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# Viaduct
Viaduct is our HTTP request library, which can make requests either via a
rust-based (reqwest) networking stack (used on iOS and for local desktop use,
for tests and the like), or using a stack that calls a function passed into it
over the FFI (on android).
For usage info, you can run `cargo +nightly doc -p viaduct` (the `+nightly` is
optional, however some intra-doc links require it), it has several examples.
## Android/FFI Backend overview
On Android, the backend works as follows:
1. During megazord initialization, we are passed a `Lazy<Client>` (`Client` comes
android component, and `Lazy` is from the Kotlin stdlib).
- It also sets a flag that indicates that even if the FFI backend never gets
fully initialized (e.g. with a callback), we should error rather than use
the reqwest backend (which should not be compiled in, however we've had
trouble ensuring this in the past, although at this point we have checks
in CI to ensure it is not present).
2. At this point, a JNA `Callback` instance is created and passed into Rust.
- This serves to proxy the request made by Rust to the `Client`.
- The `Callback` instance is never allowed to be GCed.
- To Rust, it's just a `extern "C"` function pointer that get's stored in an
atomic variable and never can be unset.
3. When Rust makes a request:
1. We serialize the request info into a protobuf record
2. This record is passed into the function pointer we should have by this
point (erroring if it has not been set yet).
3. The callback (on the Java side now) deserializes the protobuf record,
converts it to a concept-fetch Request instance, and passes it to the
4. The response (or error) is then converted into a protobuf record. The
java code then asks Rust for a buffer big enough to hold the serialized
response (or error).
5. The response is written to the buffer, and returned to Rust.
6. Rust then decodes the protobuf, and converts it to a
`viaduct::Response` object that it returns to the caller.
Some notes:
- This "request flow" is entirely synchronous, simplifying the implementation
- Cookies are explicitly not supported at the moment, adding them would
require a separate security review.
- Generally, this is the way the FFI backend is expected to work on any
platform, but for concreteness (and because it's the only one currently using
the FFI backend), we explained it for Android.
- Most of the code in `viaduct` is defining a ergonomic HTTP facade, and is
unrelated to this (or to the reqwest backend). This code is more or less
entirely (in the Kotlin layer and) in `src/backend/`.