Add "random" min compatibility testing for OpenSearch versions

Signed-off-by: Nicholas Walter Knize <nknize@apache.org>

server/src/test/java/org/opensearch/VersionTests.java

@@ -211,6 +211,23 @@ public void testMinCompatVersion() {
         assertEquals(0, LegacyESVersion.V_7_0_0.minimumCompatibilityVersion().revision);
     }
 
+    public void testOpenSearchMinCompatVersion() {
+        Version opensearchVersion = VersionUtils.randomOpenSearchVersion(random());
+        // opensearch 1.x minCompat is Legacy 6.8.0
+        // opensearch 2.x minCompat is Legacy 7.10.0
+        // opensearch 3.x minCompat is 1.{last minor version}.0
+        // until 3.0 is staged the following line will only return legacy versions
+        List<Version> candidates = opensearchVersion.major >= 3 ? VersionUtils.allOpenSearchVersions() : VersionUtils.allLegacyVersions();
+        int opensearchMajor = opensearchVersion.major;
+        int major = opensearchMajor - 1;
+        if (opensearchMajor == 1) {
+            major = 7;
+        } else if (opensearchMajor == 2) {
+            major = 8;
+        }
+        assertEquals(VersionUtils.lastFirstReleasedMinorFromMajor(candidates, major - 1), opensearchVersion.computeMinCompatVersion());
+    }
+
     public void testToString() {
         // with 2.0.beta we lowercase
         assertEquals("2.0.0-beta1", LegacyESVersion.fromString("2.0.0-beta1").toString());

test/framework/src/main/java/org/opensearch/test/VersionUtils.java

@@ -39,6 +39,7 @@
 
 import java.util.ArrayList;
 import java.util.Collections;
+import java.util.Comparator;
 import java.util.List;
 import java.util.Map;
 import java.util.Optional;
@@ -149,6 +150,8 @@ private static Version moveLastToUnreleased(List<List<Version>> versions, List<V
     private static final List<Version> RELEASED_VERSIONS;
     private static final List<Version> UNRELEASED_VERSIONS;
     private static final List<Version> ALL_VERSIONS;
+    private static final List<Version> ALL_OPENSEARCH_VERSIONS;
+    private static final List<Version> ALL_LEGACY_VERSIONS;
 
     static {
         Tuple<List<Version>, List<Version>> versions = resolveReleasedVersions(Version.CURRENT, LegacyESVersion.class);
@@ -159,6 +162,9 @@ private static Version moveLastToUnreleased(List<List<Version>> versions, List<V
         allVersions.addAll(UNRELEASED_VERSIONS);
         Collections.sort(allVersions);
         ALL_VERSIONS = Collections.unmodifiableList(allVersions);
+        // @todo remove this when legacy support is no longer needed
+        ALL_OPENSEARCH_VERSIONS = ALL_VERSIONS.stream().filter(v -> v.major < 6).collect(Collectors.toList());
+        ALL_LEGACY_VERSIONS = ALL_VERSIONS.stream().filter(v -> v.major >= 6).collect(Collectors.toList());
     }
 
     /**
@@ -182,6 +188,16 @@ public static List<Version> allVersions() {
         return ALL_VERSIONS;
     }
 
+    /** Returns an immutable, sorted list containing all opensearch versions; released and unreleased */
+    public static List<Version> allOpenSearchVersions() {
+        return ALL_OPENSEARCH_VERSIONS;
+    }
+
+    /** Returns an immutable, sorted list containing all legacy versions; released and unreleased */
+    public static List<Version> allLegacyVersions() {
+        return ALL_LEGACY_VERSIONS;
+    }
+
     /**
      * Get the released version before {@code version}.
      */
@@ -229,6 +245,25 @@ public static Version randomVersion(Random random) {
         return ALL_VERSIONS.get(random.nextInt(ALL_VERSIONS.size()));
     }
 
+    /**
+     * Return a random {@link Version} from all available opensearch versions.
+     **/
+    public static Version randomOpenSearchVersion(Random random) {
+        return ALL_OPENSEARCH_VERSIONS.get(random.nextInt(ALL_OPENSEARCH_VERSIONS.size()));
+    }
+
+    /** Returns the first released (e.g., patch version 0) {@link Version} of the last minor from the requested major version
+     *  e.g., for version 1.0.0 this would be legacy version (7.10.0); the first release (patch 0), of the last
+     *  minor (for 7.x that is minor version 10) for the desired major version (7)
+     **/
+    public static Version lastFirstReleasedMinorFromMajor(List<Version> allVersions, int major) {
+        Map<Integer, List<Version>> majorVersions = allVersions.stream().collect(Collectors.groupingBy(v -> (int)v.major));
+        Map<Integer, List<Version>> groupedByMinor = majorVersions.get(major).stream().collect(
+            Collectors.groupingBy(v -> (int)v.minor));
+        List<Version> candidates = Collections.max(groupedByMinor.entrySet(), Comparator.comparing(Map.Entry::getKey)).getValue();
+        return candidates.get(0);
+    }
+
     /** Returns a random {@link Version} from all available versions, that is compatible with the given version. */
     public static Version randomCompatibleVersion(Random random, Version version) {
         final List<Version> compatible = ALL_VERSIONS.stream().filter(version::isCompatible).collect(Collectors.toList());