Move cache snapshot loading to Visit#start()

The cache interface changed in #949

Since the cache can be loaded from disk using the browser Cache API, it is
possible that the cache snapshot will be loaded asynchronously. This means
that the calls to load cache snapshots need to be awaited.

We also changed visit.hasCachedSnapshot() to be async and return a Promise.
However, [hasCachedSnapshot is used in the iOS adapter](https://github.com/hotwired/turbo-ios/blob/c476bac66f260adbfe930ed9a151e7637973ff99/Source/WebView/turbo.js#L119)
and serialized into and data object we send to the native side via
postMessage. When postMessagereceives a Promise instead of a boolean
value it fails with a DataCloneError since it can't serialize the Promise.

This commit moves the cache snapshot loading to Visit#start() and stores
the result in a cachedSnapshot property. This allows us to keep the
hasCachedSnapshot() method synchronous and return a boolean value.

It means that Visit.start is now async, but I haven't found any callers
that rely on it being synchronous.

src/core/drive/navigator.js

@@ -21,7 +21,7 @@ export class Navigator {
       referrer: this.location,
       ...options
     })
-    this.currentVisit.start()
+    return this.currentVisit.start()
   }
 
   submitForm(form, submitter) {

src/core/drive/visit.js

@@ -105,10 +105,11 @@ export class Visit {
     return this.isSamePage
   }
 
-  start() {
+  async start() {
     if (this.state == VisitState.initialized) {
       this.recordTimingMetric(TimingMetric.visitStart)
       this.state = VisitState.started
+      this.cachedSnapshot = await this.getCachedSnapshot()
       this.adapter.visitStarted(this)
       this.delegate.visitStarted(this)
     }
@@ -231,13 +232,12 @@ export class Visit {
     }
   }
 
-  async hasCachedSnapshot() {
-    return (await this.getCachedSnapshot()) != null
+  hasCachedSnapshot() {
+    return this.cachedSnapshot != null
   }
 
   async loadCachedSnapshot() {
-    const snapshot = await this.getCachedSnapshot()
-    if (snapshot) {
+    if (this.cachedSnapshot) {
       const isPreview = await this.shouldIssueRequest()
       this.render(async () => {
         this.cacheSnapshot()
@@ -246,7 +246,7 @@ export class Visit {
         } else {
           if (this.view.renderPromise) await this.view.renderPromise
 
-          await this.renderPageSnapshot(snapshot, isPreview)
+          await this.renderPageSnapshot(this.cachedSnapshot, isPreview)
 
           this.adapter.visitRendered(this)
           if (!isPreview) {
@@ -395,7 +395,7 @@ export class Visit {
     if (this.isSamePage) {
       return false
     } else if (this.action === "restore") {
-      return !(await this.hasCachedSnapshot())
+      return !this.hasCachedSnapshot()
     } else {
       return this.willRender
     }

src/tests/unit/deprecated_adapter_support_tests.js

@@ -50,7 +50,7 @@ test("test visit proposal location includes deprecated absoluteURL property", as
 })
 
 test("test visit start location includes deprecated absoluteURL property", async () => {
-  Turbo.navigator.startVisit(window.location.toString(), "123")
+  await Turbo.navigator.startVisit(window.location.toString(), "123")
   assert.equal(adapter.locations.length, 1)
 
   const [location] = adapter.locations

src/tests/unit/native_adapter_support_tests.js

@@ -89,7 +89,7 @@ test("test visit proposal external location is proposed to adapter", async () =>
 test("test visit started notifies adapter", async () => {
   const locatable = window.location.toString()
 
-  Turbo.navigator.startVisit(locatable)
+  await Turbo.navigator.startVisit(locatable)
   assert.equal(adapter.startedVisits.length, 1)
 
   const [visit] = adapter.startedVisits
@@ -99,7 +99,7 @@ test("test visit started notifies adapter", async () => {
 test("test visit completed notifies adapter", async () => {
   const locatable = window.location.toString()
 
-  Turbo.navigator.startVisit(locatable)
+  await Turbo.navigator.startVisit(locatable)
 
   const [startedVisit] = adapter.startedVisits
   startedVisit.complete()
@@ -111,7 +111,7 @@ test("test visit completed notifies adapter", async () => {
 test("test visit request started notifies adapter", async () => {
   const locatable = window.location.toString()
 
-  Turbo.navigator.startVisit(locatable)
+  await Turbo.navigator.startVisit(locatable)
 
   const [startedVisit] = adapter.startedVisits
   startedVisit.startRequest()
@@ -124,7 +124,7 @@ test("test visit request started notifies adapter", async () => {
 test("test visit request completed notifies adapter", async () => {
   const locatable = window.location.toString()
 
-  Turbo.navigator.startVisit(locatable)
+  await Turbo.navigator.startVisit(locatable)
 
   const [startedVisit] = adapter.startedVisits
   startedVisit.recordResponse({ statusCode: 200, responseHTML: "responseHtml", redirected: false })
@@ -137,7 +137,7 @@ test("test visit request completed notifies adapter", async () => {
 test("test visit request failed notifies adapter", async () => {
   const locatable = window.location.toString()
 
-  Turbo.navigator.startVisit(locatable)
+  await Turbo.navigator.startVisit(locatable)
 
   const [startedVisit] = adapter.startedVisits
   startedVisit.recordResponse({ statusCode: 404, responseHTML: "responseHtml", redirected: false })
@@ -150,7 +150,7 @@ test("test visit request failed notifies adapter", async () => {
 test("test visit request finished notifies adapter", async () => {
   const locatable = window.location.toString()
 
-  Turbo.navigator.startVisit(locatable)
+  await Turbo.navigator.startVisit(locatable)
 
   const [startedVisit] = adapter.startedVisits
   startedVisit.finishRequest()
@@ -181,7 +181,7 @@ test("test visit follows redirect and proposes replace visit to adapter", async
   const locatable = window.location.toString()
   const redirectedLocation = "https://example.com"
 
-  Turbo.navigator.startVisit(locatable)
+  await Turbo.navigator.startVisit(locatable)
 
   const [startedVisit] = adapter.startedVisits
   startedVisit.redirectedToLocation = redirectedLocation