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contractcourt/contract_resolvers: fix subscribe preimage race

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This commit fixes a potential race condition within the
IncomingContestResolver, that could cause us to miss a
preimage that was delivered in time.

Currently we query the db for the preimage, and then
subscribe for notifications. This permits the following
ordering of events:
 - query for preimage, returns nothing
 - preimage is added and delivered to subscribers
 - subscribe to preimages
 - preimage never comes through!!

We fix this by reordering to subscribe for preimages and
then query just in case it already exists. The effect is
that the query will always return a valid read of the
preimages that are currently queued for delivery.
This commit is contained in:
Conner Fromknecht 2018-07-24 21:37:48 -07:00
parent e0baa49690
commit aa6e5bdd2a
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GPG Key ID: E7D737B67FA592C7
1 changed files with 20 additions and 15 deletions
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35
contractcourt/contract_resolvers.go
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@ -8,12 +8,12 @@ import (
"io"
"io/ioutil"
"github.com/btcsuite/btcd/txscript"
"github.com/btcsuite/btcd/wire"
"github.com/davecgh/go-spew/spew"
"github.com/lightningnetwork/lnd/chainntnfs"
"github.com/lightningnetwork/lnd/lnwallet"
"github.com/lightningnetwork/lnd/lnwire"
"github.com/btcsuite/btcd/txscript"
"github.com/btcsuite/btcd/wire"
)
var (
@ -1012,20 +1012,13 @@ func (h *htlcIncomingContestResolver) Resolve() (ContractResolver, error) {
copy(h.htlcResolution.Preimage[:], preimage[:])
}
// If the HTLC hasn't yet expired, then we'll query to see if we
// already know the preimage.
preimage, ok := h.PreimageDB.LookupPreimage(h.payHash[:])
if ok {
// If we do, then this means we can claim the HTLC! However,
// we don't know how to ourselves, so we'll return our inner
// resolver which has the knowledge to do so.
applyPreimage(preimage[:])
return &h.htlcSuccessResolver, nil
}
// If the HTLC hasn't expired yet, then we may still be able to claim
// it if we learn of the pre-image, so we'll wait and see if it pops
// up, or the HTLC times out.
// it if we learn of the pre-image, so we'll subscribe to the preimage
// database to see if it turns up, or the HTLC times out.
//
// NOTE: This is done BEFORE opportunistically querying the db, to
// ensure the preimage can't be delivered between querying and
// registering for the preimage subscription.
preimageSubscription := h.PreimageDB.SubscribeUpdates()
blockEpochs, err := h.Notifier.RegisterBlockEpochNtfn()
if err != nil {
@ -1035,6 +1028,18 @@ func (h *htlcIncomingContestResolver) Resolve() (ContractResolver, error) {
preimageSubscription.CancelSubscription()
blockEpochs.Cancel()
}()
// With the epochs and preimage subscriptions initialized, we'll query
// to see if we already know the preimage.
preimage, ok := h.PreimageDB.LookupPreimage(h.payHash[:])
if ok {
// If we do, then this means we can claim the HTLC! However,
// we don't know how to ourselves, so we'll return our inner
// resolver which has the knowledge to do so.
applyPreimage(preimage[:])
return &h.htlcSuccessResolver, nil
}
for {
select {