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Triggering a GitHub Action from an external source


TIL: a GitHub Action on a given repository can trigger Actions on other GitHub repositories - which is really handy, and enables fancy scenarios of cooperation between repositories!

Setting up a self-updating GitHub profile, following the steps shared by Simon Willison

Yesterday I've set up a custom GitHub profile, and that README file includes a "code-generated" part that automatically displays the last entries from this devblog.
It was fun! ๐Ÿ™‚

I've followed the instructions from GitHub themselves, as well as the content generously shared by Simon Willison on his blog about explaining how his own (shiny ๐ŸŒŸ) GitHub profile is automatically updated once an hour with quite a lot of dynamic content:

So far so good, by copy-pasting most of his code and adapting it to my own case I got my own GitHub profile also updated automatically once an hour.


The main difference between Simon Willison's build script and mine is that the dynamic content I have in my own README is much smaller, as I only want to display the last 10 items of this devblog there.
As a result the process can be simpler - and even managed only with Python and its standard library!

Using the standard library to fetch the RSS feed of this devblog over HTTP and then parse it is a bit less straightforward than what I could have done with higher level packages like Requests and feedparser, but it's still pretty simple ๐Ÿ™‚

The main drawback is probably that Python's xml.etree.ElementTree has a big red warning saying it is "not secure against maliciously constructed data" - but in my case what I parse is the RSS feed from my own blog, so it shouldn't be a problem ๐Ÿคž

My own "README update" script is there:

Updating the GitHub profile only when the devblog is updated

My friend Yann noticed that triggering this README generation once an hour doesn't really make sense, since contrary to Simon Willison the only source of dynamic data in this profile is the devblog: so what would be ideal would be to automatically update the README only when the devblog is updated.

Two hours in the GitHub Actions documentation later, I got it working - let's keep a note of how to do this, so I can do it again later on more easily ๐Ÿ˜…

Triggering a GitHub Action from an external source

It seems that the only way to trigger a GitHub Action from an event that is not on the GitHub repository itself is to use the repository_dispatch event.

In my case the emitter of the event will be a GitHub Action living on this devblog repository, while the receiver will be another GitHub Action, living in the olivierphi repository.
(since my GitHub profile is "olivierphi", according to the doc my GitHub profile must be a README file living in a GitHub repository that has the sane name)

Of course, triggering a GitHub Action in a given repository must not be something that anyone can do, as it would be annoying to have such Actions triggered randomly by 3rd-party people or scripts - and even more annoying, running an Action can leak sensitive information ๐Ÿ˜ฌ

Which is why the emitter of a repository_dispatch event have to authenticate itself.
GitHub's Personal Access Tokens are a way to do this.

Generating a GitHub Personal Access Token

Unfortunately, it seems that Personal Access Tokens have quite a poor granularity: the emitter of a repository_dispatch event must use a token with the repo scope, which gives it "full access to repositories, including private repositories" โš 


I thought I could create such a Token that would be limited to my GitHub profile repository, but it seems that Personal Access Tokens don't have such a level of granularity, so I do have to give such a "god-like" access to my devblog repository ๐Ÿ˜”

The steps to generate such a token are documented here:

Storing the Token on the event emitter's side

This Token having such great power, I should store it in a safe place...
It seems that GitHub Actions' Encrypted Secrets is what I need! ๐Ÿ™‚

Sending the repository_dispatch event when this DevBlog is updated

Right, now I have a (overpowered โšก) Token, and it's safely stored in the devblog repository as an Encrypted Secret...
Now all I have to do is to use it to send such an event to the repo that hosts my GitHub profile!

There are multiple ways to do this, and I went for the one that was looking the most straightforward to me.
So at the end of the GitHub Action file that deploys the blog generated by Material for MKDocs when I push an update to the git repo, I just added the following step:

- name: Notify my Github profile repo
    TOKEN: ${{ secrets.MY_TOKEN_SECRET_NAME }} # (1)
  run:  # (2) |-
    curl \
      --silent \
      -X POST \
      -H "Accept: application/vnd.github+json" \
      -H "Authorization: token ${TOKEN}" \
      "" \ 
      -d '{"event_type":"devblog-gh-pages-pushed","client_payload":{"wait_for_deployment":true}}'

  1. We ask GitHub to extract the encrypted secret to an environment variable that I name "TOKEN"
  2. The target URL includes olivierphi/olivierphi: the first one is the name of my GitHub profile, while the second one is the name of the GitHub repository.
    They have to be identical in the case of the GitHub profile.

I copy-pasted this curl command from there:

Receiving the repository_dispatch event on the GitHub profile repo

I already have a GitHub Action file that is in charge of re-building dynamically the content of my GitHUb profile's README when I push some content.

All I have to do now is to remove the hourly build, and subscribe to the repository_dispatch event:

  workflow_dispatch: # (1)

  # This is removed:
    - cron:  '33 * * * *' # rebuilt once an hour at xx:33

  # This is added:
    # triggered by my "devblog" repo when something is pushed on the GH Pages branch
    types: [devblog-gh-pages-pushed] 

  1. Thanks to this workflow_dispatch we can also manually trigger the GitHub Action.

Conditionally waiting for the DevBlog's deployment

There is one last thing I have to manage: at the time when this GitHub Action is triggered, the DevBlog's static content was just pushed to the gh-pages branch by MKDocs, so the updated DevBlog is not online yet!

So just doing this is not enough, and we need to wait for the RSS feed to be up-to-date before re-generating the README of the GitHub profile... ๐Ÿ•ฐ

It seems that most of the time this deployment takes about 30 to 40 seconds, but sometimes it took a little more than a minute; right, let's wait for 90 seconds before reading the RSS feed, and it should do the job. ๐Ÿคž

However, I don't want to have this waiting time when I push an updated version of the README file itself, or when I trigger the GitHub Action manually!

15 browser tabs opened on the GH Actions documentation later, I found a solution ๐Ÿ˜…

- name: Wait for devblog deployment on GitHub Pages
  if: ${{ github.event_name == 'repository_dispatch' && github.event.client_payload.wait_for_deployment }}
  run: |-
    echo "Deployment can take up to 1 minute, let's wait for 90 seconds"
    sleep 90
- name: Update README
  run: |-

So the Action will sleep for 90 seconds before running the Python script that fetches the blog's content and update the README accordingly, but only if:

  • The Action was triggered by a "repository_dispatch" event.
    We can detect this with:
    github.event_name == 'repository_dispatch'
  • The emitter sent a parameter - that I arbitrarily called wait_for_deployment - in its JSON payload, with a truthy value (which is what I did in the curl command earlier โ˜).
    We can detect this with:

Job's done! โœŒ

Now when I push to this DevBlog repo it sends a repository_dispatch event to the repository of my GitHub profile, which will trigger the generation of the README after having waited a bit to let some time for the up-to-date RSS feed to be online ๐Ÿ™‚

In the end, all this happens between these 2 YAML files: