Add "etag" response header config option (#94)

* Add "etag" response header

Utilize the `etag` module on npm to send an "etag" response header.

* Revert "Add "etag" response header"

This reverts commit 3c20819.

* Update README with `etag` option

* Implement sha1 etags

* Remove extra newline

* Strong ETag and always send Last-Modified

* Cast mtime to number

* Make Last-Modified be either/or again

* Document that it's a strong etag

README.md

@@ -47,18 +47,19 @@ await handler(request, response, {
 
 You can use any of the following options:
 
-| Property                                             | Description                                               |
-|------------------------------------------------------|-----------------------------------------------------------|
-| [`public`](#public-string)                           | Set a sub directory to be served                          |
-| [`cleanUrls`](#cleanurls-booleanarray)               | Have the `.html` extension stripped from paths            |
-| [`rewrites`](#rewrites-array)                        | Rewrite paths to different paths                          |
-| [`redirects`](#redirects-array)                      | Forward paths to different paths or external URLs         |
-| [`headers`](#headers-array)                          | Set custom headers for specific paths                     |
-| [`directoryListing`](#directorylisting-booleanarray) | Disable directory listing or restrict it to certain paths |
-| [`unlisted`](#unlisted-array)                        | Exclude paths from the directory listing                  |
-| [`trailingSlash`](#trailingslash-boolean)            | Remove or add trailing slashes to all paths               |
-| [`renderSingle`](#rendersingle-boolean)              | If a directory only contains one file, render it          |
-| [`symlinks`](#symlinks-boolean)                      | Resolve symlinks instead of rendering a 404 error         |
+| Property                                             | Description                                                           |
+|------------------------------------------------------|-----------------------------------------------------------------------|
+| [`public`](#public-string)                           | Set a sub directory to be served                                      |
+| [`cleanUrls`](#cleanurls-booleanarray)               | Have the `.html` extension stripped from paths                        |
+| [`rewrites`](#rewrites-array)                        | Rewrite paths to different paths                                      |
+| [`redirects`](#redirects-array)                      | Forward paths to different paths or external URLs                     |
+| [`headers`](#headers-array)                          | Set custom headers for specific paths                                 |
+| [`directoryListing`](#directorylisting-booleanarray) | Disable directory listing or restrict it to certain paths             |
+| [`unlisted`](#unlisted-array)                        | Exclude paths from the directory listing                              |
+| [`trailingSlash`](#trailingslash-boolean)            | Remove or add trailing slashes to all paths                           |
+| [`renderSingle`](#rendersingle-boolean)              | If a directory only contains one file, render it                      |
+| [`symlinks`](#symlinks-boolean)                      | Resolve symlinks instead of rendering a 404 error                     |
+| [`etag`](#etag-boolean)                              | Calculate a strong `ETag` response header, instead of `Last-Modified` |
 
 ### public (String)
 
@@ -274,6 +275,18 @@ However, this behavior can easily be adjusted:
 
 Once this property is set as shown above, all symlinks will automatically be resolved to their targets.
 
+### etag (Boolean)
+
+HTTP response headers will contain a strong [`ETag`][etag] response header, instead of a [`Last-Modified`][last-modified] header. Opt-in because calculating the hash value may be computationally expensive for large files.
+
+Sending an `ETag` header is disabled by default and can be enabled like this:
+
+```js
+{
+  "etag": true
+}
+```
+
 ## Error templates
 
 The handler will automatically determine the right error format if one occurs and then sends it to the client in that format.
@@ -317,3 +330,7 @@ Since it comes with support for `serve-handler` out of the box, you can create a
 ## Author
 
 Leo Lamprecht ([@notquiteleo](https://twitter.com/notquiteleo)) - [ZEIT](https://zeit.co)
+
+
+[etag]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag
+[last-modified]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Last-Modified

src/index.js

@@ -1,6 +1,7 @@
 // Native
 const {promisify} = require('util');
 const path = require('path');
+const {createHash} = require('crypto');
 const {realpath, lstat, createReadStream, readdir} = require('fs');
 
 // Packages
@@ -18,6 +19,20 @@ const parseRange = require('range-parser');
 const directoryTemplate = require('./directory');
 const errorTemplate = require('./error');
 
+const etags = new Map();
+
+const calculateSha = (handlers, absolutePath) =>
+	new Promise((resolve, reject) => {
+		const hash = createHash('sha1');
+		const rs = handlers.createReadStream(absolutePath);
+		rs.on('error', reject);
+		rs.on('data', buf => hash.update(buf));
+		rs.on('end', () => {
+			const sha = hash.digest('hex');
+			resolve(sha);
+		});
+	});
+
 const sourceMatches = (source, requestPath, allowSegments) => {
 	const keys = [];
 	const slashed = slasher(source);
@@ -177,7 +192,8 @@ const appendHeaders = (target, source) => {
 	}
 };
 
-const getHeaders = async (customHeaders = [], current, absolutePath, stats) => {
+const getHeaders = async (handlers, config, current, absolutePath, stats) => {
+	const {headers: customHeaders = [], etag = false} = config;
 	const related = {};
 	const {base} = path.parse(absolutePath);
 	const relativePath = path.relative(current, absolutePath);
@@ -199,7 +215,6 @@ const getHeaders = async (customHeaders = [], current, absolutePath, stats) => {
 
 	if (stats) {
 		defaultHeaders = {
-			'Last-Modified': stats.mtime.toUTCString(),
 			'Content-Length': stats.size,
 			// Default to "inline", which always tries to render in the browser,
 			// if that's not working, it will save the file. But to be clear: This
@@ -210,6 +225,17 @@ const getHeaders = async (customHeaders = [], current, absolutePath, stats) => {
 			'Accept-Ranges': 'bytes'
 		};
 
+		if (etag) {
+			let [mtime, sha] = etags.get(absolutePath) || [];
+			if (Number(mtime) !== Number(stats.mtime)) {
+				sha = await calculateSha(handlers, absolutePath);
+				etags.set(absolutePath, [stats.mtime, sha]);
+			}
+			defaultHeaders['ETag'] = `"${sha}"`;
+		} else {
+			defaultHeaders['Last-Modified'] = stats.mtime.toUTCString();
+		}
+
 		const contentType = mime.contentType(base);
 
 		if (contentType) {
@@ -479,7 +505,7 @@ const sendError = async (absolutePath, response, acceptsJSON, current, handlers,
 		try {
 			stream = await handlers.createReadStream(errorPage);
 
-			const headers = await getHeaders(config.headers, current, errorPage, stats);
+			const headers = await getHeaders(handlers, config, current, errorPage, stats);
 
 			response.writeHead(statusCode, headers);
 			stream.pipe(response);
@@ -490,7 +516,7 @@ const sendError = async (absolutePath, response, acceptsJSON, current, handlers,
 		}
 	}
 
-	const headers = await getHeaders(config.headers, current, absolutePath, null);
+	const headers = await getHeaders(handlers, config, current, absolutePath, null);
 	headers['Content-Type'] = 'text/html; charset=utf-8';
 
 	response.writeHead(statusCode, headers);
@@ -704,7 +730,7 @@ module.exports = async (request, response, config = {}, methods = {}) => {
 		return internalError(absolutePath, response, acceptsJSON, current, handlers, config, err);
 	}
 
-	const headers = await getHeaders(config.headers, current, absolutePath, stats);
+	const headers = await getHeaders(handlers, config, current, absolutePath, stats);
 
 	// eslint-disable-next-line no-undefined
 	if (streamOpts.start !== undefined && streamOpts.end !== undefined) {

test/integration.js

@@ -1079,7 +1079,7 @@ test('automatically handle ETag headers for normal files', async t => {
 	const name = 'object.json';
 	const related = path.join(fixturesFull, name);
 	const content = await fs.readJSON(related);
-	const value = 'd2ijdjoi29f3h3232';
+	const value = '"d2ijdjoi29f3h3232"';
 
 	const url = await getUrl({
 		headers: [{
@@ -1329,3 +1329,16 @@ test('allow symlinks by setting the option', async t => {
 
 	t.is(text, spec);
 });
+
+test('etag header is set', async t => {
+	const directory = 'single-directory';
+	const url = await getUrl({
+		renderSingle: true,
+		etag: true
+	});
+	const response = await fetch(`${url}/${directory}`);
+	t.is(
+		response.headers.get('etag'),
+		'"4e5f19df3bfe8db7d588edfc3960991aa0715ccf"'
+	);
+});