problem: a Websocket based upstream lacks current height until it loaded through Websocket

solution: load initial block through RPC, then continue with Websocket

src/main/kotlin/io/emeraldpay/dshackle/upstream/ChainUpstreams.kt

@@ -59,10 +59,12 @@ class ChainUpstreams (
             upstream.setLag(0)
             upstream.getHead()
         } else {
-            val newHead = EthereumHeadMerge(upstreams.map { it.getHead() })
-            newHead.start()
-            val lagObserver = HeadLagObserver(newHead, upstreams)
-            lagObserver.start()
+            val newHead = EthereumHeadMerge(upstreams.map { it.getHead().getFlux() }).apply {
+                this.start()
+            }
+            val lagObserver = HeadLagObserver(newHead, upstreams).apply {
+                this.start()
+            }
             this.lagObserver = lagObserver
             newHead
         }

src/main/kotlin/io/emeraldpay/dshackle/upstream/EthereumHeadMerge.kt

@@ -2,6 +2,7 @@ package io.emeraldpay.dshackle.upstream
 
 import io.infinitape.etherjar.domain.TransactionId
 import io.infinitape.etherjar.rpc.json.BlockJson
+import org.reactivestreams.Publisher
 import org.slf4j.LoggerFactory
 import org.springframework.context.Lifecycle
 import reactor.core.Disposable
@@ -11,7 +12,7 @@ import java.io.Closeable
 import java.util.concurrent.atomic.AtomicReference
 
 class EthereumHeadMerge(
-        upstreams: List<EthereumHead>
+        fluxes: Iterable<Publisher<BlockJson<TransactionId>>>
 ): EthereumHead, Lifecycle {
 
     private val log = LoggerFactory.getLogger(EthereumHeadMerge::class.java)
@@ -20,7 +21,6 @@ class EthereumHeadMerge(
     private var subscription: Disposable? = null
 
     init {
-        val fluxes = upstreams.map { it.getFlux() }
         flux = Flux.merge(fluxes)
                 .distinctUntilChanged {
                     it.hash

src/main/kotlin/io/emeraldpay/dshackle/upstream/EthereumUpstream.kt

@@ -5,7 +5,6 @@ import io.emeraldpay.grpc.Chain
 import org.slf4j.LoggerFactory
 import org.springframework.context.Lifecycle
 import reactor.core.Disposable
-import java.io.Closeable
 
 open class EthereumUpstream(
         val chain: Chain,
@@ -57,7 +56,17 @@ open class EthereumUpstream(
 
     open fun createHead(): EthereumHead {
         return if (ethereumWs != null) {
-            EthereumWsHead(ethereumWs).apply {
+            // load current block through RPC then listen for following blocks through WS
+            val ws = EthereumWsHead(ethereumWs).apply {
+                this.start()
+            }
+            val rpc = EthereumRpcHead(api).apply {
+                this.start()
+            }
+            val currentHead = rpc.getHead().doFinally {
+                rpc.stop()
+            }
+            EthereumHeadMerge(listOf(currentHead, ws.getFlux())).apply {
                 this.start()
             }
         } else {