In the scenario where the requested channel does not have enough balance
and another channel towards the same node generates a different failure,
we erroneously returned UnknownNextPeer instead of the expected
TemporaryChannelFailure.
This commit rewrites the non-strict forwarding logic in the switch to
return the proper failure message. Part of this is moving the link
balance check inside the link.
The previous limit of 1008 proved to be low, given that almost 50% of
the network still advertises CLTV deltas of 144 blocks, possibly
resulting in routes with many hops failing.
In this commit, we update the tower+link logic to tag a commitment as
the new (tweakless) format if it applies. In order to do this, the
BackupTask method has gained an additional parameter to indicate the
type of commitment that we're attempting to upload. This new tweakless
bool is then threaded through all the way to back up task creation to
ensure that we make the proper input.Input.
Finally, we've added a new test case for each existing test case to test
each case w/ and w/o the tweakless modifier.
In this commit, we update the funding workflow to be aware of the new
channel type that doesn't tweak the remote party's output within the
non-delay script on their commitment transaction. To do this, we now
allow the caller of `InnitChannelReservation` to signal if they want the
old or new (tweakless) commitment style.
The funding tests are also updated to test both funding variants, as
we'll still need to understand the legacy format for older nodes.
In this commit, we update the channel state machine to be aware of
tweakless commits. In several areas, we'll now check the channel's type
to see if it's `SingleFunderTweakless`. If so, then we'll opt to use the
remote party's non-delay based point directly in the script, skipping
any additional cryptographic operations. Along the way we move the
`validateCommitmentSanity` method to be defined _before_ it's used as is
cutomary within the codebase.
Notably, within the `NewUnilateralCloseSummary` method, we'll now _blank
out_ the `SingleTweak` value if the commitment is tweakless. This
indicates to callers the witness type they should map to, as the value
isn't needed at all any longer when sweeping a non-delay output.
We also update the signing+verification tests to also test that we're
able to properly generate a valid witness for the new tweakless
commitment format.
Earlier this delay was needed to increase the likelihood that the DLP
scanario was successfully completed. Since we would risk the connection
being torn down, and the link exit, we could end up with the remote
marking the channel borked, but not finishing the force close.
With the previous set of commits, we should now trigger the force close
before we merk the channel borked, which should ensure we'll resume the
orocess on next restart/connect.
Instead of marking the channel Borked in cases where we want to force
close it, we immediately let the peer fail the link. The channel state
will instead be updated by the channel arbitrator, which will transition
to StateBroadcastCommit, marking the channel borked, then marking the
commitment tx broadcasted right before publishing the force close tx. We
do this to avoid the case where we would mark it Borked, but go down
before being able to publish the closing tx.
Storing the force close tx ensures it will be re-published on startup.
Instead of marking the database state when processing the channel
reestablishment message, we wait for the result of this processing to
arrive in the link, and mark it accordingly in the database here.
We do this move the logic determining whether we should force close the
channel or not, and what state to mark it in the DB, to the same place,
as these need to be consistent.
This commit converts the ErrCommitSyncLocalDataLoss error into a struct,
that also holds the received last unrevoked commit point from the remote
party.
In this commit, we update the router and link to support users
updating the max HTLC policy for their channels. By updating these internal
systems before updating the RPC server and lncli, we protect users from
being shown an option that doesn't actually work.
The policy update logic that resided part in the gossiper and
part in the rpc server is extracted into its own object.
This prepares for additional validation logic to be added for policy
updates that would otherwise make the gossiper heavier.
It is also a small first step towards separation of our own channel data
from the rest of the graph.
Now that the link will remain ineligible until it receives
channel_reestablish from the remote peer, we can remove the channel
reestablish timeout entirely.
This commit modifies the link's EligibleToForward() method only return
true once the peers have successfully exchanged channel reestablish
messages. This is a preliminary step to increasing the reestablish
timeout, ensuring the switch won't try to forward over links while
we're waiting for the remote peer to resume the connection.
Since we will now wait to deliver the event after channel reestablish,
notifying when the link is added to the switch will no longer be
sufficient. Later, we will add receiving reestablish as an additional
requirement for EligibleToForward returning true.
The inactive ntfn is also moved, to ensure that we don't fire inactive
notifications if no corresponding active notification was sent.
Extends the invalid payment details failure with the new accept height
field. This allows sender to distinguish between a genuine invalid
details situation and a delay caused by intermediate nodes.
