Generalize BWC logic (#28505)

Generalizing BWC building so that there is less code to modify for a release. This ensures we do not
need to think about what major or minor version is in the gradle code. It follows the general rules of the
elastic release structure. For more information on the rules, see the VersionCollection's javadoc.

This also removes the additional bwc snapshots that will never be released, such as 6.0.2, which were
being built and tested against every time we ran bwc tests.

Additionally, it creates 4 new projects that correspond to the different types of snapshots that may exist
for a given version. Its possible to now run those individual tasks to work out bwc logic whereas
previously it was impossible and the entire suite of bwc tests had to be run to work out any logic
changes in the build tools' bwc project. Please note that if the project does not make sense for the 
version that is current, that an error will be thrown from that individual project if an attempt is made to 
run it.

This should allow for automating the version bumps as well, since it removes all the hardcoded version
logic from the configs.

build.gradle

@@ -103,7 +103,7 @@ allprojects {
     isIdea = System.getProperty("idea.active") != null || gradle.startParameter.taskNames.contains('idea') || gradle.startParameter.taskNames.contains('cleanIdea')
 
     // for BWC testing
-    versionCollection = versions
+    bwcVersions = versions
 
     buildMetadata = buildMetadataMap
   }
@@ -122,13 +122,13 @@ task verifyVersions {
     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 >= versions.currentVersion.major - 1 && it.before(VersionProperties.elasticsearch) }
+    knownVersions = knownVersions.findAll { it.major >= bwcVersions.currentVersion.major - 1 && 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<>(versions.versionsIndexCompatibleWithCurrent.findAll { false == it.snapshot })
+    Set<Version> actualVersions = new TreeSet<>(bwcVersions.indexCompatible.findAll { false == it.snapshot })
 
     // Finally, compare!
     if (knownVersions.equals(actualVersions) == false) {
@@ -219,13 +219,14 @@ subprojects {
     "org.elasticsearch.plugin:rank-eval-client:${version}": ':modules:rank-eval',
   ]
 
-  for (final Version version : versionCollection.versionsIndexCompatibleWithCurrent) {
-    if (version.branch != null) {
-      final String snapshotProject = ":distribution:bwc-snapshot-${version.branch}"
-      project(snapshotProject).ext.bwcVersion = version
-      ext.projectSubstitutions["org.elasticsearch.distribution.deb:elasticsearch:${version}"] = snapshotProject
-      ext.projectSubstitutions["org.elasticsearch.distribution.rpm:elasticsearch:${version}"] = snapshotProject
-      ext.projectSubstitutions["org.elasticsearch.distribution.zip:elasticsearch:${version}"] = snapshotProject
+  bwcVersions.snapshotProjectNames.each { snapshotName ->
+    Version snapshot = bwcVersions.getSnapshotForProject(snapshotName)
+    if (snapshot != null ) {
+      String snapshotProject = ":distribution:bwc:${snapshotName}"
+      project(snapshotProject).ext.bwcVersion = snapshot
+      ext.projectSubstitutions["org.elasticsearch.distribution.deb:elasticsearch:${snapshot}"] = snapshotProject
+      ext.projectSubstitutions["org.elasticsearch.distribution.rpm:elasticsearch:${snapshot}"] = snapshotProject
+      ext.projectSubstitutions["org.elasticsearch.distribution.zip:elasticsearch:${snapshot}"] = snapshotProject
     }
   }
 

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

@@ -34,24 +34,29 @@ public class Version {
     final int revision
     final int id
     final boolean snapshot
-    final String branch
     /**
-     * Suffix on the version name. Unlike Version.java the build does not
-     * consider alphas and betas different versions, it just preserves the
-     * suffix that the version was declared with in Version.java.
+     * Suffix on the version name.
      */
     final String suffix
 
     public Version(int major, int minor, int revision,
-            String suffix, boolean snapshot, String branch) {
+            String suffix, boolean snapshot) {
         this.major = major
         this.minor = minor
         this.revision = revision
         this.snapshot = snapshot
         this.suffix = suffix
-        this.branch = branch
-        this.id = major * 100000 + minor * 1000 + revision * 10 +
-            (snapshot ? 1 : 0)
+
+        int suffixOffset = 0
+        if (suffix.contains("alpha")) {
+          suffixOffset += Integer.parseInt(suffix.substring(6))
+        } else if (suffix.contains("beta")) {
+          suffixOffset += 25 + Integer.parseInt(suffix.substring(5))
+        } else if (suffix.contains("rc")) {
+          suffixOffset += 50 + Integer.parseInt(suffix.substring(3));
+        }
+
+        this.id = major * 1000000 + minor * 10000 + revision * 100 + suffixOffset
     }
 
     public static Version fromString(String s) {
@@ -60,7 +65,7 @@ public class Version {
             throw new InvalidUserDataException("Invalid version [${s}]")
         }
         return new Version(m.group(1) as int, m.group(2) as int,
-            m.group(3) as int, m.group(4) ?: '', m.group(5) != null, null)
+            m.group(3) as int, m.group(4) ?: '', m.group(5) != null)
     }
 
     @Override