Interactive Debugging

See Emscripten’s page about debugging which has extensive info about the various debugging options available.

Chromium has support for DWARF info which can be very helpful for debugging in certain circumstances. To build Pyodide with DWARF, you should set EXTRA_CFLAGS="-g4" and EXTRA_LD_FLAGS="-g4 --source-map-base "http://localhost:<port>/" (substitute in your favorite port). Make sure to rebuild CPython with these flags set (it isn’t necessary to rebuild emsdk).

Once you have done this, when you load Pyodide, you will see 404 errors for requests for the various source maps. In order to load the source maps correctly, you will need to run a custom server that “fixes” the source map urls. The following debugging server seems to work for both CPython and numpy. Run it in the Pyodide root directory. If you need to debug other C extensions, you will need to update the server. It should be clear what to do based by looking at the 404s generated by the server and locating those files in the file tree, perhaps by using find.

import socket
import socketserver
from http.server import SimpleHTTPRequestHandler
import pathlib

alternative_bases=["cpython/build/Python-3.8.2/","src/", "build/"]
def fixup_url(path):
    if pathlib.Path("." + path).exists():
        return path
    for base in alternative_bases:
        q = pathlib.Path(base + path)
        if q.exists():
            return str(q)
    # Numpy source maps can be in a bunch of different places inside of the
    # directory tree, so we need to glob for them.
    dir = list(
        pathlib.Path("packages/numpy/build/numpy-1.17.5/").glob("**" + path)
    if dir:
        return str(dir[0])
    return path

class MyTCPServer(socketserver.TCPServer):
    def server_bind(self):
        """Use socket.SO_REUSEADDR to allow faster restart"""
        self.socket.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)

class Handler(SimpleHTTPRequestHandler):
    def end_headers(self):
        # Enable Cross-Origin Resource Sharing (CORS)
        self.send_header('Access-Control-Allow-Origin', '*')

    def do_GET(self):
        self.path = fixup_url(self.path)

if __name__ == '__main__':
    port = 8000
    with MyTCPServer(("", port), Handler) as httpd:
        print("Serving at:{}".format(port))