This commit modifies hodl htlc notification from invoice registry from a
single notification per hash to distinct notifications per htlc. This
prepares for htlc-specific information (accept height) to be added to the
notification.
In this commit, we fix a bug where if a user updates a forwarding policy to be
zero, the update will be applied to the policy correctly on-disk, but not
in-memory.
We solve this issue by having the gossiper return the list of on-disk updated
policies and passing these policies to the switch, so the switch can assume
that zero-valued fields are intentional and not just uninitialized.
Logging for the hop package is controlled via the parent htlcswitch
package. Unfortunately the parent package only controlled the
initialization, but didn't enable the logger when the log level came
in from the main lnd package.
From BOLT 04:
The writer:
- MUST include amt_to_forward and outgoing_cltv_value for every node.
- MUST include short_channel_id for every non-final node.
- MUST NOT include short_channel_id for the final node.
This commit adds an additional return value to
Stream.DecodeWithParsedTypes, which returns the set of types that were
encountered during decoding. The set will contain all known types that
were decoded, as well as unknown odd types that were ignored.
The rationale for the return value (rather than an internal member) is
so that the stream remains stateless.
This return value can be used by callers during decoding to make
assertions as to whether specific types were included in the stream.
This is need, for example, when parsing onion payloads where certain
fields must be included/omitted depending on the hop type.
The original Decode method would incur the additional performance hit of
needing to track the parsed types, so we can selectively enable this
functionality when a decoder requires it by using a helper which
conditionally tracks the parsed types.
Currently the invoice registry cannot tell apart the htlcs that pay to
an invoice. Because htlcs may also be replayed on startup, it isn't
possible to determine the total amount paid to an invoice.
This commit is a first step towards fixing that. It reports the circuit
keys of htlcs to the invoice registry, which forms the basis for
accurate invoice accounting.
In this commit, we begin to enforce a maximum channel commitment fee for
channel initiators when attempting to update their commitment fee. Now,
if the new commitment fee happens to exceed their maximum, then a fee
update of the maximum fee allocation will be proposed instead if needed.
A default of up to 50% of the channel initiator's balance is enforced
for the maximum channel commitment fee. It can be modified through the
`--max-channel-fee-allocation` CLI flag.
In this commit, we update the `HopIterator` to gain awareness of the new
TLV hop payload. The default `HopIterator` will now hide the details of
the TLV from the caller, and return the same `ForwardingInfo` struct in
a uniform manner. We also add a new method: `ExtraOnionBlob` to allow
the caller to obtain the raw EOB (the serialized TLV stream) to pass
around.
Within the link, we'll now pass the EOB information into the invoice
registry. This allows the registry to parse out any additional
information from the EOB that it needs to settle the payment, such as a
preimage shard in the AMP case.
htlcs
config: Adding RejectHTLC field in config struct
This commit adds a RejectHTLC field in the config struct in config.go.
This allows the user to run lnd as a node that does not accept onward
HTLCs.
htlcswitch/switch: Adding a field RejectHTLC to the switch config
This commit adds a field RejectHTLC to the switch config. When the
switch receives an HTLC it will check this flag and if the HTLC is not
from the source hop, the HTLC will be rejected.
htlcswitch/switch: adding check for RejectHTLC flag and incomingChanID
This commit adds a check when receiving UpdateAddHTLC. The check looks
for the RejectHTLC flag set and whether the HTLC is from the sourceHop
(the local switch). If the HTLC is not from the sourceHop, then we
reject the HTLC and return a FailChannelDisabled error.
server: adding RejectHTLC field to initialization of switch
lnd_test: adding test for RejectHTLC
This commit adds a test which tests that a node with the --rejecthtlc
flag will reject any onward HTLCs but still can receive direct HTLCs and
can send HTLCs.
Previously a temporary channel failure was returning for unexpected
malformed htlc failures. This is not what we want to communicate to the
sender, because the sender may apply a penalty to us only.
Returning the temporary channel failure is especially problematic if we
ourselves are the sender and the malformed htlc failure comes from our
direct peer. When interpretating the failure, we aren't able to
distinguish anymore between our channel not having enough balance and
our peer sending an unexpected failure back.
Debug invoices are rarely used nowadays, but keep asking for maintenance
every time refactoring in primarily the invoice registry occurs. We have
passed the cost/benefit tipping point, so therefore the debug invoice
concept is removed in this commit.
