Make current Version pluggable for serverless (elastic#96539)

Version.CURRENT is statically loaded as a constant early during startup.
Yet serverless needs to be able to override the current Version so it
can add additional versions. This commit makes Version.CURRENT pluggable
via SPI. Note that the only way for this to be plugged in is via an
additional jar on the boot layer.

Author: rjernst

server/src/main/java/module-info.java

@@ -277,6 +277,7 @@
     exports org.elasticsearch.indices.recovery.plan;
     exports org.elasticsearch.indices.store;
     exports org.elasticsearch.ingest;
+    exports org.elasticsearch.internal to org.elasticsearch.serverless.version;
     exports org.elasticsearch.lucene.analysis.miscellaneous;
     exports org.elasticsearch.lucene.grouping;
     exports org.elasticsearch.lucene.queries;
@@ -382,6 +383,7 @@
     uses org.elasticsearch.reservedstate.ReservedClusterStateHandlerProvider;
     uses org.elasticsearch.jdk.ModuleQualifiedExportsService;
     uses org.elasticsearch.node.internal.TerminationHandlerProvider;
+    uses org.elasticsearch.internal.VersionExtension;
 
     provides org.apache.lucene.codecs.PostingsFormat
         with

server/src/main/java/org/elasticsearch/TransportVersion.java

@@ -12,6 +12,7 @@
 import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.common.io.stream.StreamOutput;
 import org.elasticsearch.core.Assertions;
+import org.elasticsearch.internal.VersionExtension;
 
 import java.io.IOException;
 import java.lang.reflect.Field;
@@ -136,7 +137,7 @@ private static TransportVersion registerTransportVersion(int id, String uniqueId
      * Reference to the most recent transport version.
      * This should be the transport version with the highest id.
      */
-    public static final TransportVersion CURRENT = V_8_500_008;
+    public static final TransportVersion CURRENT = findCurrent(V_8_500_008);
 
     /**
      * Reference to the earliest compatible transport version to this version of the codebase.
@@ -264,6 +265,15 @@ public static TransportVersion fromString(String str) {
         return TransportVersion.fromId(Integer.parseInt(str));
     }
 
+    // finds the pluggable current version, or uses the given fallback
+    private static TransportVersion findCurrent(TransportVersion fallback) {
+        var versionExtension = VersionExtension.load();
+        if (versionExtension == null) {
+            return fallback;
+        }
+        return new TransportVersion(versionExtension.getCurrentTransportVersionId());
+    }
+
     @Override
     public int compareTo(TransportVersion other) {
         return Integer.compare(this.id, other.id);

server/src/main/java/org/elasticsearch/internal/VersionExtension.java

@@ -0,0 +1,37 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0 and the Server Side Public License, v 1; you may not use this file except
+ * in compliance with, at your election, the Elastic License 2.0 or the Server
+ * Side Public License, v 1.
+ */
+
+package org.elasticsearch.internal;
+
+import java.util.ServiceLoader;
+
+/**
+ * Allows plugging in current version elements.
+ */
+public interface VersionExtension {
+    /**
+     * Returns the version id of the current transport version.
+     * Note this cannot return a TransportVersion object because it needs to be
+     * called during static initialization of TransportVersion.
+     */
+    int getCurrentTransportVersionId();
+
+    /**
+     * Loads a single VersionExtension, or returns {@code null} if none are found.
+     */
+    static VersionExtension load() {
+        var loader = ServiceLoader.load(VersionExtension.class);
+        var extensions = loader.stream().toList();
+        if (extensions.size() > 1) {
+            throw new IllegalStateException("More than one version extension found");
+        } else if (extensions.size() == 0) {
+            return null;
+        }
+        return extensions.get(0).get();
+    }
+}