Ignore $ref keyword referencing HTTP resources (close #238)

modules/core/src/main/scala/com.snowplowanalytics.iglu/client/validator/CirceValidator.scala

@@ -19,7 +19,11 @@ import com.snowplowanalytics.iglu.client.resolver.StorageTime
 import com.snowplowanalytics.iglu.core.circe.MetaSchemas
 // Scala
 import com.fasterxml.jackson.databind.JsonNode
+import com.networknt.schema.uri.URIFetcher
 import com.snowplowanalytics.iglu.client.resolver.Resolver.SchemaLookupResult
+import java.io.{ByteArrayInputStream, InputStream}
+import java.net.URI
+import java.nio.charset.StandardCharsets
 import scala.jdk.CollectionConverters._
 
 // Cats
@@ -55,12 +59,21 @@ object CirceValidator extends Validator[Json] {
 
   private val V4SchemaInstance = JsonSchemaFactory.getInstance(SpecVersion.VersionFlag.V4)
 
+  private val fakeUrlFetcher = new URIFetcher {
+    override def fetch(uri: URI): InputStream = {
+      // No effect on validation, because we return empty JSON Schema which matches any data.
+      val emptyJsonObject = Json.obj()
+      new ByteArrayInputStream(emptyJsonObject.toString().getBytes(StandardCharsets.UTF_8))
+    }
+  }
+
   private val IgluMetaschemaFactory =
     JsonSchemaFactory
       .builder(V4SchemaInstance)
       .addMetaSchema(IgluMetaschema)
       .forceHttps(false)
       .removeEmptyFragmentSuffix(false)
+      .uriFetcher(fakeUrlFetcher, "http", "https")
       .build()
 
   private val SchemaValidatorsConfig: SchemaValidatorsConfig = {

modules/core/src/test/scala/com.snowplowanalytics.iglu.client/validator/ValidatingWithRefSpec.scala

@@ -0,0 +1,36 @@
+package com.snowplowanalytics.iglu.client.validator
+
+import io.circe.literal._
+import org.specs2.mutable.Specification
+
+class ValidatingWithRefSpec extends Specification {
+
+  val schema =
+    json"""
+         {
+           "$$schema": "http://iglucentral.com/schemas/com.snowplowanalytics.self-desc/schema/jsonschema/1-0-0#",
+           "description": "Test schema using $$ref with 'http'/'https' protocols",
+           "self": {
+             "vendor": "com.test",
+             "name": "test",
+             "format": "jsonschema",
+             "version": "1-0-0"
+           },
+           "properties": {
+             "id": {
+               "allOf": [
+                 {"$$ref": "http://json-schema.org/draft-04/schema#"},
+                 {"$$ref": "https://json-schema.org/draft-04/schema"},
+                 {"$$ref": "http://anything"},
+                 {"$$ref": "https://anything"}
+               ]
+             }
+           }
+         } 
+      """
+
+  "Validator should ignore '$ref' keyword" in {
+    val data = json"""{"id": "can_be_anything1234"}"""
+    CirceValidator.validate(data, schema) must beRight(())
+  }
+}