Build: Move verifyVersions to new branchConsistency task (#25009)

This commit adds a new `branchConsistency` task which will run in CI
once a day, instead of on every commit. This allows `verifyVersions` to
not break immediately once a new version is released in maven.

Author: rjernst

build.gradle

@@ -76,7 +76,7 @@ for (String line : versionLines) {
     int minor = Integer.parseInt(match.group(2))
     int bugfix = Integer.parseInt(match.group(3))
     boolean unreleased = match.group(4) != null
-    Version foundVersion = new Version(major, minor, bugfix, false, unreleased)
+    Version foundVersion = new Version(major, minor, bugfix, unreleased, unreleased)
     if (currentVersion != foundVersion) {
       versions.add(foundVersion)
     }
@@ -112,6 +112,41 @@ allprojects {
   }
 }
 
+task verifyVersions {
+  doLast {
+    if (gradle.startParameter.isOffline()) {
+      throw new GradleException("Must run in online mode to verify versions")
+    }
+    // Read the list from maven central
+    Node xml
+    new URL('https://repo1.maven.org/maven2/org/elasticsearch/elasticsearch/maven-metadata.xml').openStream().withStream { s ->
+        xml = new XmlParser().parse(s)
+    }
+    Set<Version> knownVersions = new TreeSet<>(xml.versioning.versions.version.collect { it.text() }.findAll { it ==~ /\d\.\d\.\d/ }.collect { Version.fromString(it) })
+
+    // Limit the known versions to those that should be index compatible, and are not future versions
+    knownVersions = knownVersions.findAll { it.major >= 2 && it.before(VersionProperties.elasticsearch) }
+
+    /* Limit the listed versions to those that have been marked as released.
+     * Versions not marked as released don't get the same testing and we want
+     * to make sure that we flip all unreleased versions to released as soon
+     * as possible after release. */
+    Set<Version> actualVersions = new TreeSet<>(indexCompatVersions.findAll { false == it.snapshot })
+
+    // Finally, compare!
+    if (knownVersions.equals(actualVersions) == false) {
+      throw new GradleException("out-of-date released versions\nActual  :" + actualVersions + "\nExpected:" + knownVersions +
+        "\nUpdate Version.java. Note that Version.CURRENT doesn't count because it is not released.")
+    }
+  }
+}
+
+task branchConsistency {
+  description 'Ensures this branch is internally consistent. For example, that versions constants match released versions.'
+  group 'Verification'
+  dependsOn verifyVersions
+}
+
 subprojects {
   project.afterEvaluate {
     // include license and notice in jars

buildSrc/src/main/groovy/org/elasticsearch/gradle/Version.groovy

@@ -19,9 +19,12 @@
 
 package org.elasticsearch.gradle
 
+import groovy.transform.Sortable
+
 /**
  * Encapsulates comparison and printing logic for an x.y.z version.
  */
+@Sortable
 public class Version {
 
     final int major