fix(recv): dispatch a resent message once

[jlucaso1]

Summary

A consumer answered a single group message two or four times, intermittently and only for some senders. The cause is upstream of us: a sender whose network is bad re-runs its own outbox and resends the same message id, re-encrypted on a new sender-key iteration. get_sender_key (wacore/libsignal/src/protocol/group_cipher.rs:157-172) returns DuplicatedMessage only when the same iteration comes back, which covers the byte-identical stanza the server replays and nothing else, so every resend decrypted cleanly and became another Event::Messages. Nothing anywhere on handle_decrypted_plaintext → dispatch_parsed_message → commit_or_batch_inbound asked whether that message identity had already been delivered.

The fix is a dispatch-once gate keyed on SenderMessageId { chat, id, sender }, reusing the idiom the UndecryptableMessage gate already established in src/message/retry.rs. It reads immediately before dispatch_parsed_message and claims the id where a committed batch is dispatched, suppresses through the same ack_or_replay_to_hook the ratchet-level duplicate uses, and adds no durable storage.

Reproduction

resent_group_message_dispatches_once is the reproducer, and it is the wire scenario rather than a stub: a real SKDM installs the sender key, the remote peer's own MemSenderKeyStore encrypts the same plaintext twice (two iterations of one chain, exactly what an outbox retry produces), both stanzas carry the same id, and both enter through the production path Client::handle_incoming_message. It asserts one Event::Messages, two delivery receipts (one per delivery), and one counted suppression.

Before the fix it fails on the dispatch count, not on compilation:

thread 'message::tests::resent_group_message_dispatches_once' panicked:
  left: (2, 2)
 right: (1, 1)

Reverting only the gate line (} else if self.message_already_dispatched(info).await { → } else if false {) reproduces that failure plus suppressed_resend_still_installs_the_sender_key_it_carries (left: 2, right: 1), while the other tests still pass.

Every test added in review was checked the same way, each time confirming that exactly the intended tests fail and nothing else moves: restoring the pre-review claim site and key fails resend_after_a_deferred_batch_commits_is_suppressed and resend_with_a_device_qualified_participant_is_suppressed; removing the batch collapse fails two_resends_inside_one_deferred_batch_dispatch_once; removing its capacity-0 guard fails zero_capacity_disables_the_batch_collapse; removing the collapse's suppression count and the key-share reschedule fails those two tests and nothing else; building delete_keys from the collapsed slice again fails a_collapsed_batch_clears_every_arrived_pending_row; removing the envelope guard on the claim fails an_unresolved_secret_envelope_is_not_claimed; collapsing on identity alone fails two_msmsg_parts_under_one_id_both_reach_the_consumer and a_batch_keeps_the_resolved_copy_of_an_unresolved_envelope; dropping the secret capture from the suppressed branch fails a_suppressed_resend_still_captures_the_message_secret; reverting the comparison bound, the same-stanza exemption, the replay's commit check and the presence-based envelope test fails exactly an_id_reused_past_the_bound_stops_being_collapsed, two_identical_msmsg_parts_under_one_id_both_reach_the_consumer, a_replay_that_fails_to_commit_counts_as_a_suppression and a_malformed_secret_envelope_is_not_claimed.

That discipline earned its keep. The first version of the envelope test drove dispatch_parsed_message directly, which never consults the gate, so it passed with the fix reverted: a green test that proved nothing. It was rewritten to go through handle_decrypted_plaintext, and only then did the revert fail it. A later bound test was vacuous the same way (distinct payloads never collapse, bound or no bound) and was rewritten to send a repeat past the bound, which is the behaviour the bound actually changes.

Production numbers

Regenerated from the two logs (ids and JIDs redacted throughout; only counts and time deltas are quoted). Note the published one-liner undercounts, because its from [^:]* stops at the colon in a device-qualified JID; these numbers match on \S+ instead.

	bot A	bot B
distinct group message ids decrypted	3050	12857
ids decrypted more than once	31 (1.0%)	42 (0.33%)
deliveries observed for one id	up to 3	up to 6
busiest 5-minute decrypt burst	65	278

Repeated ids from the same sender have gaps of median 11.7s, p90 189s, and 285s for the longest plausible resend. Five of the repeated ids are duplicated in both logs, from two different bots, which places the resend at the sender and not in our client. Filtering one id shows the shape: successive arrivals with different sts and t, different ciphertext length, a successful decrypt each time, and one delivery receipt of ours per arrival. No <retry> of ours appears for any of them.

The same parse also turned up the case that decides the key: two different participants of one group used the same 32-character id, 116 seconds apart. A (chat, id) key with any TTL above two minutes would have swallowed the second participant's message.

Protocol evidence

Both sources are at whatspec waVersion 2.3000.1045368834, which is also the version pinned in tools/whatspec-codegen/whatspec.lock.json (rev 12dbeeb), so the IR and the bundle agree here and no staleness caveat applies. The raw bundle referenced in the task as docs/captured-js/ is not in this tree; whatspec commits bundle URLs and hashes rather than sources, so the bundle set was fetched from generated/bundles.lock.json and read directly.

