Rename isIdempotent to isSafe (#883)

The method name was a bit misleading, as what we're actually checking
for here is whether the method is [safe][0].

We can also now make use of this method in a couple of places where we
clear the snapshot cache after form submissions. The logic there is to
clear the cache when performing an operation that is unsafe.

[0]: https://developer.mozilla.org/en-US/docs/Glossary/Safe/HTTP

src/core/drive/form_submission.ts

@@ -110,8 +110,8 @@ export class FormSubmission {
     return formEnctypeFromString(this.submitter?.getAttribute("formenctype") || this.formElement.enctype)
   }
 
-  get isIdempotent() {
-    return this.fetchRequest.isIdempotent
+  get isSafe() {
+    return this.fetchRequest.isSafe
   }
 
   get stringFormData() {
@@ -151,7 +151,7 @@ export class FormSubmission {
   // Fetch request delegate
 
   prepareRequest(request: FetchRequest) {
-    if (!request.isIdempotent) {
+    if (!request.isSafe) {
       const token = getCookieValue(getMetaContent("csrf-param")) || getMetaContent("csrf-token")
       if (token) {
         request.headers["X-CSRF-Token"] = token
@@ -239,11 +239,11 @@ export class FormSubmission {
   }
 
   requestMustRedirect(request: FetchRequest) {
-    return !request.isIdempotent && this.mustRedirect
+    return !request.isSafe && this.mustRedirect
   }
 
   requestAcceptsTurboStreamResponse(request: FetchRequest) {
-    return !request.isIdempotent || hasAttribute("data-turbo-stream", this.submitter, this.formElement)
+    return !request.isSafe || hasAttribute("data-turbo-stream", this.submitter, this.formElement)
   }
 
   get submitsWith() {

src/core/drive/navigator.ts

@@ -1,6 +1,5 @@
 import { Action } from "../types"
 import { getVisitAction } from "../../util"
-import { FetchMethod } from "../../http/fetch_request"
 import { FetchResponse } from "../../http/fetch_response"
 import { FormSubmission } from "./form_submission"
 import { expandURL, getAnchor, getRequestURL, Locatable, locationIsVisitable } from "../url"
@@ -85,7 +84,7 @@ export class Navigator {
     if (formSubmission == this.formSubmission) {
       const responseHTML = await fetchResponse.responseHTML
       if (responseHTML) {
-        const shouldCacheSnapshot = formSubmission.method == FetchMethod.get
+        const shouldCacheSnapshot = formSubmission.isSafe
         if (!shouldCacheSnapshot) {
           this.view.clearSnapshotCache()
         }

src/core/frames/frame_controller.ts

@@ -268,7 +268,7 @@ export class FrameController
     frame.delegate.proposeVisitIfNavigatedWithAction(frame, formSubmission.formElement, formSubmission.submitter)
     frame.delegate.loadResponse(response)
 
-    if (formSubmission.method !== FetchMethod.get) {
+    if (!formSubmission.isSafe) {
       session.clearCache()
     }
   }

src/http/fetch_request.ts

@@ -144,7 +144,7 @@ export class FetchRequest {
       credentials: "same-origin",
       headers: this.headers,
       redirect: "follow",
-      body: this.isIdempotent ? null : this.body,
+      body: this.isSafe ? null : this.body,
       signal: this.abortSignal,
       referrer: this.delegate.referrer?.href,
     }
@@ -156,8 +156,8 @@ export class FetchRequest {
     }
   }
 
-  get isIdempotent() {
-    return this.method == FetchMethod.get
+  get isSafe() {
+    return this.method === FetchMethod.get
   }
 
   get abortSignal() {