Instrument the warmServer method more precisely (#16698)

* Instrument the `warmServer` method more precisely

This way we only report on it if we have to actually prime it vs. with every call to get the cached results

* Skip the extra variable

* Use Boolean to make intent more explicit

Author: JamesMGreene

lib/warm-server.js

@@ -1,29 +1,69 @@
 const statsd = require('./statsd')
 const fetchEarlyAccessPaths = require('./fetch-early-access-paths')
+const loadPages = require('./pages')
+const loadRedirects = require('./redirects/precompile')
+const loadSiteData = require('./site-data')
+const loadSiteTree = require('./site-tree')
+
+// For local caching
 let pages, site, redirects, siteTree, earlyAccessPaths
 
+function isFullyWarmed () {
+  return Boolean(pages && site && earlyAccessPaths && redirects && siteTree)
+}
+
+function getWarmedCache () {
+  return {
+    pages,
+    site,
+    redirects,
+    siteTree,
+    earlyAccessPaths
+  }
+}
+
 async function warmServer () {
-  if (!pages) {
-    if (process.env.NODE_ENV !== 'test') {
-      console.log('Priming context information')
-    }
+  const startTime = Date.now()
+
+  if (process.env.NODE_ENV !== 'test') {
+    console.log('Priming context information...')
+  }
 
+  if (!pages || !site || !earlyAccessPaths) {
     // Promise.all is used to load multiple things in parallel
     [pages, site, earlyAccessPaths] = await Promise.all([
-      require('./pages')(),
-      require('./site-data')(),
-      fetchEarlyAccessPaths()
+      pages || loadPages(),
+      site || loadSiteData(),
+      earlyAccessPaths || fetchEarlyAccessPaths()
     ])
+  }
 
-    redirects = await require('./redirects/precompile')(pages)
-    siteTree = await require('./site-tree')(pages, site, redirects)
+  if (!redirects) {
+    redirects = await loadRedirects(pages)
   }
 
-  return {
-    pages, site, redirects, siteTree, earlyAccessPaths
+  if (!siteTree) {
+    siteTree = await loadSiteTree(pages, site, redirects)
   }
+
+  if (process.env.NODE_ENV !== 'test') {
+    console.log(`Context primed in ${Date.now() - startTime} ms`)
+  }
+
+  return getWarmedCache()
 }
 
 // Instrument the `warmServer` function so that
 // it's wrapped in a timer that reports to Datadog
-module.exports = statsd.asyncTimer(warmServer, 'warm_server')
+const instrumentedWarmServer = statsd.asyncTimer(warmServer, 'warm_server')
+
+// We only want statistics if the priming needs to occur, so let's wrap the
+// real method and return early [without statistics] whenever possible
+module.exports = async function warmServerWrapper () {
+  // Bail out early if everything is properly ready to use
+  if (isFullyWarmed()) {
+    return getWarmedCache()
+  }
+
+  return instrumentedWarmServer()
+}