-validate snap/gui/prompting-client.desktop -> no warning, no error ``` ### Additional context _No response_