From the IR

• generated/incoming/index.json, tag: "message" (incomingMsgParser, WAWebHandleMsgParser): the official client reads from, participant, t, type and a repeated <enc> whose type is the CiphertextType enum (skmsg/pkmsg/msg/msmsg, unknownValue: reject) plus optional count, decrypt-fail, state, session_type. count (the per-<enc> retry count) and t exist on the wire but are read as message metadata, not as identity; identity is MsgKey, and the sibling incomingMsgParserForAckOnly reads exactly type, offline, id, from, participant, confirming which fields name a stanza. Nothing in the envelope makes a resend a different message.
• generated/stanza/index.json: no edit or revoke mixin writes id. WASmaxOutMessagePublishAdminEditMixin sets edit="3", WASmaxOutMessagePublishRevokeMixin edit="7", WASmaxOutSpamMessageEditMixin edit="1", and so on; every one of them is attributes-only. Each message stanza's id comes from the sending record's own id (msgRecord.data.id.id in WAWebSendMsgCreateFanoutStanza, WAWebSendMsgCreateDeviceStanza, WAWebBroadcastMessageRPC, …). generated/proto/WAProto.proto puts the target in ProtocolMessage.key (a MessageKey), field 1. An edit, revoke, reaction or poll vote therefore carries its own stanza id and points at the target from inside the protobuf, so the gate cannot swallow one. Cross-checked against the logs: no repeated id there was an edit.
• generated/stanza/index.json, receipts: WAWebSendDeliveryReceiptJob (exported as sendDeliveryReceiptsAfterDecryption) carries id, to, participant, recipient, type, class and a <list> child. type and class are dynamic there, and our plain delivery receipt omits both; the id/to/participant/recipient set matches attribute for attribute. generated/incoming/index.json, tag: "receipt" (WAWebHandleMsgReceiptParser) reads id, from, offline, type, participant, recipient, t and friends. Neither side has any attribute distinguishing a first delivery from a redelivery, and the job's own name says it fires after each decryption, so one receipt per delivery is conformance, not noise.

From the raw bundle (runtime logic the IR does not model)

• WAWebMsgProcessingApiUtils.messageInfoToKey is what settles the key's shape. For a group message it returns new MsgKey({ remote: chat, fromMe: …, participant: asUserWidOrThrow(author), id: externalId }): a device-less user wid. WA Web's own message identity is therefore (chat, fromMe, id, bare author), which is what the gate now uses; keeping the device made our key narrower than the official one (see Design 3).
• WAWebPendingMessageKey.createPendingMessageKey(msgKey, ts, encs) returns `${msgKey}_${ts}_${encs.map(e => e.e2eType + ":" + e.retryCount).join(",")}`. It includes ts, and the resends in our logs carry a different t per attempt, so this key would not collapse the cases we see even if it were used for that.
• WAWebMessageDedupUtils is a plain Map of counts with addPendingMessage (returns the running count), hasPendingMessage and maybeClearPendingMessages, behind AB prop web_pending_message_cache_enabled (code 8353, bool, default false, altDefault true). In the captured set, the only cross-module caller is maybeClearPendingMessages(count) from the offline info-bulletin handler; hasPendingMessage has no caller at all. The count surfaces as msgReceivedTimes, used in exactly one place: aggregateDeliveryReceipts treats msgReceivedTimes > 1 as isInDB: true, skipping the DB existence check for a SIGNAL_OLD_COUNTER_ERROR duplicate. So the cache is receipt bookkeeping and measurement, not a dispatch gate.
• The collapse a WA Web user actually sees happens a layer below, at MsgKey identity in the message store: the second arrival updates the same row instead of creating a message. We have no message store, so the collapse has to be at dispatch, and the key has to be message identity.
• WAWebMsgProcessingDecryptionHandler's canDecryptNext / E2EProcessResult flow (which should_process_skmsg_after_session mirrors) is untouched by the chosen position: the gate sits after both passes, so nothing about pkmsg-then-skmsg ordering changes.

Design

1. Where the gate cuts. At dispatch, in handle_decrypted_plaintext immediately before dispatch_parsed_message. Cutting before PASS 2 instead would save the duplicate's group_decrypt but leave the iteration unconsumed, and the next real message would then buffer that skipped key via add_skipped_message_key into the persisted sender key record: an entry on disk per duplicate, forever, to save 0.3% to 1.0% of receive CPU. Measured, the crypto being paid for is ~44 µs (bench_group_recv) on 0.3-1.0% of messages, i.e. under 0.5 µs per message amortised, against a persisted write per duplicate. There is also a correctness reason the pre-decrypt position cannot work as stated: the gate must fire only when a dispatch already happened, and before decrypting we cannot tell a resend of a delivered message from a resend of one that failed to decrypt.

2. What replaces the dispatch. ack_or_replay_to_hook (src/message/durability.rs), the same helper the DuplicatedMessage branch uses. A bare ack would lose the message for anyone with an InboundDurabilityHook whose earlier commit failed; the helper replays the buffered copy instead. Status broadcast is skipped exactly as in that branch, since the should_ack gate already acked it.

