Fix grant_immediate overwriting active read grants#78
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grant_immediate() forcibly evicts front messages from the ring buffer without checking read_in_progress. When a subscriber holds a FrameGrantR (raw pointer into the buffer), the evicted region gets overwritten by the new message, causing data corruption. Changes: - grant_immediate: check read_in_progress on front message before evicting; return Err(InsufficientSize) when the front is held by a reader. The caller (stack.rs) already handles this by discarding the incoming publish and skipping PUBACK, so the broker redelivers. - free_space / grant_with_context: fix off-by-one in inverted check (< to <=). When back_end == front.start the buffer is full, not empty. - Add regression test: grant_immediate_refuses_to_evict_read_in_progress
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Summary
grant_immediate()forcibly evicts front messages from the ring buffer without checkingread_in_progress. When a subscriber holds aFrameGrantR(raw pointer into the buffer), the evicted region gets overwritten by the new message, causing data corruption. This manifests as intermittent hash mismatches in rustot's OTA integration test.read_in_progresson the front message before evicting. ReturnsErr(InsufficientSize)when the front is held by a reader. The caller (stack.rs:238-254) already handles this error by discarding the incoming publish and skipping PUBACK — the broker redelivers for QoS 1.free_space()andgrant_with_context(): whenback_end == front.start(buffer wraps to exactly touch the front), this was incorrectly treated as non-inverted, returning a wildly inflated free space value.Test plan
grant_immediate_refuses_to_evict_read_in_progress