WebAssembly

WebAssembly allows you to compile native code into a platform-independent bytecode format, which, for example, permits you to run native C, C++, or Rust code in a web browser.

This feature is still developing, so may be rough around the edges.

Debugging support for WebAssembly is an ongoing effort by the WebAssembly community. Support for it in Sentry is limited by what's possible in that ecosystem.

Requirements

As of now it's possible to get crash reports uploaded to Sentry if your compiler can output WASM DWARF debug information alongside the binary code. In addition, you need to upload your WebAssembly debug information files to Sentry or use a symbol server as you would for our other platforms.

For this process to work, we require debug information files to be augmented by a build_id custom section. We provide a tool called wasm-split that can automatically add this section if missing and splits off the debug information so it isn't sent to customers.

Using wasm-split

Once your compiler has produced a .wasm file, you can use our wasm-split tool to add a build_id (if missing) and split the file into a debug information file and a final stripped .wasm file without debug information:

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wasm-split /path/to/file.wasm -d /path/to/file.debug.wasm --strip

This command modifies the file.wasm in place to add the build_id, then removes all debug information. The debug information is put in a file.debug.wasm which then needs to be uploaded to Sentry. For more information, see Debug Information Files.

Crash Reporting

WebAssembly does not currently have a runtime that can produce stack traces. We do not yet provide a ready-to-use SDK, though we have a small integration for the Sentry JavaScript SDK to enhance error reports in the browser environment which include WASM frames, called wasm-support.js, which needs to be loaded after the JavaScript SDK and before you start loading your WASM modules:

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<script
  src="https://browser.sentry-cdn.com/6.10.0/bundle.min.js"
  integrity="sha384-nsIkfmMh0uiqg+AwegHcT1SMiPNWnhZmjFDwTshLTxur6ZPNaGT8vwT+vHwI5Jag"
  crossorigin="anonymous"
></script>
<script src="wasm-support.js"></script>

If you then throw a JavaScript exception you can get the stack trace. If you want to capture an error from within your WebAssembly application you can export a utility function like this:

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function captureError() {
  try {
    throw new Error();
  } catch (e) {
    Sentry.captureException(e);
  }
}

async function loadWasm(url) {
  const importObj = {
    env: {
      capture_error: captureError,
    },
  };
  return await WebAssembly.instantiateStreaming(fetch(url), importObj);
}

Custom Crash Reports

It's likely that if you're working with WebAssembly, you want to send custom crash reports; for example, if you don't execute WebAssembly in a browser, but in your own runtime. In that case, you can follow the general protocol documentation which also covers how to describe WebAssembly frames: stack trace interface.

Here is an example event for WebAssembly:

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{
  "platform": "native",
  "debug_meta": {
    "images": [
      {
        // the URL where the wasm file lives
        "code_file": "http://localhost:8002/demo.wasm",
        // indicates a web assembly module
        "type": "wasm",
        // the contents of the build_id section in hex,
        // truncated to 32 characters with a 0 appended
        "debug_id": "9e5cbc5553b44e8b9c69d18cdd7bfd2e0"
      }
    ]
  },
  "exception": {
    "values": [
      {
        "stacktrace": {
          "frames": [
            // wasm frame
            {
              "function": "wasm_func",
              "abs_path": "http://localhost:8002/demo.wasm",
              // the absolute offset in the wasm file for the instruction
              "instruction_addr": "0x8c",
              // indicates that the instruction_addr is relative to the 1st
              // module in the debug_info images list.
              "addr_mode": "rel:0"
            },
            // javascript frame
            {
              "function": "crash",
              "abs_path": "http://localhost:8002/index.html",
              "lineno": 24,
              "platform": "javascript"
            }
          ]
        },
        "type": "Error"
      }
    ]
  }
}
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