Add a fallback to the slack notification provider

[hloeung]

This PR adds a short "fallback" message consisting of the components which is used to form the AuthorName as well as the first line of event.Message which is used as the Text.

Slack documentation says that the fallback text is used in clients that don't show formatted text (e.g. mobile notifications). Keeping it to use the first line of event.Message should keep it short enough for mobile notifications and such..

In Mattermost, fallback is actually a requirement, see Mattermost Message Attachment options. Various deployments are using the Slack notification provider for Mattermost notifications - we're one of them.

[matheuscscp]

@hloeung Nice one! Can you please test this e2e and print a screenshot from a mobile notification?

[hloeung]

We're currently using image: ghcr.io/fluxcd/notification-controller:v1.7.5. Any chance of getting an image built and published with this change for testing (under a different tag)?

[matheuscscp]

Sure, I checked out your branch, rebased with upstream main and pushed so I could trigger this:

https://github.com/fluxcd/notification-controller/actions/runs/27192646039/job/80276362068

Keep an eye on the Publish images step, it will print the image with ghcr.io/...:rc-<git sha>@<digest>

[hloeung]

Thanks for that.

Using ghcr.io/fluxcd/notification-controller:rc-aab38700, I can confirm that with this change, a fallback field and message is added.

 kubectl describe po notification-controller-6c7884998c-vhl72 -n flux-system | grep ghcr
    Image:           ghcr.io/fluxcd/notification-controller:rc-aab38700
    Image ID:        ghcr.io/fluxcd/notification-controller@sha256:14fcfa99c1d494a83c026d17bd68b32007eedad681305ca82c39425946184a28
  Normal  Pulling    59m   kubelet            spec.containers{manager}: Pulling image "ghcr.io/fluxcd/notification-controller:rc-aab38700"
  Normal  Pulled     59m   kubelet            spec.containers{manager}: Successfully pulled image "ghcr.io/fluxcd/notification-controller:rc-aab38700" in 6.561s (6.561s including waiting). Image size: 51972671 bytes.

I'm using a Mattermost to IRC bridge (https://github.com/42wim/matterircd) and added some additional logging.

With logging, I can confirm the presence of fallback and is being used:

[2026-06-10T02:11:24Z]  WARN bridge/mattermost: Using fallback text: 'terraform/stg-REDACTED.flux-terraform: error running Init: rpc error: code = Internal desc = There are some problems with the CLI configuration:'

|12:11 <is-flux-bot> [wc544…] terraform/stg-REDACTED.flux-terraform: error running Init: rpc error: code = Internal desc = There are some problems with the CLI configuration:
|12:11 <is-flux-bot> ┃ terraform/stg-REDACTED.flux-terraform
|12:11 <is-flux-bot> ┃ error running Init: rpc error: code = Internal desc = There are some problems with the CLI configuration:
|12:11 <is-flux-bot> ┃ ╷
|12:11 <is-flux-bot> ┃ │ Error: The specified plugin cache dir /home/runner/.terraform.d/plugin-cache/ cannot be opened: stat /home/runner/.terraform.d/plugin-cache/: no such file or directory
|12:11 <is-flux-bot> ┃ │
|12:11 <is-flux-bot> ┃ ╵
|12:11 <is-flux-bot> ┃

As well as an earlier applied successfully message:

[2026-06-10T02:05:31Z]  WARN bridge/mattermost: Using fallback text: 'terraform/stg-REDACTED.flux-terraform: Applied successfully'

|12:05 <is-flux-bot> [ghej3…] terraform/stg-REDACTED.flux-terraform: Applied successfully
|12:05 <is-flux-bot> ┃ terraform/stg-REDACTED.flux-terraform
|12:05 <is-flux-bot> ┃ Applied successfully
|12:05 <is-flux-bot> ┃ summary
|12:05 <is-flux-bot> ┃ Terraform event detected

Messages shown via the Mattermost UI is unchanged, as expected.

[matheuscscp]

LGTM! 🚀

Just one nit. Please fix, rebase with upstream main and force-push 🙏

[hloeung]

Done! Thanks @matheuscscp, much appreciated.