feat: elect build leader by highest node version

BREAKING CHANGE: The first job in the build matrix is no longer
automatically elected to be the build leader.
Instead the build job with the highest node version is selected.
If you want to control which job is the build leader set the
environment variable `BUILD_LEADER_ID`.

elect-build-leader.js

@@ -0,0 +1,23 @@
+const semver = require('semver')
+
+module.exports = versions => {
+  // if there is only one candidate, then it's the winner
+  if (!Array.isArray(versions) || versions.length === 1) return 1
+
+  // if there is latest stable it's the winner
+  const stable = versions.indexOf('node') + 1
+  if (stable) return stable
+
+  // otherwise we use the lower bound of all valid semver ranges
+  const validRanges = versions.filter(semver.validRange)
+  const lowVersionBoundaries = validRanges
+  .map(semver.Range)
+  .map(r => r.set[0][0].semver.version)
+
+  // then we find the highest of those
+  const highestVersion = semver.sort(Array.from(lowVersionBoundaries)).pop()
+  const highestRange = validRanges[lowVersionBoundaries.indexOf(highestVersion)]
+
+  // and make its build job the winner
+  return versions.indexOf(highestRange) + 1
+}

index.js

@@ -3,11 +3,11 @@ const {promisify} = require('util')
 const request = require('axios')
 const Travis = require('travis-ci')
 
+const electBuildLeader = require('./elect-build-leader')
+
 module.exports = async function travisDeployOnce (env = process.env) {
   if (!env.GH_TOKEN) throw new Error('GitHub token missing')
   if (env.TRAVIS !== 'true') throw new Error('Not running on Travis')
-  if (!env.TRAVIS_JOB_NUMBER.endsWith('.1')) return null
-  if (env.TRAVIS_TEST_RESULT === '1') return false
   if (env.TRAVIS_TEST_RESULT !== '0') throw new Error('Not running in Travis after_success hook')
 
   const {private: pro} = await request({
@@ -32,7 +32,14 @@ module.exports = async function travisDeployOnce (env = process.env) {
 
   const buildId = parseInt(env.TRAVIS_BUILD_ID, 10)
   const buildApi = travis.builds(buildId)
-  const {build: {job_ids: jobs}} = await promisify(buildApi.get.bind(buildApi))()
+  const {build: {config, job_ids: jobs}} = await promisify(buildApi.get.bind(buildApi))()
+
+  const buildLeader = env.BUILD_LEADER_ID || (config.node_js
+    ? electBuildLeader(config.node_js)
+    : 1)
+
+  if (!env.TRAVIS_JOB_NUMBER.endsWith(`.${buildLeader}`)) return null
+  if (env.TRAVIS_TEST_RESULT === '1') return false
 
   const currentJobId = parseInt(env.TRAVIS_JOB_ID, 10)
   let attempt = 0