Add disk cache store

This commit extends the Turbo cache API to allow for the use of
different cache stores. The default store is still the in-memory store,
but a new persistent cache store is now available.

The disk store uses the [Cache API](https://developer.mozilla.org/en-US/docs/Web/API/CacheStorage)
to store and retrieve snapshots from the browser's cache. This allows
for the snapshots to be persisted across different tabs, page reloads
and even browser restarts.

The disk store is not enabled by default. To enable it, you need to set
the `Turbo.cache.store` property to `"disk"`.

```js
Turbo.cache.store = "disk"
```

This is also a stepping stone to implement offline support with Service
Workers. With a Service Worker in place, and the disk cache store
enabled, we can still serve cached snapshots even when the browser is
offline.

src/core/cache.js

@@ -1,12 +1,13 @@
 import { setMetaContent } from "../util"
+import { SnapshotCache } from "./drive/snapshot_cache"
 
 export class Cache {
   constructor(session) {
     this.session = session
   }
 
   clear() {
-    this.session.clearCache()
+    this.store.clear()
   }
 
   resetCacheControl() {
@@ -21,6 +22,18 @@ export class Cache {
     this.#setCacheControl("no-preview")
   }
 
+  set store(store) {
+    if (typeof store === "string") {
+      SnapshotCache.setStore(store)
+    } else {
+      SnapshotCache.currentStore = store
+    }
+  }
+
+  get store() {
+    return SnapshotCache.currentStore
+  }
+
   #setCacheControl(value) {
     setMetaContent("turbo-cache-control", value)
   }

src/core/drive/cache_stores/disk_store.js

@@ -0,0 +1,64 @@
+import { PageSnapshot } from "../page_snapshot"
+
+export class DiskStore {
+  _version = "v1"
+
+  constructor() {
+    if (typeof caches === "undefined") {
+      throw new Error("windows.caches is undefined. CacheStore requires a secure context.")
+    }
+
+    this.storage = this.openStorage()
+  }
+
+  async has(location) {
+    const storage = await this.openStorage()
+    return (await storage.match(location)) !== undefined
+  }
+
+  async get(location) {
+    const storage = await this.openStorage()
+    const response = await storage.match(location)
+
+    if (response && response.ok) {
+      const html = await response.text()
+      return PageSnapshot.fromHTMLString(html)
+    }
+  }
+
+  async put(location, snapshot) {
+    const storage = await this.openStorage()
+
+    const response = new Response(snapshot.html, {
+      status: 200,
+      statusText: "OK",
+      headers: {
+        "Content-Type": "text/html"
+      }
+    })
+    await storage.put(location, response)
+    return snapshot
+  }
+
+  async clear() {
+    const storage = await this.openStorage()
+    const keys = await storage.keys()
+    await Promise.all(keys.map((key) => storage.delete(key)))
+  }
+
+  openStorage() {
+    this.storage ||= caches.open(`turbo-${this.version}`)
+    return this.storage
+  }
+
+  set version(value) {
+    if (value !== this._version) {
+      this._version = value
+      this.storage ||= caches.open(`turbo-${this.version}`)
+    }
+  }
+
+  get version() {
+    return this._version
+  }
+}

src/core/drive/cache_stores/memory_store.js

@@ -0,0 +1,56 @@
+import { toCacheKey } from "../../url"
+
+export class MemoryStore {
+  keys = []
+  snapshots = {}
+
+  constructor(size) {
+    this.size = size
+  }
+
+  async has(location) {
+    return toCacheKey(location) in this.snapshots
+  }
+
+  async get(location) {
+    if (await this.has(location)) {
+      const snapshot = this.read(location)
+      this.touch(location)
+      return snapshot
+    }
+  }
+
+  async put(location, snapshot) {
+    this.write(location, snapshot)
+    this.touch(location)
+    return snapshot
+  }
+
+  async clear() {
+    this.snapshots = {}
+  }
+
+  // Private
+
+  read(location) {
+    return this.snapshots[toCacheKey(location)]
+  }
+
+  write(location, snapshot) {
+    this.snapshots[toCacheKey(location)] = snapshot
+  }
+
+  touch(location) {
+    const key = toCacheKey(location)
+    const index = this.keys.indexOf(key)
+    if (index > -1) this.keys.splice(index, 1)
+    this.keys.unshift(key)
+    this.trim()
+  }
+
+  trim() {
+    for (const key of this.keys.splice(this.size)) {
+      delete this.snapshots[key]
+    }
+  }
+}

src/core/drive/page_snapshot.js

@@ -40,6 +40,10 @@ export class PageSnapshot extends Snapshot {
     return new PageSnapshot(clonedElement, this.headSnapshot)
   }
 
+  get html() {
+    return `${this.headElement.outerHTML}\n\n${this.element.outerHTML}`
+  }
+
   get headElement() {
     return this.headSnapshot.element
   }

src/core/drive/page_view.js

@@ -6,7 +6,7 @@ import { PageSnapshot } from "./page_snapshot"
 import { SnapshotCache } from "./snapshot_cache"
 
 export class PageView extends View {
-  snapshotCache = new SnapshotCache(10)
+  snapshotCache = new SnapshotCache()
   lastRenderedLocation = new URL(location.href)
   forceReloaded = false
 
@@ -32,6 +32,10 @@ export class PageView extends View {
     return this.render(renderer)
   }
 
