fuzz: add force-close support to chanmon_consistency

fc-crashes.md

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+# Force-close fuzzer LDK crashes
+
+Minimized crash sequences found by the chanmon_consistency fuzzer with
+force-close support. All crashes are `debug_assert` or `panic!` inside
+LDK, not in the fuzzer harness. Byte 0 encodes monitor styles (bits
+0-2) and channel type (bits 3-4: 0=Legacy, 1=KeyedAnchors).
+
+## 1. channelmonitor.rs:2727 - HTLC input not found in transaction
+
+```
+debug_assert!(htlc_input_idx_opt.is_some());
+```
+
+When resolving an HTLC spend, the monitor searches for the HTLC
+outpoint in the spending transaction's inputs but doesn't find it.
+Falls back to index 0 in release mode, which would produce incorrect
+tracking.
+
+Minimized (17 bytes):
+```
+0x40 0xff 0xff 0xff 0xff 0xff 0xff 0xff 0xff 0xff 0xff 0xff 0xff 0xff 0xdc 0xde 0xff
+```
+
+Byte 0 = 0x40: Legacy channels, no async monitors. The sequence is
+mostly 0xff (settlement) repeated, with height advances (0xdc, 0xde)
+near the end. This suggests the crash happens during settlement when
+processing on-chain HTLC spends after repeated settlement attempts.
+
+## 2. onchaintx.rs:913 - Duplicate claim ID in pending requests
+
+```
+debug_assert!(self.pending_claim_requests.get(&claim_id).is_none());
+```
+
+The OnchainTxHandler registers a claim event with a claim_id that
+already exists in the pending_claim_requests map.
+
+Minimized (10 bytes):
+```
+0x08 0xd2 0x70 0x70 0x71 0x70 0x10 0x19 0xde 0xff
+```
+
+Byte 0 = 0x08: KeyedAnchors channels, no async monitors.
+- 0xd2: B force-closes the A-B channel
+- 0x70/0x71: disconnect/reconnect peers
+- 0x10, 0x19: process messages on nodes A and B
+- 0xde: advance chain 200 blocks
+- 0xff: settle
+
+B force-closes, peers disconnect and reconnect, messages are exchanged,
+then height advances and settlement triggers the duplicate claim.
+
+## 3. onchaintx.rs:1025 - Inconsistent internal maps
+
+```
+panic!("Inconsistencies between pending_claim_requests map and claimable_outpoints map");
+```
+
+The OnchainTxHandler detects that its `pending_claim_requests` and
+`claimable_outpoints` maps are out of sync.
+
+Minimized (14 bytes):
+```
+0x00 0x3c 0x11 0x19 0xd0 0xde 0xff 0xff 0x19 0x21 0x19 0xde 0x26 0xff
+```
+
+Byte 0 = 0x00: Legacy channels, all monitors completed.
+- 0x3c: send hop payment A->B->C (1M msat)
+- 0x11, 0x19: process messages to commit HTLC on A-B
+- 0xd0: A force-closes A-B
+- 0xde: advance 200 blocks
+- 0xff: settle (first round)
+- 0xff: settle again (second round, processes more messages)
+- 0x19, 0x21, 0x19: continue processing B and C messages
+- 0xde: advance 200 more blocks
+- 0x26: process events on node C
+- 0xff: settle (third round)
+
+A hop payment partially committed, then A force-closes. Multiple
+settlement rounds with continued message processing in between triggers
+the internal map inconsistency.
+
+## 4. test_channel_signer.rs:395 - Signing revoked commitment
+
+```
+panic!("can only sign the next two unrevoked commitment numbers, revoked={} vs requested={}")
+```
+
+The test channel signer is asked to sign an HTLC transaction for a
+commitment number that has already been revoked.
+
+Minimized (18 bytes):
+```
+0x22 0x71 0x71 0x71 0x71 0x71 0x71 0x71 0xff 0xff 0xff 0xff 0xff 0xff 0xde 0xde 0xb5 0xff
+```
+
+Byte 0 = 0x22: Legacy channels, async monitors on node B.
+- 0x71: disconnect B-C peers (repeated, only first effective)
+- 0xff: settle (repeated 6 times)
+- 0xde 0xde: advance 400 blocks
+- 0xb5: restart node B with alternate monitor state
+- 0xff: settle
+
+Async monitors on B with peer disconnection, repeated settlements,
+height advances, and a node restart with a different monitor state.
+The stale monitor combined with the restart puts B's signer in a state
+where it's asked to sign for an already-revoked commitment.
+
+## 5. channelmanager.rs:9836 - Payment blocker not found
+
+```
+debug_assert!(found_blocker);
+```
+
+During payment processing, the ChannelManager expects to find a
+specific blocker entry for an in-flight payment but it's missing.
+
+Minimized (13 bytes):
+```
+0x00 0x3c 0x11 0x19 0x11 0x1f 0x19 0x21 0x19 0x27 0x27 0xde 0xff
+```
+
+Byte 0 = 0x00: Legacy channels, all monitors completed.
+- 0x3c: send hop A->B->C (1M msat)
+- 0x11, 0x19, 0x11: commit HTLC on A-B
+- 0x1f: B processes events (forwards HTLC to C)
+- 0x19, 0x21, 0x19: commit HTLC on B-C
+- 0x27, 0x27: C processes events (claims payment)
+- 0xde: advance 200 blocks
+- 0xff: settle
+
+A straightforward A->B->C hop payment that completes normally (C
+claims), followed by a height advance and settlement. No force-close
+in this sequence, so the height advance before settlement may cause
+HTLC timeout processing that conflicts with the claim path.
+
+## 6. channelmanager.rs:19484 - Monitor update ID ordering violation
+
+```
+debug_assert!(update.update_id >= pending_update.update_id);
+```
+
+A ChannelMonitorUpdate has an update_id that is less than a pending
+update's id, violating the expected monotonic ordering.
+
+Minimized (10 bytes):
+```
+0x84 0x70 0x11 0x19 0x11 0x1f 0xd0 0x11 0x1f 0xba
+```
+
+Byte 0 = 0x84: Legacy channels, no async monitors, high bits set
+(bits 3-4 = 0, bits 7 and 2 set).
+- 0x70: disconnect A-B peers
+- 0x11, 0x19, 0x11: process messages (likely reestablish after setup)
+- 0x1f: process B events
+- 0xd0: A force-closes A-B channel
+- 0x11: process A messages
+- 0x1f: process B events
+- 0xba: restart node B with alternate monitor state
+
+A force-close followed by continued message/event processing and a
+node B restart triggers a monitor update with an out-of-order ID.

