feat(s2n-quic-core): add branch tracing support

[camshaft]

Description of changes:

This change adds a branch-tracing feature to s2n-quic-core which hooks into the ensure! macro and logs each decision point with the outcome and the action it took in case of a failure. This is really useful for debugging why something happened or how often a check occurred.

Here's an example output:

0:00:00.300000 s2n_quic_core::buffer::receive_buffer: branch="self.max_recv_offset <= final_offset" result=true file="quic/s2n-quic-core/src/buffer/receive_buffer.rs" line=212
0:00:00.300000 s2n_quic_core::buffer::receive_buffer: branch="excess.is_empty()" result=true file="quic/s2n-quic-core/src/buffer/receive_buffer.rs" line=238
0:00:00.300000 s2n_quic_core::buffer::receive_buffer: branch="!request.is_empty()" result=true file="quic/s2n-quic-core/src/buffer/receive_buffer.rs" line=241
0:00:00.300000 s2n_quic_core::buffer::receive_buffer: branch="!request.is_empty()" result=false outcome="return" file="quic/s2n-quic-core/src/buffer/receive_buffer.rs" line=522
0:00:00.300000 s2n_quic_core::buffer::receive_buffer: branch="!request.is_empty()" result=false outcome="return" file="quic/s2n-quic-core/src/buffer/receive_buffer.rs" line=522
0:00:00.300000 s2n_quic_core::buffer::receive_buffer: branch="slot.is_occupied(offset)" result=true file="quic/s2n-quic-core/src/buffer/receive_buffer.rs" line=492
0:00:00.300000 s2n_quic_core::buffer::receive_buffer: branch="slot.is_occupied(self.prev_end)" result=true file="quic/s2n-quic-core/src/buffer/receive_buffer.rs" line=682
0:00:00.300000 s2n_quic_core::buffer::receive_buffer: branch="slot.is_occupied(self.prev_end)" result=true file="quic/s2n-quic-core/src/buffer/receive_buffer.rs" line=682
0:00:00.300000 s2n_quic_core::buffer::receive_buffer: branch="slot.is_occupied(self.prev_end)" result=true file="quic/s2n-quic-core/src/buffer/receive_buffer.rs" line=682
0:00:00.300000 s2n_quic_core::buffer::receive_buffer: branch="slot.is_occupied(self.prev_end)" result=true file="quic/s2n-quic-core/src/buffer/receive_buffer.rs" line=682
0:00:00.300000 s2n_quic_core::buffer::receive_buffer: branch="slot.is_occupied(offset)" result=true file="quic/s2n-quic-core/src/buffer/receive_buffer.rs" line=492
0:00:00.300000 s2n_quic:client:conn: rx_stream_progress: bytes=710 id=0
0:00:00.300000 s2n_quic:client: platform_event_loop_sleep: timeout=Some(25ms) processing_duration=1µs
0:00:00.300000 s2n_quic_core::buffer::receive_buffer: branch="slot.is_occupied(self.prev_end)" result=true file="quic/s2n-quic-core/src/buffer/receive_buffer.rs" line=682
0:00:00.300000 s2n_quic_core::buffer::receive_buffer: branch="slot.is_occupied(self.start_offset)" result=true file="quic/s2n-quic-core/src/buffer/receive_buffer.rs" line=452
0:00:00.300000 s2n_quic_core::buffer::receive_buffer: branch="watermark > 0" result=true file="quic/s2n-quic-core/src/buffer/receive_buffer.rs" line=390
0:00:00.300000 s2n_quic_core::buffer::receive_buffer: branch="len > 0" result=true file="quic/s2n-quic-core/src/buffer/receive_buffer.rs" line=460
0:00:00.300000 s2n_quic_core::buffer::receive_buffer: branch="slot.is_occupied(offset)" result=false outcome="return offset" file="quic/s2n-quic-core/src/buffer/receive_buffer.rs" line=492
0:00:00.300000 s2n_quic_core::buffer::receive_buffer: branch="slot.is_occupied(self.prev_end)" result=false outcome="return None" file="quic/s2n-quic-core/src/buffer/receive_buffer.rs" line=682
0:00:00.300000 s2n_quic_core::buffer::receive_buffer: branch="slot.is_occupied(self.prev_end)" result=false outcome="return None" file="quic/s2n-quic-core/src/buffer/receive_buffer.rs" line=682
0:00:00.300000 s2n_quic_core::buffer::receive_buffer: branch="slot.is_occupied(self.prev_end)" result=false outcome="return None" file="quic/s2n-quic-core/src/buffer/receive_buffer.rs" line=682

Testing:

I've enabled the feature on the integration tests to make it easy to debug. Note that you do need to pass S2N_LOG=trace to get the output.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[jmayclin]

nit: I think the tracing subscriber can handle this logic for you: default_directive

[camshaft]

yeah probably worth switching to that i guess