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BOLT 5: don't fulfill offered HTLCs if peer not committed to it.

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Nasty corner case which I got wrong; we can fulfill but then we risk
a reorg removing it.  And anyway, fulfilling reveals that we are
the endpoint in practice.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
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Rusty Russell 2017-09-18 10:03:11 +09:30
parent 578573f92f
commit 36c099e0d4
1 changed files with 25 additions and 5 deletions
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05-onchain.md
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@ -276,19 +276,39 @@ reason `permanent_channel_failure`) as detailed in [BOLT
Each HTLC output can only be spent by us if we have the payment
preimage, or them if it has timed out.
There are actually several possible cases for an offered HTLC:
1. The other node is not irrevocably committed to it; this can only
happen if we have not received `revoke_and_ack` so that the other
node holds two valid commitment transactions, one with the HTLC and
one without. We won't normally know the preimage here, unless it's
a payment to ourselves, and revealing that would be an information leak,
so it's best to allow the HTLC to time out in this case.
2. The other node is irrevocably committed to it, but we haven't yet
committed to an outgoing HTLC. In this case we can either forward
or timeout.
3. We have committed to an outgoing HTLC for this incoming one. In
this case we have to use the preimage if we receive it from the
outgoing HTLC, otherwise we will lose funds by making an outgoing
payment without redeeming the incoming one.
## Requirements
If the node receives (or already knows) a payment preimage for an
unresolved HTLC output it was offered, it MUST *resolve* the output by
spending it. If the transaction is the nodes' own commitment transaction, then the it MUST use the HTLC-success transaction, and the
unresolved HTLC output it was offered for which it has committed to an
outgoing HTLC, it MUST *resolve* the output by spending it.
Otherwise, if the other node is not irrevocably committed to the HTLC,
it MUST NOT *resolve* the output by spending it.
To spend an offered HTLC output: if the transaction is the nodes' own commitment transaction, then it MUST use the HTLC-success transaction, and the
HTLC-success transaction output MUST be *resolved* as described in
"On-chain HTLC Transaction Handling". Otherwise, it MUST *resolve* the output
by spending it to a convenient address.
"On-chain HTLC Transaction Handling", otherwise, it MUST spending the output to a convenient address.
Otherwise, if the HTLC output has expired, it is considered
If not otherwise resolved, once the HTLC output has expired, it is considered
*irrevocably resolved*.