fuzz/.gitignore

@@ -2,3 +2,4 @@ hfuzz_target
 target
 hfuzz_workspace
 corpus
+artifacts

fuzz/FC-INFO.md

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+# Force-Close Fuzzing Notes
+
+This file records the current contract for `chanmon_consistency` force-close
+coverage. It is intentionally short. Keep branch history and one-off debugging
+notes elsewhere.
+
+## Goal
+
+Force-close fuzzing here should:
+
+- exercise realistic off-chain to on-chain transitions
+- keep force-close from changing the eventual outcome of claimed payments
+- only allow claimed-payment sender failures when force-close dust touched a
+  used payment path
+- allow unclaimed HTLCs to resolve by CLTV timeout
+- drive the harness far enough that it observes real terminal outcomes
+- avoid manufacturing timeout wins by starving message delivery or claim
+  propagation
+
+## Hard-Mode Invariant
+
+The current hard mode is:
+
+- once the harness calls `claim_funds`, that HTLC must eventually produce
+  `PaymentClaimed` at the receiver
+- after that claim, the sender must eventually produce a terminal outcome,
+  `PaymentSent` or `PaymentFailed`
+- if the sender produces `PaymentFailed` for a claimed payment, some used
+  force-close path for that payment must have been dust-trimmed
+- force-close dust on a used path is not, by itself, enough to require
+  `PaymentFailed`; the payment may still end in `PaymentSent`
+- if no used force-close path for the claimed payment was dust-trimmed, the
+  sender must eventually produce `PaymentSent`
+- going on-chain does not create any broader exception than that dust case
+- unclaimed HTLCs may still fail by CLTV expiry
+- CSV waits on force-close outputs are normal and expected; they are not
+  payment outcome changes
+- a payment disappearing from `list_recent_payments()` is not enough, the
+  harness must observe or drive the terminal outcome directly
+
+In this mode, the following are harness failures:
+
+- `HTLCHandlingFailed::Receive` after we already chose to claim the HTLC
+- a receiver-side claim without the receiver later getting `PaymentClaimed`
+- a claimed HTLC without any sender-side terminal event
+- a claimed HTLC getting `PaymentFailed` without any dust-trimmed used
+  force-close path
+- a claimed HTLC that should fulfill resolving by CLTV timeout instead
+- cleanup stopping while live balances or other pending work still show that
+  more progress is possible
+
+## Timeouts
+
+Do not conflate CSV and CLTV:
+
+- CSV is normal force-close settlement latency
+- CLTV expiry changes the HTLC outcome
+
+The harness should keep driving through CSV waits. It should only protect
+claimed HTLCs that should still fulfill from CLTV-expiry resolution.
+
+## Harness Rules
+
+The main rules for preserving the invariant are:
+
+- advance large height jumps one block at a time, with bounded draining before
+  and after each block
+- process queued messages and events before confirming newly broadcast
+  transactions, so preimages can propagate before timeout paths win
+- keep sender-side payment bookkeeping independent of
+  `list_recent_payments()`
+- track which channels each payment actually used, and when force-closing,
+  snapshot which used payment paths become dust-blocked on the closer's
+  commitment
+- keep driving while `ClaimableOnChannelClose`, HTLC-related claimable balances,
+  queued messages, pending monitor updates, or pending broadcasts still show
+  unresolved work
+- only stop before a CLTV boundary when crossing it would let a claimed HTLC
+  that has not yet reached a sender terminal event expire instead
+- do not hide pending-payment state behind unrelated auto-driving before an
+  explicit force-close opcode; a bounded pre-close drain is acceptable when it
+  is only making already-queued work visible
+
+## Review Checklist
+
+When changing this harness, verify:
+
+- claimed HTLCs still require `PaymentClaimed`
+- claimed HTLCs still require a sender-side terminal event
+- claimed HTLCs only allow `PaymentFailed` when some used force-close path was
+  dust-trimmed
+- claimed HTLCs without dust-trimmed used force-close paths still require
+  `PaymentSent`
+- unclaimed HTLCs may still time out on-chain
+- force-close opcodes still act on the currently pending state
+- large synthetic height jumps do not become blind timeout buttons again
+- sender-side obligations are not reconciled away through local caches
+
+## Verification
+
+The standard check is:
+
+```bash
+~/repo/rl-tools/run_fuzz_runner.sh --timeout-secs 20
+```
+
+Re-run the full corpus after any meaningful force-close harness change.