Blocking IO in AsyncClient init

[cbornet]

It seems to me that some blocking IO calls are done in AsyncHTTPTransport init that calls create_ssl_context:

• ssl.load_cert_chain
• os.stat (through various calls to Path.exists(), Path.isfile(), Path.isdir(), ...)

So instantiating an AsyncClient blocks the event loop which is bad.

[davidhuser]

Also seeing it:

...
async with MyClient(base_url) as client:
    self.client = httpx.AsyncClient()
 
/usr/local/lib/python3.12/site-packages/httpx/_client.py:1442: in __init__
    self._transport = self._init_transport(
/usr/local/lib/python3.12/site-packages/httpx/_client.py:1490: in _init_transport
    return AsyncHTTPTransport(
/usr/local/lib/python3.12/site-packages/httpx/_transports/default.py:280: in __init__
    ssl_context = create_ssl_context(verify=verify, cert=cert, trust_env=trust_env)
/usr/local/lib/python3.12/site-packages/httpx/_config.py:55: in create_ssl_context
    return SSLConfig(
/usr/local/lib/python3.12/site-packages/httpx/_config.py:79: in __init__
    self.ssl_context = self.load_ssl_context()
/usr/local/lib/python3.12/site-packages/httpx/_config.py:91: in load_ssl_context
    return self.load_ssl_context_verify()
/usr/local/lib/python3.12/site-packages/httpx/_config.py:146: in load_ssl_context_verify
    if ca_bundle_path.is_file():
/usr/local/lib/python3.12/pathlib.py:892: in is_file
    return S_ISREG(self.stat().st_mode)
/usr/local/lib/python3.12/pathlib.py:840: in stat
    return os.stat(self, follow_symlinks=follow_symlinks)