What is not suppressed is the recovery work the message carried, and there are two pieces of it. An app_state_sync_key_request gets its key share scheduled again, because our own phone repeating the request is what it does when the first share never arrived. And maybe_capture_inbound_msg_secret still runs, because dispatch_parsed_message is otherwise its only caller: capture is write-behind and can drop an entry when its backend is down, so the resend is the second chance, and without it every later encrypted edit, reaction or comment on that parent stays unopenable. Both are no-ops when the message carries neither. The rule is the same in both cases: the event is a duplicate, the recovery is not.

3. What enters the cache, and its key. Only a real dispatch, and only one that handed the consumer the actual content. Two exclusions, both the same rule: UndecryptableMessage placeholders never enter (if we dispatched a placeholder and the resend now decrypts, the real content has to arrive; undecryptable_dispatched stays a separate cache), and neither does a message carrying a secret-encrypted envelope, because what the consumer got was the envelope and not the edit, reaction or comment inside it. That second test is presence, not extractability: extract_secret_encrypted returns None both for a plain message and for a tagged envelope that is malformed (missing field, IV not 12 bytes), and a malformed envelope is no more readable to a consumer than an unopenable one, so carries_secret_encrypted asks only whether one of the three envelope fields is set.

The claim is taken in commit_inbound_batch, immediately after its Event::Messages dispatch. Two constraints pin it there. It has to be at the batch, because a batch deferred by the drain never claimed at the call site (so a resend after the drain dispatched twice), and a claim taken for a commit that then fails would suppress and ack the redelivery with nothing ever handed to a consumer - the existing app_state_key_share_waits_outside_the_offline_message_lane test caught that. And it has to be after the dispatch rather than before it, because the claim's cache write is an await and this function can be cancelled by a bounded teardown flush: between the acks and the event, a cancellation would leave the batch acked with its event never sent, which for a consumer without a hook is a lost message. After the dispatch, a cancellation merely loses the claim and a later resend dispatches twice.

Because the read and the claim are at two points in time, a drain can accumulate both deliveries of one message before either claim exists, so the batch itself also collapses on the way out, keeping the first of each and counting what it drops as a suppression. The collapse changes only what the hook and the consumer see: everything that describes what arrived still runs over the pre-collapse slice, both the acks and the pending-inbound cleanup. That second half matters as much as the first, because the identities that collapse can be spelled differently on the wire (bare vs device-qualified participant); clearing only the surviving spelling's row would leave a stale row for a later resend to replay as an already-committed message. Live batches hold one message and return untouched without allocating, and a batch with nothing to collapse keeps borrowing the keys it already built.

A repeat is identity and content, from a different stanza, and only up to a bound. Each of those three qualifiers is there because identity alone loses real content:

• Content, because one stanza id can carry several genuinely different payloads. The msmsg loop in handle_incoming_message dispatches every bot-reply part under one info, and a drain batch can hold an unresolved envelope beside the retry that resolved to the inner message. A resend re-encrypts the same message, so its wire form is identical; those are not. The comparison is on the encoded bytes, through the waproto::codec entry points this file already calls for the byte cap and the durable write, and never on the decoded proto: wa::Message derives PartialEq but nothing else calls it, so a single == makes the whole message tree's equality code live and costs 236 KiB of binary, over the per-PR size budget. Encoding is lazy and at most once per side, so a batch whose identities are all distinct never encodes anything for this.
• Different stanza, because two parts of one stanza are not repeats of each other even when their payloads are equal. Every part is handed the same Arc<MessageInfo>, and two separate stanzas always allocate separate infos, so Arc::ptr_eq on the info is an exact "same stanza" test that can never exempt a genuine resend. It is best-effort in the other direction: dispatch_parsed_message may Arc::make_mut the info, and a part whose info was copied on write falls back to the content compare. What that residue can cost is the multiplicity of an identical repeat, never content the consumer did not otherwise get; per-addon identity is the complete fix and is a recorded follow-up.
• A bound, because comparing every payload against every earlier one sharing an identity is quadratic, and a participant chooses its own stanza ids: reusing one across a full 400-message drain batch would otherwise force tens of thousands of comparisons on the path holding the processing permit. Past eight distinct payloads for one identity the collapse gives up on it and keeps everything. That is also the more honest statement of intent, since a resend repeats one message and is found against the first or second entry; anything past that is not the pattern this exists for. Giving up errs toward a duplicate.

The key drops the sender's device (to_non_ad) and keeps the PN/LID namespace exactly as it arrived. Dropping the device matches messageInfoToKey above, and it has to: the server sends skmsg with a bare participant and pkmsg with a device-qualified one, so a resend bundling a rotated SKDM would otherwise be spelled differently from the delivery it repeats and slip past.

The check and the claim cannot be an atomic get_with the way the sibling gate is, for the reasons above. chat_lanes serializes incoming processing per chat and closes the window for ordinary traffic, but two workers for one chat can coexist after a lane eviction; the race they leave is a second dispatch of one message, which is the behaviour this gate improves on rather than a regression, and it errs in the safe direction. This is stated in the code rather than implied.

