From dc81373cc9ad4ca1fd7c26e3d64f130d386ad29d Mon Sep 17 00:00:00 2001
From: Glen Mailer <glenjamin@gmail.com>
Date: Tue, 8 Sep 2015 22:22:36 +0100
Subject: [PATCH] Skip compression when response has Cache-Control:
 no-transform

closes #51
closes #53
---
 HISTORY.md          |  1 +
 README.md           |  4 ++++
 index.js            | 27 +++++++++++++++++++++++++++
 test/compression.js | 32 ++++++++++++++++++++++++++++++++
 4 files changed, 64 insertions(+)

diff --git a/HISTORY.md b/HISTORY.md
index 23d65933..270e6d69 100644
--- a/HISTORY.md
+++ b/HISTORY.md
@@ -1,6 +1,7 @@
 unreleased
 ==========
 
+  * Skip compression when response has `Cache-Control: no-transform`
   * deps: accepts@~1.3.0
     - deps: mime-types@~2.1.7
     - deps: negotiator@0.6.0
diff --git a/README.md b/README.md
index 4d0abdff..a5d599db 100644
--- a/README.md
+++ b/README.md
@@ -31,6 +31,10 @@ Returns the compression middleware using the given `options`. The middleware
 will attempt to compress response bodies for all request that traverse through
 the middleware, based on the given `options`.
 
+This middleware will never compress responses that include a `Cache-Control`
+header with the [`no-transform` directive](https://tools.ietf.org/html/rfc7234#section-5.2.2.4),
+as compressing will transform the body.
+
 #### Options
 
 `compression()` accepts these properties in the options object. In addition to
diff --git a/index.js b/index.js
index 8848294d..acfc2a21 100644
--- a/index.js
+++ b/index.js
@@ -29,6 +29,13 @@ var zlib = require('zlib')
 module.exports = compression
 module.exports.filter = shouldCompress
 
+/**
+ * Module variables.
+ * @private
+ */
+
+var cacheControlNoTransformRegExp = /(?:^|,)\s*?no-transform\s*?(?:,|$)/
+
 /**
  * Compress response data with gzip / deflate.
  *
@@ -135,6 +142,12 @@ function compression(options) {
         return
       }
 
+      // determine if the entity should be transformed
+      if (!shouldTransform(req, res)) {
+        nocompress('no transform')
+        return
+      }
+
       // vary
       vary(res, 'Accept-Encoding')
 
@@ -246,3 +259,17 @@ function shouldCompress(req, res) {
 
   return true
 }
+
+/**
+ * Determine if the entity should be transformed.
+ * @private
+ */
+
+function shouldTransform(req, res) {
+  var cacheControl = res.getHeader('Cache-Control')
+
+  // Don't compress for Cache-Control: no-transform
+  // https://tools.ietf.org/html/rfc7234#section-5.2.2.4
+  return !cacheControl
+    || !cacheControlNoTransformRegExp.test(cacheControl)
+}
diff --git a/test/compression.js b/test/compression.js
index 1efd4334..26e0ca26 100644
--- a/test/compression.js
+++ b/test/compression.js
@@ -492,6 +492,38 @@ describe('compression()', function(){
     })
   })
 
+  describe('when "Cache-Control: no-transform" response header', function () {
+    it('should not compress response', function (done) {
+      var server = createServer({ threshold: 0 }, function (req, res) {
+        res.setHeader('Cache-Control', 'no-transform')
+        res.setHeader('Content-Type', 'text/plain')
+        res.end('hello, world')
+      })
+
+      request(server)
+      .get('/')
+      .set('Accept-Encoding', 'gzip')
+      .expect('Cache-Control', 'no-transform')
+      .expect(shouldNotHaveHeader('Content-Encoding'))
+      .expect(200, 'hello, world', done)
+    })
+
+    it('should not set Vary headerh', function (done) {
+      var server = createServer({ threshold: 0 }, function (req, res) {
+        res.setHeader('Cache-Control', 'no-transform')
+        res.setHeader('Content-Type', 'text/plain')
+        res.end('hello, world')
+      })
+
+      request(server)
+      .get('/')
+      .set('Accept-Encoding', 'gzip')
+      .expect('Cache-Control', 'no-transform')
+      .expect(shouldNotHaveHeader('Vary'))
+      .expect(200, done)
+    })
+  })
+
   describe('.filter', function () {
     it('should be a function', function () {
       assert.equal(typeof compression.filter, 'function')