Playwright Chromium Crash: Evaluate Error After Crash

Dec 3, 2025 by Alex Johnson 54 views

Playwright is a powerful automation library for web testing and automation. This article delves into a specific bug encountered when using Playwright with Chromium, focusing on the handling of crashes within the browser context. The core issue revolves around how Playwright manages the context and subsequent actions after a browser page crashes. This is particularly relevant for developers using Playwright for end-to-end testing, as it can lead to unexpected errors and test failures if not properly addressed.

Understanding the Bug: Evaluate Methods and Undefined Context

The central problem, as highlighted in the provided report, is that evaluate methods in Playwright, after a browser page crash, attempt to use an undefined context. This leads to a Cannot read properties of undefined (reading 'evaluateExpression') error. This unexpected behavior disrupts the normal error handling mechanisms within Playwright and can cause significant issues in automated testing environments. Let's break down the technical aspects and implications of this bug.

The Root Cause: Context Management After Crash

The root cause lies in how Playwright manages the browser context after a page crash. Specifically, the evaluateExpression method within the frames.js file is the source of the error. After a crash, the context becomes undefined, and any subsequent attempts to use evaluate methods fail. The provided code snippet from the original report pinpoints the exact line where the error occurs:

async evaluateExpression(expression, options = {}, arg) {
  const context = await this._context(options.world ?? "main");
  const value = await context.evaluateExpression(expression, options, arg);  // context is undefined here after a crash
  return value;
}

This code attempts to retrieve the context before executing an expression. However, after a crash, this context is not properly initialized or has been terminated. Subsequently, when the evaluateExpression method is called, it can't find the necessary context, leading to the error. This highlights a crucial area where Playwright's error handling needs improvement, especially concerning post-crash scenarios.

The Impact: Test Failures and Unexpected Behavior

The impact of this bug is significant, particularly in automated testing. When a page crashes, tests often rely on verifying that the expected behavior occurs. If the evaluate methods fail due to an undefined context, it can prevent these verifications from happening. This can lead to:

False negatives: Tests may fail when the actual issue is the crash itself, not the code under test.
Unclear error messages: The Cannot read properties of undefined error is not immediately indicative of a page crash, making debugging more difficult.
Inconsistent test results: Tests may behave differently depending on the timing of the crash and the subsequent actions taken.

These problems can significantly undermine the reliability and effectiveness of automated tests.

Reproducing the Bug: Step-by-Step Guide

To understand and potentially address this bug, developers can follow these steps. The test case focuses on inducing a crash and then attempting to use evaluate:

Prerequisites

Playwright Installation: Ensure Playwright is installed in your project. If you have not already done so, you can install the Playwright package using npm or yarn:
```
npm install --save-dev @playwright/test
# or
yarn add --dev @playwright/test
```
Required Libraries: Install necessary libraries like asyncio for asynchronous operations.

Test Scenario

Set up the Environment:
- Create a new Python file (e.g., crash_test.py).
- Import the necessary modules:
```
import asyncio
from playwright.async_api import async_playwright, Error as PlaywrightError
```
Launch Browser and Context:
- Use async_playwright() to create a Playwright instance.
- Launch a Chromium browser instance using p.chromium.launch(). Make sure to set headless=False for easier debugging. Also, add the argument channel="chrome" to specify the Chrome browser.
- Create a new context and page.
```
async with async_playwright() as p:
    browser = await p.chromium.launch(channel="chrome", headless=False)
    context = await browser.new_context(viewport=None)
    page = await context.new_page()
```
Set Page Content:
- Set the initial content of the page. This example uses a simple div element to trigger a crash.
```
await page.set_content("<div>This page should crash</div>")
```
Crash Event Handling:
- Create an asyncio.Future to capture the crash event.
- Define a function on_crash to set the future when the crash event is detected.
- Attach the crash handler to the page using page.on("crash", on_crash).
```
crash_event = asyncio.Future()

def on_crash(page_obj):
    if not crash_event.done():
        crash_event.set_result(True)

page.on("crash", on_crash)
```
Induce Crash:
- Navigate the page to chrome://crash. This URL is designed to crash the Chromium browser.
- Handle the exception that may arise from the crash, for example by using try...except block.
```
try:
    await page.goto("chrome://crash")
except:
    pass
```
Wait for Crash and Execute Evaluate:
- Wait for the crash event to be triggered using await crash_event.
- Attempt to execute an evaluate method after the crash.
- Catch the PlaywrightError to verify the error message.
```
await crash_event

try:
    await page.evaluate("() => {}")
except PlaywrightError as ex:
    print(f"crash message = {ex}")
```
Close Browser:
- Close the browser instance.
```
await browser.close()
```

Running the Test

Execute the Python script using a Python interpreter. Observe the output and verify that it matches the expected behavior: the error message Page.evaluate: Cannot read properties of undefined (reading 'evaluateExpression').

Proposed Solutions and Workarounds

Addressing this bug involves several potential solutions and workarounds. These strategies can help mitigate the impact of the bug and improve the reliability of automated tests.

Solution: Patching the Driver

One potential solution is to patch the driver to correctly handle the post-crash context. The main goal of this patch is to ensure that when a crash occurs, the driver is able to gracefully handle subsequent calls to the evaluate method. This would prevent the undefined context error from occurring. This might involve:

Checking for Crash Status:
- Modify the evaluateExpression method to check if the page has crashed before attempting to access the context.
Handling Undefined Context:
- If a crash is detected, the method should handle this gracefully, either by throwing a more specific error or returning a default value.
Context Re-initialization:
- Investigate the possibility of re-initializing the context or creating a new page instance after a crash, and then attempt the evaluation.

Workarounds: Test Modifications

Until a proper fix is implemented, test developers can employ several workarounds to mitigate the issue. These modifications can help to make tests more resilient to crashes.

Error Handling:
- Implement robust error handling around any actions that could potentially trigger a crash. Catch PlaywrightError exceptions and log detailed error messages.
```
try:
    await page.evaluate("() => {}")
except PlaywrightError as ex:
    print(f"An error occurred: {ex}")
```
Crash Detection:
- Use event listeners to detect page crashes and adjust the test flow accordingly. If a crash is detected, skip subsequent steps that depend on the crashed page.
```
page.on("crash", lambda page: print("Page crashed"))
```
Page Re-creation:
- If a crash occurs, consider closing the current page and creating a new one. This can help to reset the context and ensure that subsequent tests can run smoothly.
```
try:
    await page.goto("chrome://crash")
except:
    await page.close()
    page = await context.new_page()
```
Minimize Crash-Prone Actions:
- Identify actions in tests that frequently cause crashes. If possible, refactor the test to avoid those actions or find alternative methods to achieve the same result.

Conclusion: Improving Playwright's Resilience

The issue of the evaluate method using an undefined context after a crash presents a significant challenge for Playwright users. The current behavior can lead to test failures and hinder the effectiveness of automated testing. By understanding the root cause, developers can implement the proposed solutions and workarounds to enhance test reliability and improve the overall quality of automated testing workflows.

This bug underscores the importance of robust error handling and proper context management in the Playwright library, especially when dealing with potentially unstable browser environments. Addressing these issues will contribute to more reliable and efficient end-to-end testing practices.

For further reading on Playwright and browser crashes, consider exploring these resources:

Playwright Documentation: The official Playwright documentation offers comprehensive guides and API references: Playwright Documentation
Chromium Crash Reporting: Understanding how Chromium handles crashes can help you better understand the scenarios that can trigger them: Chromium Crash Reporting