4. Persistence: none. The two duplicate modes are disjoint. A byte-identical redelivery survives a restart (it comes from the server's offline queue) and is already rejected by the persisted ratchet as DuplicatedMessage. A re-encrypted resend is live and short-lived: median 11.7s between attempts, 285s for the longest plausible one. So the new gate only has to cover a short in-memory window, and the durable cost is zero. Declared gap: a sender still retrying across a restart of ours escapes the gate and dispatches twice. PortableCache is runtime-agnostic and compiles for wasm32, so the bridge target is unaffected.

5. Sizing. CacheConfig::dispatched_messages, default CacheEntryConfig::new(five_min, 1_000), on by default. The 5-minute TTL comes from the measured resend window, not from a round number: of the 56 same-sender gaps between deliveries in the logs, 54 are under 300s, and the two that are not carry 41-character ids from a namespace that never reaches this gate. Lengthening it would catch nothing more and only widen the id-collision exposure the 116s cross-sender case illustrates (the sender is in the key, so that case is safe regardless). Capacity 1000 is ~3.6x the busiest 5-minute burst measured (278) and ~14x the p99 (72). max_capacity == Some(0) short-circuits the insert (src/portable_cache.rs), so capacity 0 is both the off switch and a cheap early-out, as recent_messages already uses. The batch collapse honours the same switch, or capacity 0 would restore the old behaviour for live traffic only.

6. Observability. stats().messages_suppressed_duplicate counts suppressions for the client's lifetime (client-level, not session-level: the sender's retry window does not end because our socket did), memory_report().dispatched_messages reports the cache occupancy next to its neighbours, wacore::telemetry::recv("duplicate_resend") labels the metrics facade, and the suppressed path logs at debug with the message id. Without these, the next "a message disappeared" report could not be told apart from this fix, which is why the counter is careful in both directions: it counts what the batch collapse drops (that reaches no consumer at all), and it skips a replay that reached the consumer — but only one that actually did. ack_or_replay_to_hook reports a replay from what its commit returned, so a replay whose commit failed dispatched nothing and is counted as the suppression it is; Deferred still counts as a replay, because its batch dispatches later. Received minus suppressed is what consumers saw. The Event::Messages doc comment, which is the stability policy for that payload, states the collapse and its limits too.

7. Scope. The gate covers everything reaching handle_decrypted_plaintext: group (skmsg), 1:1 (msg/pkmsg) and status. Newsletters are excluded and unreachable from here anyway; they branch off in classify_incoming_message into handle_newsletter_message and reach dispatch_parsed_message through its own early return, never entering the commit pipeline. Status broadcast keeps its own ack gate, which the suppressed path respects.

The msmsg bot-reply dispatch (src/message/msg_secret.rs) does not consult the gate, and that is a decision rather than an oversight: an msmsg delivery is an addon under a bot stanza, and nothing in the logs or in whatspec establishes that one stanza id carries at most one dispatchable addon. If a bot streams several parts under one id, a (chat, id, sender) gate would drop every part after the first, which is the failure this gate is built to avoid. Gating there needs the addon's identity, not the message's, so it is listed as a follow-up. Those parts do reach the batch, which is why the collapse carries the content and same-stanza qualifiers in Design 3.

Benchmarks & Cost

cargo bench -p wacore --bench send_receive_benchmark, medians, same machine, baseline measured with the change stashed:

bench	before	after
bench_group_recv	48.22 µs	44.44 µs
bench_dm_recv	134.9 µs	131.1 µs
bench_dm_recv_steady	4.291 µs	3.945 µs
bench_group_send_10	38.36 µs	33.44 µs
bench_group_send_skdm_256	1.425 ms	1.468 ms

The spread is noise in both directions on a loaded machine, which is the expected result: wacore has no behavioural change (its diff is two fields on StatsSnapshot and a doc comment), and the gate lives in the whatsapp-rust crate. sender_key_derivation_benchmark was not run: the chosen position does not touch the group ratchet.

Marginal cost of the gate, measured with a temporary divan target (not part of this PR), 1000-operation batches inside one block_on so runtime entry does not dominate, release profile, medians:

operation	per message
key construction alone (two Jid clones + the id)	23.0 ns
gate hit (a suppressed resend)	206.7 ns
miss + insert (every dispatched message)	517.5 ns

So ~0.52 µs on the receive path per dispatched message: ~1.2% of the group-receive benchmark's 44 µs, ~13% of the isolated steady-state DM Signal decrypt (3.9 µs, which is only the crypto and not the surrounding pipeline). to_non_ad() replaces a Jid clone in the key, so it does not change these figures materially.

The batch collapse costs one hash per item on batches larger than one, i.e. drains only; a live commit takes an early return. Message comparison runs only where identities collide, encodes each side at most once, and is bounded at eight distinct payloads per identity, so its worst case over a full batch is linear with a small constant rather than quadratic. The envelope check on the claim path is three as_option() reads on an already-decoded message, with no decode and no allocation.

Memory. size_of::<SenderMessageId>() == 88 (Jid 32 x2 + MessageId 24). MessageId's CompactString inlines up to 24 bytes; in the logs 61% of ids are 32 characters and spill to the heap (~32 bytes), while the 22-character (31%) and 20-character (8.5%) forms stay inline. At the default capacity that is 1000 x (88 B key + up to ~32 B heap id + the map's per-entry overhead), on the order of 0.15-0.20 MB worst case for a client saturating the cache, next to an identically shaped undecryptable_dispatched.