Previously the debughtlc flag also controlled whether hodl masks were
active. It is safe to remove that additional condition because the hodl
masks are still guarded by the dev build tag.
In order to prevent information leaks by nodes probing with a payment
hash, this commit changes exit hop processing so that it always returns
incorrect_or_unknown_payment_details and leaves the prober in the dark
about whether an invoice actually exists.
Align naming better with the lightning spec. Not the full name of the
failure (FailIncorrectOrUnknownPaymentDetails) is used, because this
would cause too many long lines in the code.
The current value was based on the previous default CLTV delta of 144
blocks. This has been lowered to 40 since lnd v0.6.0-beta, making the
current value of 5000 blocks a bit high. Lowering it to one week should
be more than enough to account for the other major lightning
implementations. Eclair currently has a default CLTV delta of 144, while
c-lightning's is 14.
This commit makes the outgoing link pipeline the settle to the
switch as soon as it receives it. Previously, it would wait for a
revocation before sending it, which caused increased latency on
payments as well as possibly never settling on the incoming link.
A duplicate settle is still sent to the switch, but it is handled
gracefully. A new AckEventTicker was added to the switch which
acknowledges any pending settle / fail entries in an outgoing
link's fwd pkgs in batch. This was needed in order to reduce the
number of db txn's which would have been incurred by acking whenever
we receive a duplicate settle without batching.
Methods on failure message types used to be defined on value receivers.
This allowed assignment of a failure message to ForwardingError both as
a value and as a pointer. This is error-prone, especially when using a
type switch.
In this commit the failure message methods are changed so that they
target pointer receivers.
Two instances where a value was assigned instead of a reference are
fixed.
TestSwitchGetPaymentResult tests that the switch interacts as expected
with the circuit map and network result store when looking up the result
of a payment ID. This is important for not to lose results under
concurrent lookup and receiving results.
paymentResultStore is a persistent store where we keep track of all
received payment results. This is used to ensure we don't lose results
from payment attempts on restarts.
In this commit, we fix a lingering TOOD statement in the channel arb.
Before this commitment, we would simply wipe our our local HTLC set of
the HTLC set that was on the remote commitment transaction on force
close. This was incorrect as if our commitment transaction had an HTLC
that the remote commitment didn't, then we would fail to cancel that
back, and cause both channels to time out on chain.
In order to remedy this, we introduce a new `HtlcSetKey` struct to track
all 3 possible in-flight set of HTLCs: ours, theirs, and their pending.
We also we start to tack on additional data to all the unilateral close
messages we send to subscribers. This new data is the CommitSet, or the
set of valid commitments at channel closure time. This new information
will be used by the channel arb in an upcoming commit to ensure it will
cancel back HTLCs in the case of split commitment state.
Finally, we start to thread through an optional *CommitSet to the
advanceState method. This additional information will give the channel
arb addition information it needs to ensure it properly cancels back
HTLCs that are about to time out or may time out depending on which
commitment is played.
Within the htlcswitch pakage, we modify the `SignNextCommitment` method
to return the new set of pending HTLCs for the remote party's commitment
transaction and `ReceiveRevocation` to return the latest set of
commitment transactions on the remote party's commitment as well. This
is a preparatory change which is part of a larger change to address a
lingering TODO in the cnct.
Additionally, rather than just send of the set of HTLCs after the we
revoke, we'll also send of the set of HTLCs after the remote party
revokes, and we create a pending commitment state for it.
In this commit we move handing the deobfuscator from the router to the
switch from when the payment is initiated, to when the result is
queried.
We do this because only the router can recreate the deobfuscator after a
restart, and we are preparing for being able to handle results across
restarts.
Since the deobfuscator cannot be nil anymore, we can also get rid of
that special case.
This lets us distinguish an critical error from a actual payment result
(success or failure). This is important since we know that we can only
attempt another payment when a final result from the previous payment
attempt is received.
With the following commits, it'll become important to not resuse
paymentIDs, since there is no way to tell whether the HTLC in question
has already been forwarded and settled/failed.
We clarify this in the SendHTLC comments, and alter the tests to not
attempt to resend an HTLC with a duplicate payment ID.
