CI: publish with helpful commit message

ci/publish

@@ -1,4 +1,9 @@
 #!/bin/bash
+# This script publishes the Helm chart to the JupyterHub Helm chart repo and
+# pushes associated built docker images to Docker hub using chartpress.
+# --------------------------------------------------------------------------
+
+# Exit on error and raise an error for unset environment variables
 set -eu
 
 # Decrypt a private SSH key having its public key registered on GitHub. It will
@@ -7,6 +12,7 @@ set -eu
 openssl aes-256-cbc -K $encrypted_c6b45058ffe8_key -iv $encrypted_c6b45058ffe8_iv -in ci/publish-id_rsa.enc -out ci/id_rsa -d
 chmod 0400 ci/id_rsa
 
+# Configure docker with credentials to let chartpress push images to docker hub.
 docker login -u "${DOCKER_USERNAME}" -p "${DOCKER_PASSWORD}"
 
 # Activate logging of bash commands now that the sensitive stuff is done
@@ -18,8 +24,27 @@ export GIT_SSH_COMMAND="ssh -i ${PWD}/ci/id_rsa"
 
 if [ "${TRAVIS_TAG:-}" == "" ]; then
     # Using --long, we are ensured to get a build suffix, which ensures we don't
-    # build the same tag twice.
-    chartpress --push --publish-chart --long
+    # build the same tag twice. Using --extra-message, we help automation like
+    # henchbot to submit update PRs to jupyterhub/mybinder.org-deploy.
+    #
+    # ref: https://github.com/jupyterhub/chartpress#usage ref:
+    # https://github.com/henchbot/mybinder.org-upgrades
+    #
+    # NOTE: By crafting a smart commit message for the publication commit to
+    #       jupyterhub/helm-chart, we can make GitHub help us mention the
+    #       publication commit in the PR or commit. This helps users realize
+    #       what version they need to upgrade to etc in order to be able to
+    #       benefit from the merged PR in a development release.
+    #
+    #       GitHub merge commits contain a PR reference like #123. `sed` looks
+    #       to extract either a PR reference like #123 or fall back to create a
+    #       commit hash reference like @123abcd. Combined with TRAVIS_REPO_SLUG
+    #       we craft a commit message like jupyterhub/binderhub#123 or
+    #       jupyterhub/binderhub@123abcd which will be understood as a reference
+    #       by GitHub.
+    PR_OR_HASH=$(git log -1 --pretty=%B | head -n1 | sed 's/^.*\(#[0-9]*\).*/\1/' | sed 's/^\([0-9a-f]*\)-.*/@\1/')
+    EXTRA_MESSAGE="${TRAVIS_REPO_SLUG}${PR_OR_HASH}"
+    chartpress --push --publish-chart --long --extra-message "${EXTRA_MESSAGE}"
 else
     # Setting a tag explicitly enforces a rebuild if this tag had already been
     # built and we wanted to override it.