Windows proxy support: no concurrent proxy support

[bbonaby]

Relevant area(s)

Windows

Brief description of your issue

MXC's network proxy support on Windows has two limitations users should be aware of:

1. WinHttp-only scope. MXC only inherently proxies network traffic from a sandboxed process when both of these are true:

   • The proxy property is set in the MXC configuration schema.
   • The sandboxed process makes its outbound calls through the WinHttp stack.

   Processes using other networking stacks (for example, WinINet, raw sockets, or third-party HTTP libraries that don't go through WinHttp) will not be automatically routed through the configured proxy.

2. Single concurrent sandbox. Only one sandboxed process with proxy traffic should run at a time. Starting a second sandboxed process that relies on proxy support while a first is still running can cause the first one's proxy traffic to be dropped.

A fix is planned for a future release.

Steps to reproduce

1. Configure an MXC sandbox with the proxy property set.
2. Start sandboxed process A and have it make outbound HTTP(S) requests through WinHttp.
3. While A is still running, start a second sandboxed process B with the same proxy configuration.

Expected behavior

Both sandboxed processes continue to have their WinHttp traffic routed independently through the configured proxy.

Actual behavior

When the second sandbox starts, the first sandbox's proxy traffic can be dropped.