refactor(fspy): simplify shm_io to a plain frame arena - #578
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fspy benchmarklinuxmacoswindows |
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| let serialized_size = | ||
| usize::try_from(T::serialized_size(value)?).expect("serialized size exceeds usize"); | ||
| let Some(size) = NonZeroUsize::new(serialized_size) else { | ||
| return Err(WriteEncodedError::ZeroSizedFrame); |
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When write_encoded is called after the receiver has closed with a zero-sized value such as () or a schema whose size calculation fails, these early returns produce ZeroSizedFrame or Encode instead of Claim(Closed). The previous implementation entered the gate before calculating the serialized size, and the documented post-close contract requires writes to be reported as Claim(Closed) so callers can silently discard them; perform the gate check first and hold its guard through serialization.
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The gated API reads shared memory only when the gate is closed and nothing was in flight, which proves every claimed frame was fully written and published. The three-state frame header (`0` / `+size` / `-size`), the atomic header stores and the release fences existed so a reader could tolerate crashed or in-flight writers — states that are unreachable through the gate. Delete them. A frame is now `| size: u32 | content | padding |`, written once at claim time with a plain store, and the arena keeps exactly one atomic: its end offset. Crash exclusion lives entirely in the gate, where a leaked count fails closed — the run is not cached — instead of being parsed around. The arena also stops being a surface of its own and becomes an internal of the gated module. No public API change: `GatedShmWriter`, `GatedShmReceiver`, `GatedShmReader`, `FrameMut`, `ClaimError` and `StillWriting` keep their signatures, so the channel layer, the fspy crate and both preload clients are untouched. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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Motivation
The gated API reads shared memory only when the gate is closed and nothing was in flight, which proves every claimed frame was fully written and published — the gate guard is released only after the frame drops. The three-state frame header (
0/+size/-size), the atomic header stores and the release fences inshm_ioexisted so a reader could tolerate crashed or in-flight writers: states that are unreachable through the gated API.This deletes them. The arena keeps exactly one atomic, its end-offset word; a frame becomes
| size: u32 | content | padding |, written once at claim time with a plain store, because the claim already made the range exclusive and publication is the gate release rather than the header store. Crash exclusion lives entirely in the gate, where a leaked count fails closed — the run is not cached — instead of being parsed around at read time. The arena also stops being a surface of its own and becomes an internal of the gated module.No public API change:
GatedShmWriter,GatedShmReceiver,GatedShmReader,FrameMut,ClaimErrorandStillWritingkeep their signatures, so the channel layer, the fspy crate and both preload clients are untouched.🤖 Generated with Claude Code