+  setCacheStore(cacheName) {
+    SnapshotCache.setStore(cacheName)
+  }
+
   clearSnapshotCache() {
     this.snapshotCache.clear()
   }

src/core/drive/preloader.js

@@ -30,9 +30,7 @@ export class Preloader {
   async preloadURL(link) {
     const location = new URL(link.href)
 
-    if (this.snapshotCache.has(location)) {
-      return
-    }
+    if (await this.snapshotCache.has(location)) return
 
     try {
       const response = await fetch(location.toString(), { headers: { "Sec-Purpose": "prefetch", Accept: "text/html" } })

src/core/drive/snapshot_cache.js

@@ -1,56 +1,35 @@
-import { toCacheKey } from "../url"
+import { DiskStore } from "./cache_stores/disk_store"
+import { MemoryStore } from "./cache_stores/memory_store"
 
 export class SnapshotCache {
-  keys = []
-  snapshots = {}
-
-  constructor(size) {
-    this.size = size
+  static currentStore = new MemoryStore(10)
+
+  static setStore(storeName) {
+    switch (storeName) {
+      case "memory":
+        SnapshotCache.currentStore = new MemoryStore(10)
+        break
+      case "disk":
+        SnapshotCache.currentStore = new DiskStore()
+        break
+      default:
+        throw new Error(`Invalid store name: ${storeName}`)
+    }
   }
 
   has(location) {
-    return toCacheKey(location) in this.snapshots
+    return SnapshotCache.currentStore.has(location)
   }
 
   get(location) {
-    if (this.has(location)) {
-      const snapshot = this.read(location)
-      this.touch(location)
-      return snapshot
-    }
+    return SnapshotCache.currentStore.get(location)
   }
 
   put(location, snapshot) {
-    this.write(location, snapshot)
-    this.touch(location)
-    return snapshot
+    return SnapshotCache.currentStore.put(location, snapshot)
   }
 
   clear() {
-    this.snapshots = {}
-  }
-
-  // Private
-
-  read(location) {
-    return this.snapshots[toCacheKey(location)]
-  }
-
-  write(location, snapshot) {
-    this.snapshots[toCacheKey(location)] = snapshot
-  }
-
-  touch(location) {
-    const key = toCacheKey(location)
-    const index = this.keys.indexOf(key)
-    if (index > -1) this.keys.splice(index, 1)
-    this.keys.unshift(key)
-    this.trim()
-  }
-
-  trim() {
-    for (const key of this.keys.splice(this.size)) {
-      delete this.snapshots[key]
-    }
+    return SnapshotCache.currentStore.clear()
   }
 }

src/core/drive/visit.js

@@ -215,8 +215,8 @@ export class Visit {
     }
   }
 
-  getCachedSnapshot() {
-    const snapshot = this.view.getCachedSnapshotForLocation(this.location) || this.getPreloadedSnapshot()
+  async getCachedSnapshot() {
+    const snapshot = (await this.view.getCachedSnapshotForLocation(this.location)) || this.getPreloadedSnapshot()
 
     if (snapshot && (!getAnchor(this.location) || snapshot.hasAnchor(getAnchor(this.location)))) {
       if (this.action == "restore" || snapshot.isPreviewable) {
@@ -235,8 +235,8 @@ export class Visit {
     return this.getCachedSnapshot() != null
   }
 
-  loadCachedSnapshot() {
-    const snapshot = this.getCachedSnapshot()
+  async loadCachedSnapshot() {
+    const snapshot = await this.getCachedSnapshot()
     if (snapshot) {
       const isPreview = this.shouldIssueRequest()
       this.render(async () => {

src/tests/fixtures/disk_cache.html

@@ -0,0 +1,49 @@
+<!DOCTYPE html>
+<html id="html" data-skip-event-details="turbo:submit-start turbo:submit-end">
+  <head>
+    <meta charset="utf-8">
+    <meta name="csp-nonce" content="123">
+    <link rel="icon" href="data:;base64,iVBORw0KGgo=">
+    <title>Turbo</title>
+    <script src="/dist/turbo.es2017-umd.js" data-turbo-track="reload"></script>
+    <script src="/src/tests/fixtures/test.js"></script>
+    <script>
+      Turbo.cache.store = "disk"
+
+      document.addEventListener("turbo:load", async () => {
+        await new Promise(resolve => setTimeout(resolve, 100))
+
+        const cachesList = document.getElementById("caches")
+
+        const cache = await caches.open("turbo-v1")
+        const keys = await cache.keys()
+        cachesList.innerHTML = keys.map(key => `<li>${key.url}</li>`).join("")
+
+        const clearCacheButton = document.getElementById("clear-cache")
+        clearCacheButton.addEventListener("click", async (event) => {
+          await Turbo.cache.clear()
+          cachesList.innerHTML = ""
+        })
+      })
+    </script>
+  </head>
+  <body>
+    <h1>Cached pages:</h1>
+    <ul id="caches"></ul>
+
+    <h3>Links:</h3>
+    <ul>
+      <li>
+        <a id="first-link" href="./disk_cache.html">First HTTP cached page</a>
+      </li>
+      <li>
+        <a id="second-link" href="./disk_cache.html?page=2">Second HTTP cached page</a>
+      </li>
+      <li>
+        <a id="third-link" href="./disk_cache.html?page=3">Third HTTP cached page</a>
+      </li>
+    </ul>
+
+    <button id="clear-cache">Clear cache</button>
+  </body>
+</html>