feat: cache etag support

Signed-off-by: flakey5 <73616808+flakey5@users.noreply.github.com>

Author: flakey5

lib/handler/cache-handler.js

@@ -4,7 +4,8 @@ const util = require('../core/util')
 const DecoratorHandler = require('../handler/decorator-handler')
 const {
   parseCacheControlHeader,
-  parseVaryHeader
+  parseVaryHeader,
+  isEtagUsable
 } = require('../util/cache')
 
 function noop () {}
@@ -135,15 +136,24 @@ class CacheHandler extends DecoratorHandler {
         cacheControlDirectives
       )
 
-      this.#writeStream = this.#store.createWriteStream(this.#cacheKey, {
+      /**
+       * @type {import('../../types/cache-interceptor.d.ts').default.CachedResponse}
+       */
+      const value = {
         statusCode,
         statusMessage,
         rawHeaders: strippedHeaders,
         vary: varyDirectives,
         cachedAt: now,
         staleAt,
         deleteAt
-      })
+      }
+
+      if (typeof headers.etag === 'string' && isEtagUsable(headers.etag)) {
+        value.etag = headers.etag
+      }
+
+      this.#writeStream = this.#store.createWriteStream(this.#cacheKey, value)
 
       if (this.#writeStream) {
         const handler = this

lib/interceptor/cache.js

@@ -151,7 +151,8 @@ module.exports = (opts = {}) => {
               ...opts,
               headers: {
                 ...opts.headers,
-                'if-modified-since': new Date(result.cachedAt).toUTCString()
+                'if-modified-since': new Date(result.cachedAt).toUTCString(),
+                etag: result.etag
               }
             },
             new CacheRevalidationHandler(

lib/util/cache.js

@@ -201,6 +201,48 @@ function parseVaryHeader (varyHeader, headers) {
   return output
 }
 
+/**
+ * Note: this deviates from the spec a little. Empty etags ("", W/"") are valid,
+ *  however, including them in cached resposnes serves little to no purpose.
+ *
+ * @see https://www.rfc-editor.org/rfc/rfc9110.html#name-etag
+ *
+ * @param {string} etag
+ * @returns {boolean}
+ */
+function isEtagUsable (etag) {
+  if (etag.length <= 2) {
+    // Shortest an etag can be is two chars (just ""). This is where we deviate
+    //  from the spec requiring a min of 3 chars however
+    return false
+  }
+
+  if (etag[0] === '"' && etag[etag.length - 1] === '"') {
+    if (etag[1] === '"' || etag.startsWith('"W/')) {
+      // ETag: ""asd123"" or ETag: "W/"asd123"", kinda undefined behavior in the
+      //  spec. Some servers will accept these while others don't.
+      return false
+    }
+
+    // ETag: "asd123"
+    return true
+  }
+
+  if (etag.startsWith('W/"') && etag[etag.length - 1] === '"') {
+    if (etag.length === 4) {
+      // ETag: W/"", also where we deviate from the spec & require a min of 3
+      //  chars
+      return false
+    }
+
+    // ETag: for W/"", W/"asd123"
+    return true
+  }
+
+  // Anything else
+  return false
+}
+
 /**
  * @param {unknown} store
  * @returns {asserts store is import('../../types/cache-interceptor.d.ts').default.CacheStore}
@@ -244,6 +286,7 @@ module.exports = {
   makeCacheKey,
   parseCacheControlHeader,
   parseVaryHeader,
+  isEtagUsable,
   assertCacheMethods,
   assertCacheStore
 }

test/cache-interceptor/utils.js

@@ -1,8 +1,8 @@
 'use strict'
 
 const { describe, test } = require('node:test')
-const { deepStrictEqual } = require('node:assert')
-const { parseCacheControlHeader, parseVaryHeader } = require('../../lib/util/cache')
+const { deepStrictEqual, equal } = require('node:assert')
+const { parseCacheControlHeader, parseVaryHeader, isEtagUsable } = require('../../lib/util/cache')
 
 describe('parseCacheControlHeader', () => {
   test('all directives are parsed properly when in their correct format', () => {
@@ -215,3 +215,28 @@ describe('parseVaryHeader', () => {
     })
   })
 })
+
+describe('isEtagUsable', () => {
+  const valuesToTest = {
+    // Invalid etags
+    '': false,
+    asd: false,
+    '"W/"asd""': false,
+    '""asd""': false,
+
+    // Valid etags
+    '"asd"': true,
+    'W/"ads"': true,
+
+    // Spec deviations
+    '""': false,
+    'W/""': false
+  }
+
+  for (const key in valuesToTest) {
+    const expectedValue = valuesToTest[key]
+    test(`\`${key}\` = ${expectedValue}`, () => {
+      equal(isEtagUsable(key), expectedValue)
+    })
+  }
+})

test/interceptors/cache.js

@@ -220,6 +220,72 @@ describe('Cache Interceptor', () => {
     strictEqual(await response.body.text(), 'asd123')
   })
 
+  test('revalidates request w/ etag when provided', async (t) => {
+    let requestsToOrigin = 0
+
+    const clock = FakeTimers.install({
+      shouldClearNativeTimers: true
+    })
+
+    const server = createServer((req, res) => {
+      res.setHeader('cache-control', 'public, s-maxage=1, stale-while-revalidate=10')
+      requestsToOrigin++
+
+      if (requestsToOrigin > 1) {
+        equal(req.headers['etag'], '"asd123"')
+
+        if (requestsToOrigin === 3) {
+          res.end('asd123')
+        } else {
+          res.statusCode = 304
+          res.end()
+        }
+      } else {
+        res.setHeader('etag', '"asd123"')
+        res.end('asd')
+      }
+    }).listen(0)
+
+    const client = new Client(`http://localhost:${server.address().port}`)
+      .compose(interceptors.cache())
+
+    after(async () => {
+      server.close()
+      await client.close()
+      clock.uninstall()
+    })
+
+    await once(server, 'listening')
+
+    strictEqual(requestsToOrigin, 0)
+
+    const request = {
+      origin: 'localhost',
+      method: 'GET',
+      path: '/'
+    }
+
+    // Send initial request. This should reach the origin
+    let response = await client.request(request)
+    strictEqual(requestsToOrigin, 1)
+    strictEqual(await response.body.text(), 'asd')
+
+    clock.tick(1500)
+
+    // Now we send two more requests. Both of these should reach the origin,
+    //  but now with a conditional header asking if the resource has been
+    //  updated. These need to be ran after the response is stale.
+    // No update for the second request
+    response = await client.request(request)
+    strictEqual(requestsToOrigin, 2)
+    strictEqual(await response.body.text(), 'asd')
+
+    // This should be updated, even though the value isn't expired.
+    response = await client.request(request)
+    strictEqual(requestsToOrigin, 3)
+    strictEqual(await response.body.text(), 'asd123')
+  })
+
   test('respects cache store\'s isFull property', async () => {
     const server = createServer((_, res) => {
       res.end('asd')

types/cache-interceptor.d.ts

@@ -53,6 +53,7 @@ declare namespace CacheHandler {
     statusCode: number;
     statusMessage: string;
     rawHeaders: Buffer[];
+    etag?: string;
     /**
      * Headers defined by the Vary header and their respective values for
      *  later comparison