Test query builder bwc against previous supported versions instead of…

… just the current version.

Relates to elastic#25456

qa/query-builder-bwc/build.gradle

@@ -30,12 +30,7 @@ task bwcTest {
     group = 'verification'
 }
 
-// For now test against the current version:
-Version currentVersion = Version.fromString(VersionProperties.elasticsearch.minus('-SNAPSHOT'))
-Version[] versions = [currentVersion]
-// TODO: uncomment when there is a released version with: https://github.com/elastic/elasticsearch/pull/25456
-// versions = indexCompatVersions
-for (Version version : versions) {
+for (Version version : indexCompatVersions) {
     String baseName = "v${version}"
 
     Task oldQueryBuilderTest = tasks.create(name: "${baseName}#oldQueryBuilderTest", type: RestIntegTestTask) {
@@ -48,9 +43,8 @@ for (Version version : versions) {
 
     configure(extensions.findByName("${baseName}#oldQueryBuilderTestCluster")) {
         distribution = 'zip'
-        // TODO: uncomment when there is a released version with: https://github.com/elastic/elasticsearch/pull/25456
-        // bwcVersion = version
-        // numBwcNodes = 1
+        bwcVersion = version
+        numBwcNodes = 1
         numNodes = 1
         clusterName = 'query_builder_bwc'
         setting 'http.content_type.required', 'true'
@@ -89,7 +83,7 @@ test.enabled = false // no unit tests for rolling upgrades, only the rest integr
 // basic integ tests includes testing bwc against the most recent version
 task integTest {
     if (project.bwc_tests_enabled) {
-        dependsOn = ["v${versions[-1]}#bwcTest"]
+        dependsOn = ["v${indexCompatVersions[-1]}#bwcTest"]
     }
 }
 

qa/query-builder-bwc/src/test/java/org/elasticsearch/bwc/QueryBuilderBWCIT.java

@@ -61,6 +61,16 @@
 
 import static org.elasticsearch.common.xcontent.XContentFactory.jsonBuilder;
 
+/**
+ * An integration test that tests whether percolator queries stored in older supported ES version can still be read by the
+ * current ES version. Percolator queries are stored in the binary format in a dedicated doc values field (see
+ * PercolatorFieldMapper#createQueryBuilderField(...) method). Using the query builders writable contract. This test
+ * does best effort verifying that we don't break bwc for query builders between the first previous major version and
+ * the latest current major release.
+ *
+ * The queries to test are specified in json format, which turns out to work because we tend break here rarely. If the
+ * json format of a query being tested here then feel free to change this.
+ */
 public class QueryBuilderBWCIT extends ESRestTestCase {
 
     private static final List<Object[]> CANDIDATES = new ArrayList<>();