This commit moves the responsibility of generating a unique payment ID
from the switch to the router. This will make it easier for the router
to keep track of which HTLCs were successfully forwarded onto the
network, as it can query the switch for existing HTLCs as long as the
paymentIDs are kept.
The router is expected to maintain a map from paymentHash->paymentID,
such that they can be replayed on restart. This also lets the router
check the status of a sent payment after a restart, by querying the
switch for the paymentID in question.
This commit is the final step in making the link unaware of invoices. It
now purely offers the htlc to the invoice registry and follows
instructions from the invoice registry about how and when to respond to
the htlc.
The change also fixes a bug where upon restart, hodl htlcs were
subjected to the invoice minimum cltv delta requirement again. If the
block height has increased in the mean while, the htlc would be canceled
back.
Furthermore the invoice registry interaction is aligned between link and
contract resolvers.
Now that the success resolver preimage field is always populated by the
incoming contest resolver, preimage lookups earlier in the
process (channel and channel arbitrator) can mostly be removed.
In this commit, we add a new test to ensure that we're able to properly
convert malformed HTLC errors that are sourced from multiple hops away,
or our direct channel peers. In order to test this effectively, we force
the onion decryptors of various peers to always fail which will trigger
the malformed HTLC logic.
In this commit, we now properly convert multi-hop malformed HTLC
failures. Before this commit, we wouldn't properly add a layer of
encryption to these errors meaning that the destination would fail to
decrypt the error as it was actually plaintext.
To remedy this, we'll now check if we need to convert an error, and if
so we'll encrypt it as if it we were the source of the error (the true
source is our direct channel peer).
In this commit, we fix a bug that caused us to be unable to properly
handle malformed HTLC failures from our direct link. Before this commit,
we would attempt to decrypt it and fail since it wasn't well formed. In
this commit, if its an error for a local payment, and it needed to be
converted, then we'll decode it w/o decrypting since it's already
plaintext.
In this commit, we add a new method to the ErrorEncrypter interface:
`EncryptMalformedError`. This takes a raw error (no encryption or MAC),
and encrypts it as if we were the originator of this error. This will be
used by the switch to convert malformed fail errors to regular fully
encrypted errors.
In this commit, we start the first phase of fixing an existing bug
within the switch. As is, we don't properly convert
`UpdateFailMalformedHTLC` to regular `UpdateFailHTLC` messages that are
fully encrypted. When we receive a `UpdateFailMalformedHTLC` today,
we'll convert it into a regular fail message by simply encoding the
failure message raw. This failure message doesn't have a MAC yet, so if
we sent it backwards, then the destination wouldn't be able to decrypt
it. We can recognize this type of failure as it'll be the same size as
the raw failure message max size, but it has 4 extra bytes for the
encoding. When we come across this message, we'll mark is as needing
conversion so the switch can take care of it.
In this commit, we fix a bug that would cause a node with a hodl HTLC to
cancel back the HTLC upon restart if the invoice has been settled, but
the HTLC is still present on the commitment transaction. A fix for the
HTLC still being present (not triggering a new commitment) has been
fixed recently. However, for older nodes with a lingering HTLC, on
restart it would be failed back.
In this commit, we make the check stricter by only performing these
checks for HTLCs that are in the open state. This ensures that we'll
only check this constraints the first time around, before the HTLC has
been transitioned to the accepted state.
This commit adds a brief delay when sending our channel reestablish
message if the link contains a restored channel to ensure we first have
a stable connection. Sending the message will cause the remote peer to
force close the channel, which currently may not be resumed reliably if
the connection is being torn town simultaneously. This delay can be
removed after the force close is reliable, but in the meantime it
improves the reliability of successfully closing out the channel and
allows the `channel_backup_restore/restore_during_creation` to pass
reliably.
In this commit, we modify the starting link logic to always send the
chan sync message to the remote peer in a synchronous manner. Otherwise,
it's possible that we fail very quickly below this block, and don't ever
send the message to the remote peer.
The idea of the batch counter is to increase it for commit tx updates,
so that if the commit tx cannot be updated immediately (revocation
window exhausted), the batch ticker makes sure it happens later.
The batch counter was increased for forwarded htlcs, but not for exit hop
resolutions.
This lead to the situation where the commitment tx would not be updated,
even though the htlc was settled locally. When no other changes happen
on the channel, the htlc eventually reaches its expiry and the channel
is force closed.
This commits exposes the various parameters around going to chain and
accepting htlcs in a clear way.
