Lazily initialize current transport version (#96852)

The CURRENT transport version may be plugged in via SPI. Unfortunately
since it is a static constant, this requires loading the SPI during
clinit of TransportVersion, which precludes the SPI classes from
actually referring to TransportVersion (or say, having their own
TransportVersion constant) without causing an initalization deadlock.

This commit delays initalization of CURRENT by moving it into a static
inner class. Since the visibility is private, and the only use of the
class is within the current() method, initialization of that class
should be delayed until the first call to current().

Author: rjernst

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

@@ -53,7 +53,7 @@
  * Each transport version should only be used in a single merged commit (apart from BwC versions copied from {@link Version}).
  * <p>
  * To add a new transport version, add a new constant at the bottom of the list that is one greater than the current highest version,
- * ensure it has a unique id, and update the {@link #CURRENT} constant to point to the new version.
+ * ensure it has a unique id, and update the {@link CurrentHolder#CURRENT} constant to point to the new version.
  * <h2>Reverting a transport version</h2>
  * If you revert a commit with a transport version change, you <em>must</em> ensure there is a <em>new</em> transport version
  * representing the reverted change. <em>Do not</em> let the transport version go backwards, it must <em>always</em> be incremented.
@@ -136,7 +136,18 @@ private static TransportVersion registerTransportVersion(int id, String uniqueId
     public static final TransportVersion V_8_500_010 = registerTransportVersion(8_500_010, "9818C628-1EEC-439B-B943-468F61460675");
     public static final TransportVersion V_8_500_011 = registerTransportVersion(8_500_011, "2209F28D-B52E-4BC4-9889-E780F291C32E");
 
-    private static final TransportVersion CURRENT = findCurrent(V_8_500_011);
+    private static class CurrentHolder {
+        private static final TransportVersion CURRENT = findCurrent(V_8_500_011);
+
+        // 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());
+        }
+    }
 
     /**
      * Reference to the earliest compatible transport version to this version of the codebase.
@@ -244,7 +255,7 @@ public static boolean isCompatible(TransportVersion version) {
      * This should be the transport version with the highest id.
      */
     public static TransportVersion current() {
-        return CURRENT;
+        return CurrentHolder.CURRENT;
     }
 
     public boolean after(TransportVersion version) {
@@ -272,15 +283,6 @@ 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);