Binary size is +10.25 KiB stripped (+0.10%) and +9.31 KiB .text (+0.11%), inside the per-PR budget (64 KiB / 32 KiB), with .text waproto unchanged at 0 and no new dependency (Cargo.lock crate count 468 before and after). That last part took a correction worth recording: an earlier revision compared the decoded protos with ==, which made wa::Message's derived PartialEq live across the whole message tree and cost +236 KiB stripped, most of it attributed to waproto even though no waproto file is touched. Comparing the encoded bytes through waproto::codec — whose #[inline(never)] wrappers exist precisely to keep that tree from being instantiated in calling crates, and both of which this file already called — put it back: locally measured .text 8,828,982 → 8,636,982 and stripped 10,978,328 → 10,746,776.

Not covered by any benchmark: the inbound dispatch pipeline in the whatsapp-rust crate (protobuf decode, commit batching, ack and receipt emission) has no bench target, so the gate's share of the whole inbound cost is stated as a marginal figure against a wacore decrypt baseline rather than measured end to end. benches/client_group_send.rs is send-only by construction.

Audit

Every event_bus.dispatch in src/message/ and src/handlers/, asked the same question: is the effect idempotent under message identity, or does it fire per wire delivery?

site	verdict
src/message/dispatch.rs (Event::Messages, newsletter)	Correct. Plaintext, server-identified, never re-encrypted by the sender and never in the commit pipeline.
src/message/commit_batch.rs (Event::Messages)	Now covered, and it is where the claim and the batch collapse happen.
src/message/receive.rs and src/message/msg_secret.rs (Event::DecryptedPayload)	Correct by design, stated here so nobody "fixes" it later: the event is pre-decode and per <enc>, and its whole point is to hand a consumer bytes this build may not be able to decode. Its identity is (message, enc_index), not the message. A resend is a genuinely different payload.
src/message/retry.rs (Event::EncDecryptFailed)	Correct. Same shape: a per-<enc>, per-attempt diagnostic. Collapsing it would hide a sender whose every attempt fails.
src/message/retry.rs (Event::UndecryptableMessage)	Already gated, by the precedent this change follows.
Delivery receipt (ack_received_message)	Correct as-is, and resolved against the IR rather than by taste: WAWebSendDeliveryReceiptJob is sendDeliveryReceiptsAfterDecryption, and neither the send shape nor WAWebHandleMsgReceiptParser carries anything separating a first delivery from a repeat. One receipt per delivery is what the official client does. Both the suppressed path and the collapsed batch keep sending it per delivery.
src/handlers/** (DirtyState, OfflineSyncPreview, contact/group/newsletter/mex notifications, IdentityChange, IncomingCall, Presence)	Correct. None is keyed on message identity; notifications and calls have their own ids and their own idempotency.

Investigado e correto

• A single stanza never dispatches twice today. The one shape that could is a stanza whose session <enc> and group <enc> both yield dispatchable plaintext; in practice the pkmsg is SKDM-only (is_sender_key_distribution_only returns before dispatch) and the skmsg carries the content, so exactly one dispatch happens. No such case appears in the logs.
• One stanza id can dispatch several different payloads, which is a different thing and is why the collapse carries the content and same-stanza qualifiers: the msmsg loop dispatches every bot-reply part under one info. Covered by two_msmsg_parts_under_one_id_both_reach_the_consumer and two_identical_msmsg_parts_under_one_id_both_reach_the_consumer.
• Edits, revokes, reactions and poll votes cannot be swallowed. Established from whatspec, not assumed: no edit or revoke mixin writes id, every message stanza's id comes from the sender's own record, and the target lives in ProtocolMessage.key. Cross-checked against the logs. The secret-encrypted form of those addons is covered separately, by never claiming an envelope the consumer could not read (malformed included) and by the collapse comparing content.
• A resend after a retry receipt of ours passes the gate. We never dispatch on an attempt that failed to decrypt, so no key was claimed. Covered by resend_after_our_own_decrypt_failure_dispatches.
• A resend after a failed commit passes the gate. Covered by resend_after_a_failed_commit_dispatches.
• messages_received still counts every decrypted message that entered the pipeline, including ones later suppressed. That is what it already meant for every other path that decrypts without reaching a consumer, so redefining it here would change an existing counter for reasons unrelated to this gate. The suppression counter is the one that reconciles the two.
• MemoryReport::dispatched_messages is a bare entry count, not CollectionStats, so it does not contribute to total_estimated_bytes(). That matches every neighbour of the same shape (message_retry_counts, undecryptable_dispatched, pdo_pending_requests, pdo_requested), and undecryptable_dispatched in particular holds the identical key type at the identical capacity. Promoting only the new one would make the report inconsistent; changing the convention belongs to the whole group at once.
• The long-gap "duplicates" are not resends. Two repeated ids in bot B's log (116s and ~17h apart) turn out to be different senders in the same chat reusing one id, and two more (~91 min apart) carry 41-character ids from a different id namespace. They are the argument for keeping the sender in the key, not for widening the TTL.
• Semver Checks (informational) is red, and not from this PR. Every failing item is a waproto:: path (EncryptMessageOutput::message_key and AIRichResponseContentItemMetadata fields removed, new variants on exhaustive generated enums, sync_action_data_to_vec arity), i.e. pre-existing drift between the last published 0.7.0 and the regenerated protobuf. This PR touches no waproto file, and the job's own summary says it does not block. Worth knowing when reading that job: it aborts at the first failing crate (Finished [97s] waproto, exit 100) and never reaches whatsapp-rust, so its silence about this crate is not evidence of anything — see the CacheConfig caveat below.
• Follow-ups, deliberately not fixed here. All are paths that dispatch outside the commit pipeline or under an identity this gate does not model, and in each case claiming there would trade a duplicate for a possible loss - the wrong direction for this gate:
  • History-sync notifications. A resend carrying a history_sync_notification enqueues a second MajorSyncTask and dispatches a second Event::HistorySync, since handle_history_sync runs above the gate. This cache records that a message batch committed, not that a history chunk was processed, and the download happens later and can fail. A resend here is our own phone retrying a chunk, so suppressing it after a failed download loses that history permanently.
  • PDO placeholder recovery. src/pdo.rs dispatches the recovered message's Event::Messages directly, bypassing the commit batcher, so it never claims an identity and a resent PDO response re-dispatches it. Claiming it there would suppress a later normal delivery of the same message - the delivery that goes through the durability hook, while PDO recovery is deliberately event-only.
  • Per-addon identity for msmsg. Beyond the gate itself, this is also the complete fix for the residue noted in Design 3: an InboundMessage cannot carry the addon index without widening a frozen event payload, so the same-stanza exemption is exact where it fires and falls back to content comparison where the info was copied on write.
  • Separately, our delivery receipt omits type and class (dynamic in WAWebSendDeliveryReceiptJob) and the <list> child - a conformance question for the receipt format, which this PR is scoped out of.

