Add support for caching the Slack user profiles locally

lib/slack-profile-cache.js

@@ -9,9 +9,12 @@
 //   PombeirP
 
 const MemCache = require('mem-cache')
+const fs = require('fs')
 const { WebClient } = require('@slack/client')
 
 const token = process.env.SLACK_USER_TOKEN || ''
+const cachePath = './slack-cache.json'
+var allowLoadFromCache = true
 
 module.exports = (robot) => new GitHubSlackIdMapper(robot)
 
@@ -49,6 +52,24 @@ class GitHubSlackIdMapper {
 }
 
 async function internalBuild (robot, cache) {
+  if (allowLoadFromCache) {
+    try {
+      const json = fs.readFileSync(cachePath)
+      if (json) {
+        const cacheFromFile = JSON.parse(json)
+        for (const kvp of cacheFromFile) {
+          cache.set(kvp.k, kvp.v)
+        }
+        robot.log.info(`Read Slack user cache from file (${cache.length} entries)`)
+        allowLoadFromCache = false
+        return
+      }
+    } catch (error) {
+      // Ignore
+      robot.log.info('Could not find Slack user cache')
+    }
+  }
+
   robot.log.info('Populating Slack user ID cache...')
 
   try {
@@ -117,6 +138,13 @@ async function internalBuild (robot, cache) {
     if (usersMissingGitHubInfo) {
       robot.log.warn(`The following ${usersMissingGitHubInfo.length} Slack users have no GitHub info in their profiles: ${usersMissingGitHubInfo.map(s => '@' + s).join(', ')}`)
     }
+
+    // Write cache out to JSON file for faster startup next time
+    const c = []
+    for (const key of cache.keys) {
+      c.push({ k: key, v: cache.get(key) })
+    }
+    fs.writeFileSync(cachePath, JSON.stringify(c, null, 2))
   } catch (e) {
     robot.log.error(`Error while populating Slack user ID cache: ${e}`)
   }