perf(zenoh-sync): replace event_listener with parking_lot Mutex+Condvar

[YuanYuYuan]

Summary

Replace event_listener in zenoh-sync's Event with a parking_lot Mutex + Condvar + tokio::sync::Notify triple. The previous implementation allocated a new event_listener::Listener on every wait() call, triggering a lazy DWARF/backtrace init inside event_listener that showed up as ~3% CPU overhead on the sender thread in profiling.

Key Changes

• commons/zenoh-sync/src/event.rs: replace Event + AtomicU8 with AtomicU8 flag + parking_lot::Mutex<()> + parking_lot::Condvar + tokio::sync::Notify; async path uses Notify::notified(), sync path uses the Condvar
• commons/zenoh-sync/benches/notify_wait.rs: criterion benchmark covering sticky (hot, same-thread re-check) and cross-thread wakeup paths

Benchmark Results

path	before	after	delta
sticky (hot re-check)	31.7 ns	32.3 ns	~0%
cross-thread wakeup	4.67 µs	4.39 µs	−8%

Flamegraph: std::backtrace_rs::symbolize::gimli::Context::new at 3.12% on the sender thread before, completely absent after.

Reproducing

# save baseline on main
git stash  # or checkout main
taskset -c 0,1 cargo bench -j4 -p zenoh-sync --bench notify_wait -- --save-baseline before

# compare on this branch
git stash pop  # or checkout feat/ev-fix
taskset -c 0,1 cargo bench -j4 -p zenoh-sync --bench notify_wait -- --baseline before

Results land in target/criterion/notify_wait/.

Breaking Changes

None

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Labels: enhancement

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[codecov]

Codecov Report

❌ Patch coverage is 92.40506% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.48%. Comparing base (1fab5d0) to head (9b3faca).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
commons/zenoh-sync/src/event.rs	92.40%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2513      +/-   ##
==========================================
+ Coverage   72.43%   72.48%   +0.05%     
==========================================
  Files         390      390              
  Lines       63374    63367       -7     
==========================================
+ Hits        45902    45931      +29     
+ Misses      17472    17436      -36     

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