perf(finalize-manifest): cache finalized manifests

Fixes: #35

lib/finalize-manifest.js

@@ -2,6 +2,7 @@
 
 const BB = require('bluebird')
 
+const cache = require('./cache')
 const checksumStream = require('checksum-stream')
 const dezalgo = require('dezalgo')
 const finished = require('mississippi').finished
@@ -17,12 +18,31 @@ const through = require('mississippi').through
 
 module.exports = finalizeManifest
 function finalizeManifest (pkg, spec, opts) {
-  return BB.fromNode(cb => {
-    tarballedProps(pkg, spec, opts, (err, props) => {
-      if (err) { return cb(err) }
-      // normalize should not add any fields, and once
-      // makeManifest completes, it should never be modified.
-      cb(null, new Manifest(pkg, props))
+  const key = cache.key(`${spec.type}-manifest`, pkg._resolved)
+  opts = optCheck(opts)
+  opts.memoize = true
+  return (opts.cache ? cache.get.info(opts.cache, key, opts).then(res => {
+    if (!res) { throw new Error('cache miss') }
+    return new Manifest(res.metadata)
+  }) : BB.reject()).catch(() => {
+    return BB.fromNode(cb => {
+      tarballedProps(pkg, spec, opts, function (err, props) {
+        if (err) { return cb(err) }
+        // normalize should not add any fields, and once
+        // makeManifest completes, it should never be modified.
+        var result = new Manifest(pkg, props)
+        if (opts.cache) {
+          opts.metadata = result
+          cache.put(
+            opts.cache,
+            key,
+            '.',
+            opts
+          ).then(() => cb(null, result), cb)
+        } else {
+          cb(null, result)
+        }
+      })
     })
   })
 }