Create Slack integration for Google Cloud Build

Posts notifications to a Slack channel whenever a Google Cloud Build
completes.

scripts/gcb2slack/.gcloudignore

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+# This file specifies files that are *not* uploaded to Google Cloud Platform
+# using gcloud. It follows the same syntax as .gitignore, with the addition of
+# "#!include" directives (which insert the entries of the given .gitignore-style
+# file at that point).
+#
+# For more information, run:
+#   $ gcloud topic gcloudignore
+#
+.gcloudignore
+# If you would like to upload your .git directory, .gitignore file or files
+# from your .gitignore file, remove the corresponding line
+# below:
+.git
+.gitignore
+
+node_modules

scripts/gcb2slack/.gitignore

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+/node_modules/

scripts/gcb2slack/README.md

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+# Slack Notifications for Google Cloud Build
+
+> This is not an officially supported Google product.
+
+This Google Cloud Function sends a notification to a Slack channel whenever a
+Google Cloud Build completes.
+
+![Success notification](success.png)
+![Failure notification](failure.png)
+
+## Installation
+
+### Create a Slack Webhook
+
+Follow
+[Slack's "Getting Started with Incoming Webhooks" instructions](https://api.slack.com/messaging/webhooks)
+to create a Slack app and incoming webhook URL. This URL will be required in the
+next installation step.
+
+### Deploy Google Cloud Function
+
+If you don't already have a Google Cloud project, [create one now](https://cloud.google.com/resource-manager/docs/creating-managing-projects). Ensure that you have the [Google Cloud SDK](https://cloud.google.com/sdk/) installed as well.
+
+Run the following command to deploy this Google Cloud Function, after changing
+`$PROJECT` to your
+[Google Cloud project ID](https://cloud.google.com/resource-manager/docs/creating-managing-projects#identifying_projects)
+and `$SLACK_WEBHOOK_URL` to the webhook URL you created in the previous
+installation step. You may also customise `$FUNC_NAME` and `$STAGE_BUCKET`.
+
+```shell
+# Required parameters
+$ PROJECT="YOUR-GOOGLE-CLOUD-PROJECT-ID"
+$ SLACK_WEBHOOK_URL="YOUR-SLACK-WEBHOOK-URL"
+# Customisable parameters
+$ FUNC_NAME="cloudbuild2slack"
+$ STAGE_BUCKET="${PROJECT}-cloudbuild2slack"
+
+$ gcloud --project="${PROJECT}" functions deploy "${FUNC_NAME}" \
+    --stage-bucket="${STAGE_BUCKET}" \
+    --trigger-topic="cloud-builds" \
+    --runtime="nodejs10" \
+    --set-env-vars="SLACK_WEBHOOK_URL=${SLACK_WEBHOOK_URL}"
+```

scripts/gcb2slack/index.js

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+const {
+    IncomingWebhook
+} = require('@slack/webhook');
+
+const {
+    auth
+} = require('google-auth-library');
+
+const webhook = new IncomingWebhook(process.env.SLACK_WEBHOOK_URL);
+
+// Add additional statues to list if you'd like:
+// QUEUED, WORKING, CANCELLED
+const interestingStatuses = ['SUCCESS', 'FAILURE', 'INTERNAL_ERROR', 'TIMEOUT'];
+
+// cloudbuildToSlack reads from a Google Cloud Build Pub/Sub topic and writes
+// to a Slack webhook.
+module.exports.cloudbuildToSlack = async (pubSubEvent, context) => {
+    if (!pubSubEvent.data) {
+        console.info("No `data` field in pubSubEvent");
+        return;
+    }
+    const build = eventToBuild(pubSubEvent.data);
+
+    // Skip if the current status is not in the status list.
+    if (interestingStatuses.indexOf(build.status) === -1) {
+        console.log("Build status %s is ignored", build.status);
+        return;
+    }
+
+    // Send message to Slack.
+    const message = await createSlackMessage(build);
+    await webhook.send(message);
+};
+
+// eventToBuild transforms pubsub event message to a build object.
+const eventToBuild = (data) => {
+    return JSON.parse(Buffer.from(data, 'base64').toString());
+}
+
+const getTrigger = async (triggerId) => {
+    const client = await auth.getClient({
+        scopes: 'https://www.googleapis.com/auth/cloud-platform',
+    });
+    const projectId = await auth.getProjectId();
+    const res = await client.request({
+        url: `https://cloudbuild.googleapis.com/v1/projects/${projectId}/triggers/${triggerId}`,
+    });
+    return res.data;
+}
+
+// createSlackMessage creates a message from a build object.
+const createSlackMessage = async (build) => {
+    const buildStatus = build.statusDetail || build.status;
+    const trigger = await getTrigger(build.buildTriggerId);
+
+    let {
+        REPO_NAME: repo,
+        BRANCH_NAME: branch,
+    } = build.substitutions;
+
+    // If there was no substitution containing the repository name,
+    // take it from build.source. Unfortunately, this name will be the name
+    // used by Google Cloud Source Repositories, not the name on GitHub.
+    if (!repo && build.source.repoSource) {
+        repo = build.source.repoSource.repoName;
+    }
+
+    let messagePrefix, color;
+    switch (build.status) {
+        case 'TIMEOUT':
+            messagePrefix = "Timeout of ";
+            color = 'danger';
+            break;
+        case 'FAILURE':
+            messagePrefix = "Failed to ";
+            color = 'danger';
+            break;
+        case 'INTERNAL_ERROR':
+            messagePrefix = "Internal error while trying to ";
+            color = 'warning';
+            break;
+        case 'SUCCESS':
+            messagePrefix = "Completed ";
+            color = 'good';
+            break;
+    }
+
+    let message = {
+        text: `${messagePrefix}'${trigger.description}' for ${repo} repo`,
+        attachments: [{
+            fallback: buildStatus,
+            color: color,
+            fields: [{
+                title: 'Status',
+                value: buildStatus,
+                short: true,
+            }, {
+                title: 'Build Logs',
+                value: build.logUrl,
+            }],
+        }],
+    };
+
+    if (branch) {
+        message.attachments[0].fields.push({
+            title: 'Branch',
+            value: branch,
+        })
+    }
+
+    return message
+}