test(threading): pin the pitfalls of the client threading model

src/Playwright.Tests/ThreadingModelTests.cs

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+/*
+ * MIT License
+ *
+ * Copyright (c) 2020 Darío Kondratiuk
+ * Modifications copyright (c) Microsoft Corporation.
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+namespace Microsoft.Playwright.Tests;
+
+/// <summary>
+/// Pins what the client's threading model does today; see "Threading model" in CLAUDE.md.
+/// The first test is the guarantee the model gives. The others are its pitfalls, asserted as
+/// they are rather than as they should be: when one of them starts failing, the model changed
+/// and the documentation needs an update. The check-then-subscribe race inside the library is
+/// covered by PageWaitForUrlTests.ShouldNotMissNavigationCommittedRightAfterTheCall (#3364).
+/// </summary>
+public class ThreadingModelTests : PageTestEx
+{
+    // Navigates to a page that commits at once (Firefox and WebKit need a few KB before they
+    // commit) and finishes loading only when the returned action is invoked, so that a
+    // WaitForLoadStateAsync started before that has to wait for the `load` event.
+    private async Task<Action> GotoPageThatLoadsOnDemandAsync()
+    {
+        var finishBody = new TaskCompletionSource<bool>();
+        Server.SetRoute("/slow.html", async ctx =>
+        {
+            ctx.Response.ContentType = "text/html; charset=utf-8";
+            await ctx.Response.WriteAsync("<!DOCTYPE html><html><body><!--" + new string('x', 4096) + "-->");
+            await ctx.Response.Body.FlushAsync();
+            await finishBody.Task;
+            await ctx.Response.WriteAsync("</body></html>");
+        });
+        await Page.GotoAsync(Server.Prefix + "/slow.html", new() { WaitUntil = WaitUntilState.Commit });
+        return () => finishBody.SetResult(true);
+    }
+
+    [PlaywrightTest]
+    public async Task EventsBeforeTheReplyAreHandledBeforeTheMethodReturns()
+    {
+        // Messages are dispatched one at a time and a reply only queues the continuation,
+        // so everything the `load` event did is visible when GotoAsync returns.
+        int loads = 0;
+        Page.Load += (_, _) => Interlocked.Increment(ref loads);
+        for (int i = 1; i <= 20; i++)
+        {
+            await Page.GotoAsync(Server.EmptyPage);
+            Assert.AreEqual(i, Volatile.Read(ref loads));
+        }
+    }
+
+    [PlaywrightTest]
+    public async Task EventDrivenWaitResumesOnTheReaderThread()
+    {
+        // Waiter completes a plain TaskCompletionSource, so the continuation runs synchronously
+        // inside the event handler, inside Connection.Dispatch, until its next incomplete await.
+        // A reply, by contrast, queues the continuation to the thread pool.
+        var finishLoading = await GotoPageThatLoadsOnDemandAsync();
+        var load = Page.WaitForLoadStateAsync(LoadState.Load);
+        finishLoading();
+        await load;
+        StringAssert.Contains("Connection.Dispatch", Environment.StackTrace);
+
+        await Page.GotoAsync(Server.EmptyPage);
+        StringAssert.DoesNotContain("Connection.Dispatch", Environment.StackTrace);
+    }
+
+    [PlaywrightTest]
+    public async Task PageLoadHasNotFiredWhenWaitForLoadStateReturns()
+    {
+        // Frame.OnLoadState raises Frame.LoadState, which completes the wait, before Page.Load.
+        // In the JS client the continuation is a microtask and runs after both.
+        bool loadFired = false;
+        Page.Load += (_, _) => loadFired = true;
+        var finishLoading = await GotoPageThatLoadsOnDemandAsync();
+        var load = Page.WaitForLoadStateAsync(LoadState.Load);
+        finishLoading();
+        await load;
+        Assert.False(loadFired);
+
+        // The reply to this call is dispatched only after the reader has finished OnLoadState.
+        await Page.EvaluateAsync("1");
+        Assert.True(loadFired);
+    }
+
+    [PlaywrightTest]
+    public async Task BlockingAfterAnEventDrivenWaitStallsTheConnection()
+    {
+        // Same cause: this code runs on the reader thread, so the reply to EvaluateAsync cannot
+        // be dispatched until it yields. The same Wait() after a GotoAsync completes at once.
+        var finishLoading = await GotoPageThatLoadsOnDemandAsync();
+        var load = Page.WaitForLoadStateAsync(LoadState.Load);
+        finishLoading();
+        await load;
+        var evaluate = Page.EvaluateAsync<int>("1 + 1");
+        Assert.False(evaluate.Wait(1000));
+        Assert.AreEqual(2, await evaluate);
+    }
+
+    [PlaywrightTest]
+    public async Task BlockingInAnEventHandlerStallsTheConnection()
+    {
+        // Event handlers run on the reader thread too: a synchronous wait on any Playwright
+        // call inside one is a deadlock until the wait gives up.
+        bool completed = true;
+        Page.Console += (_, _) => completed = Page.EvaluateAsync<int>("1 + 1").Wait(1000);
+        await Page.EvaluateAsync("console.log('hi')");
+        Assert.False(completed);
+    }
+
+    [PlaywrightTest]
+    public async Task ThrowingInAnEventHandlerClosesTheConnection()
+    {
+        // Handlers run inside Connection.Dispatch, whose catch-all closes the connection, so one
+        // throwing handler takes down every page and browser of the IPlaywright instance.
+        // Uses its own instance so that the shared one survives; the driver exits with it.
+        using var playwright = await Microsoft.Playwright.Playwright.CreateAsync();
+        var browser = await playwright[BrowserName].LaunchAsync(await LaunchOptionsAsync());
+        var page = await browser.NewPageAsync();
+        page.Console += (_, _) => throw new InvalidOperationException("boom");
+
+        var exception = Assert.ThrowsAsync<TargetClosedException>(() => page.EvaluateAsync("console.log('hi')"));
+        StringAssert.Contains("boom", exception.Message);
+        Assert.ThrowsAsync<TargetClosedException>(() => browser.NewPageAsync());
+    }
+}