In addition to this, it reverts those parameters to what they were
before the merge of commit d107627145.
This commit increase the expiry grace delta to a value above the
broadcast delta. This prevents htlcs from being accepted that would
immediately trigger a channel force close.
A correct delta is generated in server.go where there is access to
the broadcast delta and passed via the peer to the links.
Co-authored-by: Jim Posen <jim.posen@gmail.com>
Previously there was no minimum remaining blocks requirement on
forwarded htlcs, which may cause channel arbitrator to force
close the channel directly after forwarding the htlc.
Co-authored-by: Jim Posen <jim.posen@gmail.com>
This commit modifies the invoice registry to handle invoices for which
the preimage is not known yet (hodl invoices). In that case, the
resolution channel passed in from links and resolvers is stored until we
either learn the preimage or want to cancel the htlc.
This commit detaches signaling the invoice registry that an htlc was
locked in from the actually settling of the htlc.
It is a preparation for hodl invoices.
In further commits the behaviour of invoice registry becomes more
intrinsically connected to the link. This commit prepares for that by
allowing link and registry to be tested as a single unit.
In the TestChannelLinkMultiHopUnknownPaymentHash test, a preimage was
modified to trigger an unknown payment hash failure. The way the mock is
implemented, it would take the hash of that modified preimage and store
it. It basically stores a completely different invoice. For this test,
it is just as good to store no invoice at all.
Previously it could happen that an invoice was open at the time of the
LookupInvoice call, the htlc was settled because of that, but when the
SettleInvoice call was made eventually, it would fail because the
invoice was canceled in the mean time. The htlc would then be settled,
but the invoice not marked as such.
In this commit, we set a default max HTLC in the forwarding
policies of newly open channels.
The ForwardingPolicy's MaxHTLC field (added in this commit)
will later be used to decide whether an HTLC satisfies our policy before
forwarding it.
To ensure the ForwardingPolicy's MaxHTLC default matches the max HTLC
advertised in the ChannelUpdate sent out for this channel, we also add
a MaxPendingAmount() function to the lnwallet.Channel.
This commit makes use of the batched AddWitness
method of the WitnessCache, in order to avoid
performing one write for each accepted preimage.
Additionally, this fixes an existing hole in the
consistency guarantees since the batched writes
are now guaranteed to take place before accepting
the next CommitSig. Previously, these writes were
processed in an unsynchronized go routine that
could be delayed arbitrarily long before being
executed.
With this change, the async_payments_benchmarks
actually shows a slight improvement in
performance, presumably because we no longer do
an individual write per preimage, even though
the execution is now explicitly in the critical
path. There is likely also a marginal performance
improvement from the reduction in goroutine
overhead.
In this commit, we modify the WitnessCache's
AddPreimage method to accept a variadic number
of preimages. This enables callers to batch
preimage writes in performance critical areas
of the codebase, e.g. the htlcswitch.
Additionally, we lift the computation of the
witnesses' keys outside of the db transaction.
This saves us from having to do hashing inside
and blocking other callers, and limits extraneous
blocking at the call site.
This function will be used in the switch to retrieve the channel point for a link,
allowing the switch to retrieve individual channels from the database.
This commit is a step to split the lnwallet package. It puts the Input
interface and implementations in a separate package along with all their
dependencies from lnwallet.
In this commit, we deprecate the `IncorrectHtlcAmount` onion error.
We'll still decode this error to use when retrying paths, but we'll no
longer send this ourselves. The `UnknownPaymentHash` error has been
amended to also include the value of the payment as well. This allows us
to worry about one less error.
This commit modifies the behavior of the
HasActiveLink method within the switch to
only return true if the link is in the
link index and is eligible to forward
HTLCs.
The prior version returns true whenever
the link is found in the link index,
which may return true for pending
channels that are not actually active.
This commit adds conversion between the lnwire.UpdateFee message and the
new FeeUpdate PaymentDescriptor. We re-purpose the existing Amount field
in the PaymentDescriptor stuct to hold the feerate.
This commit is a preparation for the addition of new invoice
states. A database migration is not needed because we keep
the same field length and values.
In this commit, we fix a minor discrepancy with the spec. We should
return a FinalFailExpiryTooSoon error, rather than a
FinalFailIncorrectCltvExpiry error, when the last HTLC of a route (exit
hop) has an expiration height that is deemed too soon by the final
destination of the HTLC.
