Rename initial to unreferenced in transport versions

The concept of "initial" transport version is broader than just the
initial ids created for releases. There are several cases where ids were
skipped for various reasons. In order to ensure validation can still
work (eg density of base ids), this commit broadens the concept to a
place we can define any unreferenced transport version.

Author: rjernst

build-tools-internal/src/integTest/groovy/org/elasticsearch/gradle/internal/transport/TransportVersionManagementPluginFuncTest.groovy

@@ -46,8 +46,8 @@ class TransportVersionManagementPluginFuncTest extends AbstractGradleFuncTest {
         javaResource("myserver", "transport/definitions/named/" + name + ".csv", ids)
     }
 
-    def initialTransportVersion(String name, String id) {
-        javaResource("myserver", "transport/definitions/initial/" + name + ".csv", id)
+    def unreferencedTransportVersion(String name, String id) {
+        javaResource("myserver", "transport/definitions/unreferenced/" + name + ".csv", id)
     }
 
     def definedAndUsedTransportVersion(String name, String ids) {
@@ -101,7 +101,7 @@ class TransportVersionManagementPluginFuncTest extends AbstractGradleFuncTest {
         """
         namedTransportVersion("existing_91", "8012000")
         namedTransportVersion("existing_92", "8123000,8012001")
-        initialTransportVersion("initial_9_0_0", "8000000")
+        unreferencedTransportVersion("initial_9_0_0", "8000000")
         latestTransportVersion("9.2", "existing_92", "8123000")
         latestTransportVersion("9.1", "existing_92", "8012001")
         // a mock version of TransportVersion, just here so we can compile Dummy.java et al
@@ -303,4 +303,14 @@ class TransportVersionManagementPluginFuncTest extends AbstractGradleFuncTest {
         assertDefinitionsFailure(result, "Transport version definition file " +
             "[myserver/src/main/resources/transport/definitions/named/patch.csv] has patch version 8015001 as primary id")
     }
+
+    def "unreferenced directory is optional"() {
+        given:
+        file("myserver/src/main/resources/transport/unreferenced/initial_9_0_0.csv").delete()
+        file("myserver/src/main/resources/transport/unreferenced").deleteDir()
+        when:
+        def result = gradleRunner(":myserver:validateTransportVersionDefinitions").build()
+        then:
+        result.task(":myserver:validateTransportVersionDefinitions").outcome == TaskOutcome.SUCCESS
+    }
 }

build-tools-internal/src/main/java/org/elasticsearch/gradle/internal/transport/ValidateTransportVersionResourcesTask.java

@@ -107,10 +107,13 @@ public void validateTransportVersions() throws IOException {
         // now load all definitions, do some validation and record them by various keys for later quick lookup
         // NOTE: this must run after loading referenced names and existing definitions
         // NOTE: this is sorted so that the order of cross validation is deterministic
-        for (String subDir : List.of("initial", "named")) {
-            try (var definitionsStream = Files.list(definitionsDir.resolve(subDir)).sorted()) {
-                for (var definitionFile : definitionsStream.toList()) {
-                    recordAndValidateDefinition(readDefinitionFile(definitionFile));
+        for (String subDirName : List.of("unreferenced", "named")) {
+            Path subDir = definitionsDir.resolve(subDirName);
+            if (Files.isDirectory(subDir)) {
+                try (var definitionsStream = Files.list(subDir).sorted()) {
+                    for (var definitionFile : definitionsStream.toList()) {
+                        recordAndValidateDefinition(readDefinitionFile(definitionFile));
+                    }
                 }
             }
         }