Describe the bug

We are observing duplicate liveliness samples when using FetchingSubscriber with liveliness subscriptions. This started surfacing consistently after switching to an io_uring-based transport (likely due to timing changes making the race deterministic).

Specifically, sub.recv_async() may return the same key expression (e.g. LIVELINESS_KEYEXPR_1) twice.

Expected Behavior

Liveliness tokens should be deduplicated by key expression, consistent with the session-level invariant in Zenoh (i.e., a token should not be observed twice unless it was actually re-declared).

Actual Behavior

The same liveliness key expression is delivered twice:

• once from the fetch phase (liveliness().get(...))
• once from the live subscription stream

Root Cause (Analysis)

FetchingSubscriber merges:

1. fetched samples
2. live samples

via an internal merge queue that deduplicates only by timestamp, not by key expression.

However:

• fetched liveliness samples and live updates may have different timestamps (or synthetic ones),
• therefore both entries are preserved,
• resulting in duplicate delivery of the same liveliness token.

This is particularly visible in clique scenarios and under timing-sensitive transports like io_uring.

Notes

From discussion with maintainers:

• This is considered a bug in zenoh-ext, not in core zenoh.
• Fetching subscribers are somewhat deprecated / discouraged in favor of history(true).
• Liveliness support in fetching subscribers was likely added without full consideration of deduplication semantics.

Suggested Fix

Deduplicate liveliness samples in FetchingSubscriber by key expression, not just timestamp.

Possible approaches:

• maintain a HashSet<KeyExpr> for liveliness tokens
• or enforce the same invariant as session-level liveliness handling

Workaround

Temporarily ignoring the failing test in CI:

#[ignore = "https://github.com/eclipse-zenoh/zenoh/issues/2523"]

To reproduce

Test:
test_liveliness_fetching_subscriber_clique
with "uring" feature

Symptom:
sub.recv_async() yields duplicate LIVELINESS_KEYEXPR_1

System info

Linux