Changes

• src/message/dispatch.rs: message_already_dispatched / mark_message_dispatched / dispatch_key / dispatch_gate_enabled, keyed on SenderMessageId with the sender's device dropped.
• src/message/commit_batch.rs: the claim, taken just after a committed batch's event goes out so the ack-to-dispatch span keeps no suspension point, and skipped for a message carrying a secret envelope; the collapse of repeats inside a batch, on identity and encoded content, from different stanzas, bounded per identity, counted as suppressions. Acks and pending-inbound cleanup both still run over every stanza that arrived.
• src/message/receive.rs: the gate branch in handle_decrypted_plaintext, suppressing through ack_or_replay_to_hook, still scheduling any app-state key share the message asked for and still capturing any message secret it carries, and reporting dispatched: true so the caller's ack state machine is unchanged.
• src/message/durability.rs: ack_or_replay_to_hook now reports whether a buffered copy was replayed and will dispatch, from what its commit returned, so the suppression counter skips only a replay that reached the consumer.
• src/features/message_edit.rs: carries_secret_encrypted, presence without shape validation, for callers deciding that the consumer did not get real content.
• src/cache_config.rs: dispatched_messages, 5m TTL / 1000 entries, in Debug next to its neighbours. Capacity 0 disables both the gate and the batch collapse.
• src/portable_cache.rs: configured_capacity(), so the off switch can be honoured outside the cache.
• src/client.rs, src/client/lifecycle.rs, src/client/accessors.rs: the cache, the duplicate_dispatch_suppressed counter, and both readouts.
• wacore/src/stats.rs: StatsSnapshot::messages_suppressed_duplicate, filled client-side like resends_throttled.
• wacore/src/types/events.rs: a paragraph on Event::Messages recording the collapse and what it does not cover, since that doc comment is the payload's stability policy.
• src/message/tests.rs: twenty-two tests (below), plus capturing_client_with_cache_config so a test can build a client with the gate turned off.

One API caveat, corrected from an earlier version of this description. MemoryReport and StatsSnapshot are #[non_exhaustive], so their new fields are additive. CacheConfig is not: it carries nine pub fields, no private field and no #[non_exhaustive], so adding dispatched_messages does break a downstream exhaustive struct literal or full destructure, and the field's Default only helps callers that already write ..Default::default() (which is the idiom the struct's own doc example uses). Marking the struct #[non_exhaustive] is itself a breaking change and a wider one, so it belongs in its own change and is the maintainer's call: either this field lands as an ordinary minor break at 0.x alongside its nine siblings, which is what this PR does, or the struct gets #[non_exhaustive] first as a deliberate one-time break that prevents every future one. That review thread is left open on purpose.

