Dynamic attributes

[gyozaaaa]

Dynamic headers and proxies on an instantiated client

Both settings could only be given to the constructor and were fixed for the life of the
client. They are now readable and writable attributes on Client and
blocking.Client.

client = Client(headers={"x-api-key": "secret"}, proxies=Proxy.all("http://a:8080"))

client.proxies = Proxy.all("http://b:8080")     # a single proxy or a sequence
client.proxies = None                            # back to the system proxies

client.headers.update({"x-request-id": "1"})     # add, or replace what is there
client.headers["x-trace"] = "abc"
del client.headers["x-trace"]
client.headers = {"x-api-key": "other"}          # replace them altogether

• proxies reads back as a list, or None. Proxy.all(...) is accepted wherever a
  sequence is, on the attribute and on the constructor argument alike.
• headers reads back a ClientHeaders, a HeaderMap bound to the client, so the
  usual lookups work on it and it can be handed to another client. HeaderMap.update()
  is new and available on plain maps too.
• Changing either keeps the rest of the configuration and the cookie jar. Only the
  headers of the client are replaced, so those coming from an emulation are kept.
  Requests already in flight keep the settings they started with.

Implementation

wreq::Client bakes default headers and proxies in at build time, so a change rebuilds
the underlying client from the retained configuration and swaps it in through an
ArcSwap. The request path stays lock-free; a mutex serializes the rebuilds so that
concurrent updates of the headers and the proxies cannot lose each other. Dropping the
old connection pool is intended: connections opened through the previous proxy are not
reused.

Two details worth flagging for review:

• Builder.user_agent moved from PyBackedStr to String, because PyBackedStr is not
  Clone in pyo3 0.29 and the retained configuration has to be cloneable. One small
  allocation per client build.
• A tls_verify certificate file is now read once, at construction, rather than on every
  rebuild.

Tests

tests/proxy_test.py and the client half of tests/header_test.py are hermetic — local
servers standing in for the origin and for two forward proxies, no network. They cover
switching, clearing, scheme-specific proxy lists, per-request override, configuration and
cookie-jar preservation, emulation headers surviving a header change, concurrent updates
from threads, and the blocking client.