In this commit, we remove the per channel `sigPool` within the
`lnwallet.LightningChannel` struct. With this change, we ensure that as
the number of channels grows, the number of gouroutines idling in the
sigPool stays constant. It's the case that currently on the daemon, most
channels are likely inactive, with only a hand full actually
consistently carrying out channel updates. As a result, this change
should reduce the amount of idle CPU usage, as we have less active
goroutines in select loops.
In order to make this change, the `SigPool` itself has been publicly
exported such that outside callers can make a `SigPool` and pass it into
newly created channels. Since the sig pool now lives outside the
channel, we were also able to do away with the Stop() method on the
channel all together.
Finally, the server is the sub-system that is currently responsible for
managing the `SigPool` within lnd.
Per team decision all tests should run with "debug" flag.
Makefile updated accordingly.
A developer may still run the test from command line by "go test ...."
The commit protects against this and issue an error if needed.
In this commit we add a check to HtlcSatifiesPolicy to verify that the
time lock for the outgoing htlc that is requested in the onion packet
isn't too far in the future.
Without this check, anyone could force an unreasonably long time lock on
the forwarding node.
In this commit, we increase the fwdpkg gc interval
to avoid having it conflict with switch tests that
inspect forwarding packages. The current timeout is
a little too short on travis, and sporadically fails
TestChannelLinkCleanupSpuriousResponses, which was
added recently.
This commit moves the logic handling responses to
locally-initiated payments to be asynchronous. The
reordering of operations into handleLocalDispatch
brings a serious performance burden to the switch's
main event loop. However, the at-most once semantics
of circuit map and idempotency of cleanup methods
allows concurrent operations to run in parallel.
Prior to this commit, the async_payments_benchmark
would timeout due to the forcibly serial nature of
the prior design. With this change, there is no
perceptible difference in the benchmark OMM, even
though we've added two extra db calls.
Composes the new payment status helper methods such that
we only require one db txn per state transition. This
also allows us to remove the exclusive lock from the
control tower, and enable more concurrent requests.
In this commit, we address an issue that could arise when using the
SendToRoute RPC. In this RPC, we specify the exact hops that a payment
should take. However, within the switch, we would set a constraint for
the first hop to be any hop as long as the first peer was at the end of
it. This would cause discrepancies when attempting to use the RPC as the
payment would actually go through another hop with the same peer. We fix
this by explicitly specifying the channel ID of the first hop.
Fixes#1500.
Fixes#1515.
In this commit, we modify the readHandler w/in
the mock peer to drop messages if it is unable
to find the target link. This has led to observed
race conditions related to removing a link and still
attempting to deliver messages. By removing this,
the readHandler shouldn't fail the test as a result.
This commit increases the fwdpkg garbage collection
interval to 15s, to mitigate the likelihood of it
interfering with our unit tests related to fwdpkgs.
This commit removes the concept of "circuit deletion
forgivness" from the link. This was originally
implemented due to the strict semantics of the original
DeleteCircuit implementation, which would fail if we tried
to delete unknown circuits. Forgivness is used on startup
to ignore this error in case the circuits had already been
deleted before shutting down.
Now that the circuit deletion has been relaxed, this
behavior is no longer necessary, as requests to delete
unknown (or previously deleted) circuits will be ignored.
This is necessary for future changes regarding switch
cleanup, which may attempt to cleanup already deleted
circuits.
Previously, we would only allow deletion of circuits if all circuit keys
were found in the pending map.
In this commit, we relax this to allow for deletion of any circuits
that are found pending, and ignore those that are not found. This
is a preliminary step to cleaning up duplicate forwards that get caught
by the switch. It also allows us to gracefully handle any nodes that
are still afflicted by the split mailbox issue.
Replaces the log statement in CommitCircuits so that
it prints the circuit key of the incoming channel. This way
we avoid spewing the secp curve stored in the ErrorEncrypter.
In this commit, we thread through a link's quit channel into
routeAsync, the primary helper method allowing links to send
htlcPackets through the switch. This is intended to remove
deadlocks from happening, where the link is synchronously
blocking on forwarding packets to the switch, but also
needs to shutdown.
This commit adds a test that verifies Stop does not block
if the link is concurrently forwarding incoming Adds to
the switch. This test fails prior to the commits that
thread through the link's quit channel.