Tests added: resent_group_message_dispatches_once, same_id_from_two_participants_dispatches_twice, byte_identical_redelivery_is_still_rejected_by_the_ratchet (run with the gate at capacity 0, so only the ratchet can satisfy it), zero_capacity_disables_the_gate, zero_capacity_disables_the_batch_collapse, resend_after_our_own_decrypt_failure_dispatches, resend_after_a_deferred_batch_commits_is_suppressed, two_resends_inside_one_deferred_batch_dispatch_once, resend_with_a_device_qualified_participant_is_suppressed, resend_after_a_failed_commit_dispatches, suppressed_key_request_resend_still_schedules_the_share, a_suppressed_resend_still_captures_the_message_secret, suppressed_resend_still_installs_the_sender_key_it_carries, suppressed_resend_replays_to_a_durability_hook, a_replay_that_fails_to_commit_counts_as_a_suppression, a_collapsed_batch_clears_every_arrived_pending_row, an_unresolved_secret_envelope_is_not_claimed, a_malformed_secret_envelope_is_not_claimed, a_batch_keeps_the_resolved_copy_of_an_unresolved_envelope, two_msmsg_parts_under_one_id_both_reach_the_consumer, two_identical_msmsg_parts_under_one_id_both_reach_the_consumer, an_id_reused_past_the_bound_stops_being_collapsed.

No existing test needed adapting. app_state_key_share_waits_outside_the_offline_message_lane failed against the first version of the gate and drove where the claim is taken; it passes unmodified.

Validation

cargo fmt --all
cargo test -p whatsapp-rust --lib            # 1776 passed, 1 ignored
cargo test -p whatsapp-rust --lib message::  # 280 passed, 1 ignored
cargo test -p wacore --lib                   # 1485 passed, 1 ignored
cargo clippy -p whatsapp-rust --all-targets -- -D warnings
cargo clippy -p whatsapp-rust --all-features --all-targets -- -D warnings
cargo bench -p wacore --bench send_receive_benchmark    # baseline and after
python3 scripts/ci/measure_binary_size.py               # for the size figures above

Rest of the matrix left to CI (workspace clippy, feature matrix, wasm32, Miri, e2e, doctests), green on the current head apart from the pre-existing Semver Checks failure explained above.

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• wacore/src/types/events.rs

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---

📝 Walkthrough

Summary by CodeRabbit

• New Features

  • Added protection against delivering duplicate decrypted messages.
  • Added configurable dispatched-message tracking with five-minute retention and capacity for 1,000 entries.
  • Added duplicate-suppression counts to client statistics and memory reports.

• Bug Fixes

  • Prevented resent messages from being dispatched more than once while preserving sender-specific message identity.
  • Preserved recovery, acknowledgement, and durability behavior when message processing fails.
  • Ensured deferred, batched, retried, and duplicate messages continue to be handled correctly.

• Documentation

  • Documented duplicate-message suppression behavior and reporting.

Walkthrough

The client adds a configurable TTL cache for dispatched decrypted messages. It suppresses duplicate resends by sender-scoped message identity, preserves durability behavior, and reports suppression telemetry and cache usage.

Changes

Dispatch deduplication

Layer / File(s)	Summary
Dispatch cache and telemetry state src/cache_config.rs, src/client.rs, src/client/accessors.rs, src/client/lifecycle.rs, src/portable_cache.rs, wacore/src/stats.rs	The client initializes a five-minute, 1,000-entry dispatch cache. MemoryReport exposes its entry count. StatsSnapshot exposes suppressed duplicate counts. PortableCache exposes its configured capacity.
Durable dispatch gate src/message/dispatch.rs, src/message/receive.rs, src/message/durability.rs, src/message/commit_batch.rs, wacore/src/types/events.rs	Messages use chat, message ID, and device-less sender identity as the dispatch key. Duplicate dispatches are suppressed. Batch commits retain all arrivals for acknowledgements and pending-row deletion, while dispatching and marking only deduplicated items. Durability hooks report whether they replay a message.
Resend and recovery coverage src/message/tests.rs	Tests cover sender isolation, disabled deduplication, decrypt and commit failures, sender-key processing, deferred batches, replay, participant normalization, and acknowledgements.

Estimated code review effort: 4 (Complex) | ~45 minutes

Merge Risk: ⚪ Minimal · up to 2ef19

The PR suppresses repeated dispatches for the same inbound message while preserving delivery acknowledgements and recovery handling; no actionable merge-blocking risk remains.

Sequence Diagram(s)

sequenceDiagram
  participant MessageReceive
  participant Client
  participant InboundDurability
  MessageReceive->>Client: Check sender-scoped dispatch key
  Client-->>MessageReceive: Return dispatched status
  alt Duplicate dispatch
    MessageReceive->>InboundDurability: Acknowledge or replay message
  else New dispatch
    MessageReceive->>InboundDurability: Commit inbound batch
    InboundDurability->>Client: Mark message dispatched
    InboundDurability-->>MessageReceive: Dispatch Messages event
  end

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🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 77.27% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 22 functions across 11 files. (1 skipped:…	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly and concisely describes the primary change: preventing resent messages from being dispatched more than once.
Description check	✅ Passed	The description directly explains the duplicate-dispatch issue, implementation, design decisions, tests, and validation for the changeset.

Full details: Docstring Coverage

Explanation

Docstring coverage is 77.27% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 22 functions across 11 files. (1 skipped: 1 too large.)

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[greptile-apps]

Greptile Summary

This PR adds an in-memory dispatch-once gate for re-encrypted message resends and completes the prior deferred-batch fix by collapsing equivalent deliveries at commit time.

• Keys deduplication by chat, message ID, and device-normalized sender.
• Preserves per-delivery acknowledgments, durability-hook behavior, message-secret capture, and app-state key recovery.
• Adds cache configuration, suppression statistics, memory reporting, and focused resend/batch regression tests.

