problem: doesn't work for private ethereum networks

solution: allow configuring methods with static responses
rel: #96

docs/04-upstream-config.adoc

@@ -218,6 +218,27 @@ NOTE: It's especially useful when used together with upstream labels.If an archi
 possible to specify that the client wants to execute method `trace_transaction` only on an archive node(s), which has
 complete historical data for tracing.
 
+=== Static Methods
+
+You can overwrite existing methods or add new ones using a static response:
+
+[source, yaml]
+----
+upstreams:
+  - id: my-node
+    chain: ethereum
+    methods:
+      enabled:
+        - name: net_version
+          static: "\"100000\""
+        - name: eth_chainId
+          static: "0x186a0"
+        - name: eth_custom_array
+          static: '["custom_array_response"]'
+        - name: eth_custom_bool
+          static: "false"
+----
+
 === Authentication
 
 ==== TLS

src/main/kotlin/io/emeraldpay/dshackle/config/UpstreamsConfig.kt

@@ -173,6 +173,7 @@ open class UpstreamsConfig {
 
     class Method(
         val name: String,
-        val quorum: String? = null
+        val quorum: String? = null,
+        val static: String? = null
     )
 }

src/main/kotlin/io/emeraldpay/dshackle/config/UpstreamsConfigReader.kt

@@ -249,7 +249,8 @@ class UpstreamsConfigReader(
                 getValueAsString(m, "name")?.let { name ->
                     UpstreamsConfig.Method(
                         name = name,
-                        quorum = getValueAsString(m, "quorum")
+                        quorum = getValueAsString(m, "quorum"),
+                        static = getValueAsString(m, "static")
                     )
                 }
             }?.filterNotNull()?.toSet() ?: emptySet()

src/main/kotlin/io/emeraldpay/dshackle/startup/ConfiguredUpstreams.kt

@@ -123,6 +123,9 @@ open class ConfiguredUpstreams(
                     if (m.quorum != null) {
                         it.setQuorum(m.name, m.quorum)
                     }
+                    if (m.static != null) {
+                        it.setStaticResponse(m.name, m.static)
+                    }
                 }
             }
         } else {

src/main/kotlin/io/emeraldpay/dshackle/upstream/calls/ManagedCallMethods.kt

@@ -16,11 +16,13 @@
  */
 package io.emeraldpay.dshackle.upstream.calls
 
+import com.fasterxml.jackson.databind.ObjectMapper
 import io.emeraldpay.dshackle.quorum.AlwaysQuorum
 import io.emeraldpay.dshackle.quorum.CallQuorum
 import io.emeraldpay.dshackle.quorum.NonEmptyQuorum
 import io.emeraldpay.dshackle.quorum.NotLaggingQuorum
 import org.slf4j.LoggerFactory
+import java.io.IOException
 import java.util.Collections
 
 /**
@@ -44,6 +46,7 @@ class ManagedCallMethods(
         enabled + delegated - disabled
     )
     private val quorum: MutableMap<String, CallQuorum> = HashMap()
+    private val staticResponse: MutableMap<String, String> = HashMap()
 
     init {
         enabled.forEach { m ->
@@ -64,6 +67,10 @@ class ManagedCallMethods(
         this.quorum[method] = quorum
     }
 
+    fun setStaticResponse(method: String, response: String) {
+        this.staticResponse[method] = response
+    }
+
     override fun getQuorumFor(method: String): CallQuorum {
         return when {
             Collections.binarySearch(delegated, method) >= 0 -> delegate.getQuorumFor(method)
@@ -84,10 +91,22 @@ class ManagedCallMethods(
     }
 
     override fun isHardcoded(method: String): Boolean {
-        return delegate.isHardcoded(method)
+        return this.staticResponse.containsKey(method) || delegate.isHardcoded(method)
     }
 
     override fun executeHardcoded(method: String): ByteArray {
+        if (this.staticResponse.containsKey(method)) {
+            var json: String = this.staticResponse[method].orEmpty()
+            // Check if it's valid JSON
+            val mapper = ObjectMapper()
+            try {
+                mapper.readTree(json)
+            } catch (e: IOException) {
+                // Encode and default to string
+                json = mapper.writeValueAsString(json)
+            }
+            return json.toByteArray()
+        }
         return delegate.executeHardcoded(method)
     }
 }

src/test/groovy/io/emeraldpay/dshackle/startup/ConfiguredUpstreamsSpec.groovy

@@ -22,8 +22,8 @@ class ConfiguredUpstreamsSpec extends Specification {
         )
         def methods = new UpstreamsConfig.Methods(
                 [
-                        new UpstreamsConfig.Method("foo_bar", null),
+                        new UpstreamsConfig.Method("foo_bar", null, null),
-                        new UpstreamsConfig.Method("foo_bar", "not_empty")
+                        new UpstreamsConfig.Method("foo_bar", "not_empty", null)
                 ] as Set,
                 [] as Set
         )
@@ -35,4 +35,27 @@ class ConfiguredUpstreamsSpec extends Specification {
         act instanceof ManagedCallMethods
         act.getQuorumFor("foo_bar") instanceof NonEmptyQuorum
     }
+
+    def "Got static response from extra methods"() {
+        setup:
+        def currentUpstreams = Mock(CurrentMultistreamHolder) {
+            _ * getDefaultMethods(Chain.ETHEREUM) >> new DefaultEthereumMethods(Chain.ETHEREUM)
+        }
+        def configurer = new ConfiguredUpstreams(
+                currentUpstreams, Stub(FileResolver), Stub(UpstreamsConfig), Stub(CachesFactory)
+        )
+        def methods = new UpstreamsConfig.Methods(
+                [
+                        new UpstreamsConfig.Method("foo_bar", null, "static_response")
+                ] as Set,
+                [] as Set
+        )
+        def upstream = new UpstreamsConfig.Upstream()
+        upstream.methods = methods
+        when:
+        def act = configurer.buildMethods(upstream, Chain.ETHEREUM)
+        then:
+        act instanceof ManagedCallMethods
+        new String(act.executeHardcoded("foo_bar")) == "\"static_response\""
+    }
 }