Diagnostic Tracing and Time Travel Debugging with Node-ChakraCore

In post-mortem debugging scenarios, developers frequently find themselves hunting for failure clues (e.g. error text) in the log files and then searching for those clues in their code. Once the log statement is found in the source code, the developer is often left to re-construct the context of the error.

The cutting edge diagnostic capabilities in Node-ChakraCore allows developers to look at the faulting code within the full fidelity of the debugger with all the runtime context preserved, enabling them to deeply inspect the code as it was during the original execution. Node-Chakracore has been extended with a suite of such features to support tracing and reproducing program executions and a range of updates to improve the time-travel debugging experience.

This project is being developed in the open and we are always happy to get feedback, bug reports, functionality requests, and pull-requests from the community.

Diagnostic Tracing

Reproducing issues that depend on a specific interleaving of callbacks, specifics of a server configuration or users system, or transient network/file-system behavior can be challenging. To help simplify the process we have updated Node-ChakraCore with a suite of diagnostic tracing features. These features allow you to record an application execution using the --record flag and, then when any of the conditions listed below occur, a diagnostic trace is written out to disk (with the location printed to stderr).

Default trace emit events:

Unhandled exceptions always emit a trace.
Exit with a non-zero exit code always emits a trace.
Failed asserts probabilistically emit a trace with a backoff to prevent overloading.
Calls to console.error and console.warn probabilistically emit a trace with a backoff to prevent overloading.
A SIGINT signal will always emit a trace.

When we run with our remote-file server demo with the record flag and hit a bug in the file I/O code (followed by crtl-c on the command line) we get the output shown below.

Once written to disk these trace directories can be used locally or copied to another machine for later analysis and debugging.

The animation below shows us launching the TraceDebug configuration in the VSCode debugger with the trace directory argument set to the emitOnSigInt trace (written after seeing the file not found error). When the trace hits the point where it raised and triggered for the error message write it will break into the debugger allowing us to inspect local variables, the call stack and, set a breakpoint where the callback that received the I/O error was registered. Then we reverse-execute back-in-time to this callback registration and can inspect local variables as well as the full call stack as they existed then.

If desired we can single step-back through other statements as well or execute forward again, seeing the exact set of statements and variable values that existed during the original execution, until we have all the information we need to understand the problem.

Time-Travel Debugging

TTD functionality is available in Node-ChakraCore and is supported by VSCode. You can use VSCode to debug diagnostic traces even if you don't have the project sources available. This is done by adding the following configuration into your .vscode\launch.jsonconfiguration file.

{
    "name": "Trace Debug",
    "type": "node",
    "request": "launch",
    "stopOnEntry": true,
    "windows": { "runtimeExecutable": "nvs.cmd" },
    "osx": { "runtimeExecutable": "nvs" },
    "linux": { "runtimeExecutable": "nvs" },
    "runtimeArgs": [
        "run",
        "chakracore",
        "--nolazy",
        "--break-first",
        "--replay-debug=[path to trace directory]"
    ],
    "console": "internalConsole"
}

How to get started

To get started with diagnostic tracing and TTD you will need the following:

Install Node-ChakraCore
- (Recommended) install NVS which is a cross-platform tool for switching between different versions and forks of Node.js and will allow you to easily switch between Node-ChakraCore and other Node.js versions. Once NVS is installed simply enter the following commands in the console:
```
$ nvs remote chakracore-nightly https://nodejs.org/download/chakracore-nightly/
$ nvs add chakracore-nightly/latest
$ nvs use chakracore-nightly
```
- (Manual) Download the build for your platform/architecture and manually add the location of the binaries to your path.
Install VSCode using the latest installer.

Sample Program

The code for the Remote File System sample demo'ed at NodeInteractive is available here.

You can clone the repo to a location of your choice and npm install the dependencies. Once you launch the application it will serve up the contents of the folder testdata on port 3000. The application is designed to intermittently rename the files hello_world.js <-> helloWorld.js in the background. So, as shown in the screenshot above, a warn trace can be triggered by interacting with the application and clicking on this file at various times. You can always trigger a sigint trace via crtl-c at the command line.

Once this SIGINT trace is written it can be replay debugged in VSCode using the TraceDebug launch configuration and you can change the path if you want to debug other trace files that are written.

Feedback

Please let us know on our issues page if you have any question or comment.

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