Confidence Score: 5/5

The PR appears safe to merge because the previously reported deferred-duplicate dispatch path is now collapsed before consumer delivery without compromising acknowledgments or durability.

No blocking failure remains.

Important Files Changed

Filename	Overview
src/message/commit_batch.rs	Collapses equivalent deferred-batch deliveries while retaining acknowledgments and pending-row cleanup for every arrived stanza.
src/message/receive.rs	Suppresses already-dispatched resends while preserving durability replay, message-secret capture, and app-state key-share recovery.
src/message/dispatch.rs	Defines the normalized dispatch identity and the read/claim operations for the new dispatch-once cache.
src/message/durability.rs	Reports whether a pending inbound copy will actually replay so suppression accounting remains accurate.
src/message/tests.rs	Adds coverage for live and deferred resends, cache disabling, sender normalization, durability, envelopes, and multipart messages.

Sequence Diagram

sequenceDiagram
    participant WA as WhatsApp Server
    participant RX as Receive Pipeline
    participant Batch as Inbound Commit Batch
    participant Hook as Durability Hook
    participant App as Consumer
    WA->>RX: Original message
    WA->>RX: Re-encrypted resend with same identity
    RX->>Batch: Queue both during deferred drain
    Batch->>Batch: Collapse matching identity and content
    Batch->>Hook: Commit retained message
    Hook-->>Batch: Durable
    Batch->>WA: Ack every arrived stanza
    Batch->>App: Dispatch one Event::Messages
    Batch->>Batch: Mark identity dispatched

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Inline comments:
In `@src/message/dispatch.rs`:
- Around line 24-27: Correct the concurrency invariant documented near
dispatched_messages: separate get and insert operations are not protected when
chat_lanes entries are evicted, so duplicate dispatches can occur. Either
explicitly document that this duplicate-dispatch window is accepted, or replace
the separate operations with an atomic get_with pattern matching
dispatch_undecryptable_event.

In `@src/message/tests.rs`:
- Around line 15055-15070: The test should replace the fixed 50 ms sleep with
crate::test_utils::poll_until, polling delivery_receipts_for(&transport.sent(),
id) until the expected count of 2 is reached before asserting receipts and
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📦 Binary size report

Metric	main	PR	Δ
bin size (stripped)	10.24 MiB	10.25 MiB	+10.25 KiB (+0.10%) 🔺
bin .text	8.23 MiB	8.24 MiB	+9.31 KiB (+0.11%) 🔺
bin allocated (text+data+bss)	10.24 MiB	10.25 MiB	+12.11 KiB (+0.12%) 🔺
llvm-lines wacore	555,100	555,102	+2 (+0.00%) 🔺
llvm-lines wacore copies	18,160	18,160	0
llvm-lines whatsapp-rust lib	786,468	790,156	+3,688 (+0.47%) 🔺
llvm-lines whatsapp-rust lib copies	24,415	24,539	+124 (+0.51%) 🔺
deps crates (Cargo.lock)	468	468	0

.text per crate

Crate	main	PR	Δ
.text whatsapp_rust	1.92 MiB	1.93 MiB	+7.57 KiB (+0.39%) 🔺
.text wacore	717.32 KiB	717.70 KiB	+389 B (+0.05%) 🔺
.text wacore_binary	80.01 KiB	80.01 KiB	0
.text wacore_libsignal	178.01 KiB	178.01 KiB	0
.text wacore_appstate	24.01 KiB	24.01 KiB	0
.text wacore_noise	20.92 KiB	20.92 KiB	0
.text waproto	1.79 MiB	1.79 MiB	0
.text whatsapp_rust_sqlite_storage	543.40 KiB	543.40 KiB	0
.text whatsapp_rust_tokio_transport	40.56 KiB	40.56 KiB	0
.text whatsapp_rust_ureq_http_client	12.75 KiB	12.75 KiB	0
.text std	1012.84 KiB	1014.11 KiB	+1.27 KiB (+0.12%) 🔺
.text other deps	1.91 MiB	1.91 MiB	+73 B (+0.00%) 🔺

Top movers (cargo-bloat attribution)

Crate	main	PR	Δ
whatsapp_rust	1.92 MiB	1.93 MiB	+7.57 KiB (+0.39%)
std	1012.84 KiB	1014.11 KiB	+1.27 KiB (+0.12%)

Baseline: ed543cba3 (latest main run) · Head: 2ca604347 · Graphs

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Actionable comments posted: 2

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Inline comments:
In `@src/message/commit_batch.rs`:
- Around line 886-892: The deferred-batch dispatch path around
mark_message_dispatched must deduplicate items by dispatch_key before invoking
the hook and publishing Event::Messages, while retaining required sender-key
processing and acknowledgements for every original item. Update the relevant
batch flow and add a regression test covering a resend before
flush_inbound_commits_under_permit.
- Around line 886-889: Remove the repeated explanatory comment at the batch
dispatch call site in the commit-batch flow, while leaving the rationale comment
in mark_message_dispatched unchanged. Do not alter the claim or dispatch
behavior.

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