Thankfully this was a lot easier than tracing hiwire keys. 

I located one actual leak: if an error is thrown during `to_js` it would forget to free `proxies`. Other than that, I fixed a few "dumb leaks" in `api.js` where we leak the methods like `eval_code` or `find_imports`. So the code was already in surprisingly good shape in this way.

It's still possible that we leak some Python objects somehow, but this is as good as we can do for systematic checks short of compiling Python itself with `TRACE_MALLOC`.
Eliminate pyproxy leaks. Thankfully this was a lot easier than tracing hiwire keys. 

I located one actual leak: if an error is thrown during `to_js` it would forget to free `proxies`. Other than that, I fixed a few "dumb leaks" in `api.js` where we leak the methods like `eval_code` or `find_imports`. So the code was already in surprisingly good shape in this way.

It's still possible that we leak some Python objects somehow, but this is as good as we can do for systematic checks short of compiling Python itself with `TRACE_MALLOC`.
(debugging)=
# Interactive Debugging
See [Emscripten's page about
debugging](https://emscripten.org/docs/porting/Debugging.html) which has
extensive info about the various debugging options available.

[Chromium has support for DWARF
info](https://developer.chrome.com/blog/webassembly/) 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`.
```py
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)
        self.socket.bind(self.server_address)

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

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


if __name__ == '__main__':
    port = 8000
    with MyTCPServer(("", port), Handler) as httpd:
        print("Serving at: http://127.0.0.1:{}".format(port))
        httpd.serve_forever()
```

(debugging)=
# Interactive Debugging
See [Emscripten's page about
debugging](https://emscripten.org/docs/porting/Debugging.html) which has
extensive info about the various debugging options available.

[Chromium has support for DWARF
info](https://developer.chrome.com/blog/webassembly/) 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`.
```py
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)
        self.socket.bind(self.server_address)

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

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


if __name__ == '__main__':
    port = 8000
    with MyTCPServer(("", port), Handler) as httpd:
        print("Serving at: http://127.0.0.1:{}".format(port))
        httpd.serve_forever()
```
