fix(perf): refactor content.read to avoid lstats

lib/content/read.js

@@ -10,7 +10,8 @@ const pipe = BB.promisify(require('mississippi').pipe)
 const ssri = require('ssri')
 const Y = require('../util/y.js')
 
-BB.promisifyAll(fs)
+const lstatAsync = BB.promisify(fs.lstat)
+const readFileAsync = BB.promisify(fs.readFile)
 
 const ReadOpts = figgyPudding({
   size: {}
@@ -19,10 +20,8 @@ const ReadOpts = figgyPudding({
 module.exports = read
 function read (cache, integrity, opts) {
   opts = ReadOpts(opts)
-  return pickContentSri(cache, integrity).then(content => {
-    const sri = content.sri
-    const cpath = contentPath(cache, sri)
-    return fs.readFileAsync(cpath, null).then(data => {
+  return withContentSri(cache, integrity, (cpath, sri) => {
+    return readFileAsync(cpath, null).then(data => {
       if (typeof opts.size === 'number' && opts.size !== data.length) {
         throw sizeError(opts.size, data.length)
       } else if (ssri.checkData(data, sri)) {
@@ -39,12 +38,11 @@ module.exports.readStream = readStream
 function readStream (cache, integrity, opts) {
   opts = ReadOpts(opts)
   const stream = new PassThrough()
-  pickContentSri(
-    cache, integrity
-  ).then(content => {
-    const sri = content.sri
+  withContentSri(cache, integrity, (cpath, sri) => {
+    return lstatAsync(cpath).then(stat => ({cpath, sri, stat}))
+  }).then(({cpath, sri, stat}) => {
     return pipe(
-      fs.createReadStream(contentPath(cache, sri)),
+      fs.createReadStream(cpath),
       ssri.integrityStream({
         integrity: sri,
         size: opts.size
@@ -57,60 +55,61 @@ function readStream (cache, integrity, opts) {
   return stream
 }
 
+let copyFileAsync
 if (fs.copyFile) {
   module.exports.copy = copy
+  copyFileAsync = BB.promisify(fs.copyFile)
 }
 function copy (cache, integrity, dest, opts) {
   opts = ReadOpts(opts)
-  return pickContentSri(cache, integrity).then(content => {
-    const sri = content.sri
-    const cpath = contentPath(cache, sri)
-    return fs.copyFileAsync(cpath, dest).then(() => content.size)
+  return withContentSri(cache, integrity, (cpath, sri) => {
+    return copyFileAsync(cpath, dest)
   })
 }
 
 module.exports.hasContent = hasContent
 function hasContent (cache, integrity) {
   if (!integrity) { return BB.resolve(false) }
-  return pickContentSri(cache, integrity)
-    .catch({code: 'ENOENT'}, () => false)
-    .catch({code: 'EPERM'}, err => {
+  return withContentSri(cache, integrity, (cpath, sri) => {
+    return lstatAsync(cpath).then(stat => ({size: stat.size, sri}))
+  }).catch(err => {
+    if (err.code === 'ENOENT') { return false }
+    if (err.code === 'EPERM') {
       if (process.platform !== 'win32') {
         throw err
       } else {
         return false
       }
-    }).then(content => {
-      if (!content.sri) return false
-      return ({ sri: content.sri, size: content.stat.size })
-    })
+    }
+  })
 }
 
-module.exports._pickContentSri = pickContentSri
-function pickContentSri (cache, integrity) {
-  const sri = ssri.parse(integrity)
-  // If `integrity` has multiple entries, pick the first digest
-  // with available local data.
-  const algo = sri.pickAlgorithm()
-  const digests = sri[algo]
-  if (digests.length <= 1) {
-    const cpath = contentPath(cache, digests[0])
-    return fs.lstatAsync(cpath).then(stat => ({ sri: digests[0], stat }))
-  } else {
-    return BB.any(sri[sri.pickAlgorithm()].map(meta => {
-      return pickContentSri(cache, meta)
-    }))
-      .catch(err => {
-        if ([].some.call(err, e => e.code === 'ENOENT')) {
-          throw Object.assign(
-            new Error('No matching content found for ' + sri.toString()),
-            {code: 'ENOENT'}
-          )
-        } else {
-          throw err[0]
-        }
-      })
-  }
+function withContentSri (cache, integrity, fn) {
+  return BB.try(() => {
+    const sri = ssri.parse(integrity)
+    // If `integrity` has multiple entries, pick the first digest
+    // with available local data.
+    const algo = sri.pickAlgorithm()
+    const digests = sri[algo]
+    if (digests.length <= 1) {
+      const cpath = contentPath(cache, digests[0])
+      return fn(cpath, digests[0])
+    } else {
+      return BB.any(sri[sri.pickAlgorithm()].map(meta => {
+        return withContentSri(cache, meta, fn)
+      }, {concurrency: 1}))
+        .catch(err => {
+          if ([].some.call(err, e => e.code === 'ENOENT')) {
+            throw Object.assign(
+              new Error('No matching content found for ' + sri.toString()),
+              {code: 'ENOENT'}
+            )
+          } else {
+            throw err[0]
+          }
+        })
+    }
+  })
 }
 
 function sizeError (expected, found) {