Terminate execution on internal navigation

Currently, if there's an internal navigation event, we continue to process the
page normally. This has negative performance implication, and can result in
user-scripts producing unexpected results.

For example, imagine a page with a script that does.

```js
if (x) {
   form.submit();
}

reloadProduct();
```

The call to `form.submit()` should stop the script from executing. And, if the
page has 10 other <script> tags after this, they shouldn't be loaded nor
executed.

This code terminates the execution of the current script on an internal
navigation event, and stops the rest of the page from load.

While I believe this creates a more "correct" behavior, it also introduces new
edge cases. There's no a period of time, between the termination being stopped
and then being resumed, where executing code is not safe.

src/browser/page.zig

@@ -388,11 +388,15 @@ pub const Page = struct {
             // > immediately before the browser continues to parse the
             // > page.
             // https://developer.mozilla.org/en-US/docs/Web/HTML/Element/script#notes
-            self.evalScript(&script);
+            if (self.evalScript(&script) == false) {
+                return;
+            }
         }
 
         for (defer_scripts.items) |*script| {
-            self.evalScript(script);
+            if (self.evalScript(script) == false) {
+                return;
+            }
         }
         // dispatch DOMContentLoaded before the transition to "complete",
         // at the point where all subresources apart from async script elements
@@ -402,7 +406,9 @@ pub const Page = struct {
 
         // eval async scripts.
         for (async_scripts.items) |*script| {
-            self.evalScript(script);
+            if (self.evalScript(script) == false) {
+                return;
+            }
         }
 
         try HTMLDocument.documentIsComplete(html_doc, self);
@@ -419,10 +425,13 @@ pub const Page = struct {
         );
     }
 
-    fn evalScript(self: *Page, script: *const Script) void {
+    fn evalScript(self: *Page, script: *const Script) bool {
-        self.tryEvalScript(script) catch |err| {
+        self.tryEvalScript(script) catch |err| switch (err) {
-            log.err(.js, "eval script error", .{ .err = err, .src = script.src });
+            error.JsErr => {}, // already been logged with detail
+            error.Terminated => return false,
+            else => log.err(.js, "eval script error", .{ .err = err, .src = script.src }),
         };
+        return true;
     }
 
     // evalScript evaluates the src in priority.
@@ -436,29 +445,26 @@ pub const Page = struct {
             log.err(.browser, "clear document script", .{ .err = err });
         };
 
-        const src = script.src orelse {
+        var script_source: ?[]const u8 = null;
+        if (script.src) |src| {
+            self.current_script = script;
+            defer self.current_script = null;
+
+            // https://html.spec.whatwg.org/multipage/webappapis.html#fetch-a-classic-script
+            script_source = (try self.fetchData(src, null)) orelse {
+                // TODO If el's result is null, then fire an event named error at
+                // el, and return
+                return;
+            };
+        } else {
             // source is inline
             // TODO handle charset attribute
-            if (try parser.nodeTextContent(parser.elementToNode(script.element))) |text| {
+            script_source = try parser.nodeTextContent(parser.elementToNode(script.element));
-                try script.eval(self, text);
+        }
-            }
-            return;
-        };
 
-        self.current_script = script;
+        if (script_source) |ss| {
-        defer self.current_script = null;
+            try script.eval(self, ss);
-
+        }
-        // https://html.spec.whatwg.org/multipage/webappapis.html#fetch-a-classic-script
-        const body = (try self.fetchData(src, null)) orelse {
-            // TODO If el's result is null, then fire an event named error at
-            // el, and return
-            return;
-        };
-
-        script.eval(self, body) catch |err| switch (err) {
-            error.JsErr => {}, // nothing to do here.
-            else => return err,
-        };
 
         // TODO If el's from an external file is true, then fire an event
         // named load at el.
@@ -701,13 +707,18 @@ pub const Page = struct {
             .reason = opts.reason,
         });
         self.delayed_navigation = true;
-        const arena = self.session.transfer_arena;
+
+        const session = self.session;
+        const arena = session.transfer_arena;
         const navi = try arena.create(DelayedNavigation);
         navi.* = .{
             .opts = opts,
-            .session = self.session,
+            .session = session,
             .url = try self.url.resolve(arena, url),
         };
+
+        // In v8, this throws an exception which JS code cannot catch.
+        session.executor.terminateExecution();
         _ = try self.loop.timeout(0, &navi.navigate_node);
     }
 
@@ -822,6 +833,11 @@ const DelayedNavigation = struct {
         const initial = self.initial;
 
         if (initial) {
+            // Prior to schedule this task, we terminated excution to stop
+            // the running script. If we don't resume it before doing a shutdown
+            // we'll get an error.
+            session.executor.resumeExecution();
+
             session.removePage();
             _ = session.createPage() catch |err| {
                 log.err(.browser, "delayed navigation page error", .{
@@ -985,9 +1001,17 @@ const Script = struct {
                 }
             },
         } catch {
-            if (try try_catch.err(page.arena)) |msg| {
+            if (page.delayed_navigation) {
-                log.warn(.user_script, "eval script", .{ .src = src, .err = msg });
+                return error.Terminated;
             }
+
+            if (try try_catch.err(page.arena)) |msg| {
+                log.warn(.user_script, "eval script", .{
+                    .src = src,
+                    .err = msg,
+                });
+            }
+
             try self.executeCallback("onerror", page);
             return error.JsErr;
         };
@@ -1060,10 +1084,15 @@ fn timestamp() u32 {
 // immediately.
 pub export fn scriptAddedCallback(ctx: ?*anyopaque, element: ?*parser.Element) callconv(.C) void {
     const self: *Page = @alignCast(@ptrCast(ctx.?));
+    if (self.delayed_navigation) {
+        // if we're planning on navigating to another page, don't run this script
+        return;
+    }
+
     var script = Script.init(element.?, self) catch |err| {
         log.warn(.browser, "script added init error", .{ .err = err });
         return;
     } orelse return;
 
-    self.evalScript(&script);
+    _ = self.evalScript(&script);
 }

src/runtime/js.zig

@@ -472,6 +472,14 @@ pub fn Env(comptime State: type, comptime WebApis: type) type {
                 self.scope = null;
                 _ = self.scope_arena.reset(.{ .retain_with_limit = SCOPE_ARENA_RETAIN });
             }
+
+            pub fn terminateExecution(self: *const ExecutionWorld) void {
+                self.env.isolate.terminateExecution();
+            }
+
+            pub fn resumeExecution(self: *const ExecutionWorld) void {
+                self.env.isolate.cancelTerminateExecution();
+            }
         };
 
         const PersistentObject = v8.Persistent(v8.Object);
@@ -1562,7 +1570,15 @@ pub fn Env(comptime State: type, comptime WebApis: type) type {
             }
 
             pub fn send(self: *const Inspector, msg: []const u8) void {
-                self.session.dispatchProtocolMessage(self.isolate, msg);
+                // Can't assume there's an existing Scope (with its HandleScope)
+                // available when doing this. Pages (and thus Scopes) come and
+                // go, but CDP can keep sending messages.
+                const isolate = self.isolate;
+                var temp_scope: v8.HandleScope = undefined;
+                v8.HandleScope.init(&temp_scope, isolate);
+                defer temp_scope.deinit();
+
+                self.session.dispatchProtocolMessage(isolate, msg);
             }
 
             // From CDP docs