[now dev] Add etag response header for Lambda invocations (#2502)

This matches the behavior in production.

src/util/dev/builder-cache.ts

@@ -1,7 +1,6 @@
 import chalk from 'chalk';
 import execa from 'execa';
 import npa from 'npm-package-arg';
-import { createHash } from 'crypto';
 import { join, resolve } from 'path';
 import { funCacheDir } from '@zeit/fun';
 import cacheDirectory from 'cache-or-tmp-directory';
@@ -12,6 +11,7 @@ import {
   BuilderCacheCleanError
 } from '../errors-ts';
 import wait from '../output/wait';
+import { getSha } from '../sha';
 import { Output } from '../output';
 
 import * as staticBuilder from './static-builder';
@@ -215,9 +215,3 @@ function getPackageName(
   }
   return null;
 }
-
-function getSha(buffer: Buffer): string {
-  const hash = createHash('sha256');
-  hash.update(buffer);
-  return hash.digest('hex');
-}

src/util/dev/builder.ts

@@ -11,6 +11,7 @@ import chalk from 'chalk';
 import which from 'which';
 import ora, { Ora } from 'ora';
 
+import { getSha } from '../sha';
 import { Output } from '../output';
 import { relative } from '../path-helpers';
 import { LambdaSizeExceededError } from '../errors-ts';
@@ -354,6 +355,7 @@ export async function executeBuild(
             }
           }
         });
+        asset.sha = getSha(asset.zipBuffer, 'sha1');
       }
 
       match.buildTimestamp = Date.now();

src/util/dev/server.ts

@@ -839,7 +839,7 @@ export default class DevServer {
         return;
 
       case 'Lambda':
-        if (!asset.fn) {
+        if (!asset.fn || !asset.sha) {
           // This is mostly to appease TypeScript since `fn` is an optional prop,
           // but this shouldn't really ever happen since we run the builds before
           // responding to HTTP requests.
@@ -894,6 +894,7 @@ export default class DevServer {
 
         res.statusCode = result.statusCode;
         this.setResponseHeaders(res, nowRequestId, result.headers);
+        res.setHeader('etag', `W/"${asset.sha}"`);
 
         let resBody: Buffer | string | undefined;
         if (result.encoding === 'base64' && typeof result.body === 'string') {

src/util/dev/types.ts

@@ -59,6 +59,7 @@ export interface BuilderInputs {
 
 export type BuiltLambda = Lambda & {
   fn?: FunLambda;
+  sha?: string;
 };
 
 export type BuilderOutput = BuiltLambda | FileFsRef | FileBlob;

src/util/sha.ts

@@ -0,0 +1,7 @@
+import { createHash } from 'crypto';
+
+export function getSha(buffer: Buffer, cypher: string = 'sha256'): string {
+  const hash = createHash(cypher);
+  hash.update(buffer);
+  return hash.digest('hex');
+}

test/dev-server.unit.js

@@ -44,6 +44,7 @@ function testFixture(name, fn) {
 function validateResponseHeaders(t, res) {
   t.is(res.headers.get('cache-control'), 'public, max-age=0, must-revalidate');
   t.is(res.headers.get('x-now-trace'), 'dev1');
+  t.truthy(/^W\/"[0-9a-f]{40}"$/.test(res.headers.get('etag')));
   t.truthy(
     /^dev1:[0-9a-z]{5}-[1-9][0-9]+-[a-f0-9]{12}$/.test(
       res.headers.get('x-now-id')