Prevent deadlocks in Peer (#484)

We were using BUFFERED channels, with the default behavior where the
sender suspends when the buffer is full (64 elements by default).

The issue is that the same coroutine receives from the `input` channel
and emits potentially multiple events to that same channel. We would thus
fill the buffer, which causes us to `suspend`, which stops dequeuing
events, leading to a complete deadlock.

src/commonMain/kotlin/fr/acinq/lightning/io/Peer.kt

@@ -25,7 +25,7 @@ import fr.acinq.secp256k1.Hex
 import kotlinx.coroutines.*
 import kotlinx.coroutines.channels.BufferOverflow
 import kotlinx.coroutines.channels.Channel
-import kotlinx.coroutines.channels.Channel.Factory.BUFFERED
+import kotlinx.coroutines.channels.Channel.Factory.UNLIMITED
 import kotlinx.coroutines.channels.ReceiveChannel
 import kotlinx.coroutines.flow.*
 import org.kodein.log.newLogger
@@ -113,8 +113,10 @@ class Peer(
 
     val remoteNodeId: PublicKey = walletParams.trampolineNode.id
 
-    private val input = Channel<PeerCommand>(BUFFERED)
-    private var output = Channel<ByteArray>(BUFFERED)
+    // We use unlimited buffers, otherwise we may end up in a deadlock since we're both
+    // receiving *and* sending to those channels in the same coroutine.
+    private val input = Channel<PeerCommand>(UNLIMITED)
+    private var output = Channel<ByteArray>(UNLIMITED)
     val outputLightningMessages: ReceiveChannel<ByteArray> get() = output
 
     private val logger = MDCLogger(nodeParams.loggerFactory.newLogger(this::class), staticMdc = mapOf("remoteNodeId" to remoteNodeId))
@@ -368,7 +370,7 @@ class Peer(
 
         suspend fun respond() {
             // Reset the output channel to avoid sending obsolete messages
-            output = Channel(BUFFERED)
+            output = Channel(UNLIMITED)
             for (msg in output) send(msg)
         }