Adds a :distribution:bwc-stable project

Adds the `:distribution:bwc-stable` project which builds the previous
stable branch when `:distribution:bwc` is building an unreleased
branch. When `:distribution:bwc` builds a released branch then
`:distribution:bwc-stable` is an empty build, not used or depended
on by anything.

Relates to elastic#24798

build.gradle

@@ -99,7 +99,11 @@ if (currentVersion.bugfix == 0) {
   // and the bwc distribution will checkout and build that version.
   Version last = versions[-1]
   versions[-1] = new Version(last.major, last.minor, last.bugfix,
-      true, last.unreleased)
+    true, last.unreleased)
+  if (last.bugfix == 0) {
+    versions[-2] = new Version(
+      versions[-2].major, versions[-2].minor, versions[-2].bugfix, true, last.unreleased)
+  }
 }
 
 // injecting groovy property variables into all projects
@@ -171,12 +175,20 @@ subprojects {
     "org.elasticsearch.plugin:aggs-matrix-stats-client:${version}": ':modules:aggs-matrix-stats',
     "org.elasticsearch.plugin:percolator-client:${version}": ':modules:percolator',
   ]
-  if (wireCompatVersions[-1].snapshot) {
+  if (indexCompatVersions[-1].snapshot) {
     // if the most previous version is a snapshot, we need to connect that version to the
     // bwc project which will checkout and build that snapshot version
-    ext.projectSubstitutions["org.elasticsearch.distribution.deb:elasticsearch:${wireCompatVersions[-1]}"] = ':distribution:bwc'
-    ext.projectSubstitutions["org.elasticsearch.distribution.rpm:elasticsearch:${wireCompatVersions[-1]}"] = ':distribution:bwc'
-    ext.projectSubstitutions["org.elasticsearch.distribution.zip:elasticsearch:${wireCompatVersions[-1]}"] = ':distribution:bwc'
+    ext.projectSubstitutions["org.elasticsearch.distribution.deb:elasticsearch:${indexCompatVersions[-1]}"] = ':distribution:bwc'
+    ext.projectSubstitutions["org.elasticsearch.distribution.rpm:elasticsearch:${indexCompatVersions[-1]}"] = ':distribution:bwc'
+    ext.projectSubstitutions["org.elasticsearch.distribution.zip:elasticsearch:${indexCompatVersions[-1]}"] = ':distribution:bwc'
+    if (indexCompatVersions[-2].snapshot) {
+      /* if the version before the previous version is a snapshot, we need to
+       * connect that version to the bwc project which will checkout and build
+       * that snapshot version */
+      ext.projectSubstitutions["org.elasticsearch.distribution.deb:elasticsearch:${indexCompatVersions[-2]}"] = ':distribution:bwc-stable'
+      ext.projectSubstitutions["org.elasticsearch.distribution.rpm:elasticsearch:${indexCompatVersions[-2]}"] = ':distribution:bwc-stable'
+      ext.projectSubstitutions["org.elasticsearch.distribution.zip:elasticsearch:${indexCompatVersions[-2]}"] = ':distribution:bwc-stable'
+    }
   }
   project.afterEvaluate {
     configurations.all {

distribution/build.gradle

@@ -41,7 +41,7 @@ buildscript {
 }
 
 Collection distributions = project.subprojects.findAll {
-   it.path.contains(':tools') == false && it.name != 'bwc' }
+   it.path.contains(':tools') == false && it.path.contains(':bwc') == false }
 
 /*****************************************************************************
  *                                Notice file                                *

distribution/bwc/build.gradle

@@ -27,13 +27,16 @@ import org.elasticsearch.gradle.LoggedExec
  * without relying on snapshots.
  */
 
-String bwcVersion = wireCompatVersions[-1]
+int bwcVersionIndex = project.name == 'bwc' ? -1 : -2
+String bwcVersion = indexCompatVersions[bwcVersionIndex]
 if (bwcVersion.endsWith('-SNAPSHOT')) {
   apply plugin: 'distribution'
 
   def (String major, String minor, String bugfix) = bwcVersion.split('\\.')
-  String bwcBranch = bugfix == '0-SNAPSHOT' ? "${major}.x" : "${major}.${minor}"
+  String bwcBranch = bwcVersionIndex == -1 && bugfix == '0-SNAPSHOT' ?
+      "${major}.x" : "${major}.${minor}"
   File checkoutDir = file("${buildDir}/bwc/checkout-${bwcBranch}")
+
   task createClone(type: LoggedExec) {
     onlyIf { checkoutDir.exists() == false }
     commandLine = ['git', 'clone', rootDir, checkoutDir]

settings.gradle

@@ -16,6 +16,7 @@ List projects = [
   'benchmarks',
   'distribution:integ-test-zip',
   'distribution:bwc',
+  'distribution:bwc-stable',
   'distribution:zip',
   'distribution:tar',
   'distribution:deb',
@@ -104,6 +105,10 @@ for (String example : examplePlugins) {
   project(":example-plugins:${example}").projectDir = new File(rootProject.projectDir, "plugins/examples/${example}")
 }
 
+/* bwc and bwc-unreleased share the same build directory and build file, but
+ * apply to different backwards compatibility branches. */
+project(':distribution:bwc-stable').projectDir = project(':distribution:bwc').projectDir
+
 if (isEclipse) {
   project(":core").projectDir = new File(rootProject.projectDir, 'core/src/main')
   project(":core").buildFileName = 'eclipse-build.gradle'