||Viaduct is a way for Application Services Rust components
(https://github.com/mozilla/application-services) to make network requests
using a trusted stack (gecko).
The way it works is roughly as follows:
- First we register a callback using `viaduct_initialize`
(Viaduct::Initialize). This callback is stored on the Rust side
in a static variable, therefore Initialize() must be called only once.
- When the Rust code needs to make a network request, our callback
(Viaduct::ViaductCallback) will be called with a protocol buffer describing
the request to make on their behalf. Note 1: The callback MUST be called from
a background thread as it is blocking. Note 2: It is our side responsibility
to call `viaduct_destroy_bytebuffer` on the buffer.
- We set a semaphore to make the background thread wait while we make the
request on the main thread using nsIChannel. (ViaductRequest::MakeRequest)
- Once a response is received, we allocate a bytebuffer to store the
response using `viaduct_alloc_bytebuffer` and unlock the semaphore.
- The background thread is unlocked, and the callback returns the response to
the Rust caller. (Viaduct::ViaductCallback)
- The Rust caller will free the response buffer we allocated earlier.