[Backport 2.0] [Version] Don't spoof major for 3.0+ clusters

server/src/main/java/org/opensearch/Version.java

@@ -279,11 +279,16 @@ public boolean onOrBefore(Version version) {
         return version.id >= id;
     }
 
-    // LegacyESVersion major 7 is equivalent to Version major 1
     public int compareMajor(Version other) {
-        int m = major == 1 ? 7 : major == 2 ? 8 : major;
-        int om = other.major == 1 ? 7 : other.major == 2 ? 8 : other.major;
-        return Integer.compare(m, om);
+        // comparing Legacy 7x for bwc
+        // todo: remove the following when removing legacy support in 3.0.0
+        if (major == 7 || other.major == 7 || major == 6 || other.major == 6) {
+            // opensearch v1.x and v2.x need major translation to compare w/ legacy versions
+            int m = major == 1 ? 7 : major == 2 ? 8 : major;
+            int om = other.major == 1 ? 7 : other.major == 2 ? 8 : other.major;
+            return Integer.compare(m, om);
+        }
+        return Integer.compare(major, other.major);
     }
 
     @Override
@@ -339,12 +344,9 @@ protected Version computeMinCompatVersion() {
         } else if (major == 6) {
             // force the minimum compatibility for version 6 to 5.6 since we don't reference version 5 anymore
             return LegacyESVersion.fromId(5060099);
-        }
-        /*
-         * TODO - uncomment this logic from OpenSearch version 3 onwards
-         *
-        else if (major >= 3) {
+        } else if (major >= 3 && major < 5) {
             // all major versions from 3 onwards are compatible with last minor series of the previous major
+            // todo: remove 5 check when removing LegacyESVersionTests
             Version bwcVersion = null;
 
             for (int i = DeclaredVersionsHolder.DECLARED_VERSIONS.size() - 1; i >= 0; i--) {
@@ -358,7 +360,6 @@ else if (major >= 3) {
             }
             return bwcVersion == null ? this : bwcVersion;
         }
-         */
 
         return Version.min(this, fromId(maskId((int) major * 1000000 + 0 * 10000 + 99)));
     }
@@ -396,6 +397,10 @@ private Version computeMinIndexCompatVersion() {
             bwcMajor = major - 1;
         }
         final int bwcMinor = 0;
+        if (major == 3) {
+            return Version.min(this, fromId((bwcMajor * 1000000 + bwcMinor * 10000 + 99) ^ MASK));
+        }
+        // todo remove below when LegacyESVersion is removed in 3.0
         return Version.min(this, fromId((bwcMajor * 1000000 + bwcMinor * 10000 + 99)));
     }
 
@@ -409,16 +414,15 @@ public boolean isCompatible(Version version) {
         // OpenSearch version 2 is the functional equivalent of predecessor unreleased version "8"
         // todo refactor this logic after removing deprecated features
         int a = major;
-        if (major == 1) {
-            a = 7;
-        } else if (major == 2) {
-            a = 8;
-        }
         int b = version.major;
-        if (version.major == 1) {
-            b = 7;
-        } else if (version.major == 2) {
-            b = 8;
+
+        if (a == 7 || b == 7 || a == 6 || b == 6) {
+            if (major <= 2) {
+                a += 6; // for legacy compatibility up to version 2.x (to compare minCompat)
+            }
+            if (version.major <= 2) {
+                b += 6; // for legacy compatibility up to version 2.x (to compare minCompat)
+            }
         }
 
         assert compatible == false || Math.max(a, b) - Math.min(a, b) <= 1;

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

@@ -63,8 +63,8 @@ public void testVersionComparison() {
 
         // compare opensearch version to LegacyESVersion
         assertThat(Version.V_1_0_0.compareMajor(LegacyESVersion.V_7_0_0), is(0));
-        assertThat(Version.V_1_0_0.compareMajor(LegacyESVersion.fromString("6.3.0")), is(1));
-        assertThat(LegacyESVersion.fromString("6.3.0").compareMajor(Version.V_1_0_0), is(-1));
+        assertThat(Version.V_2_0_0.compareMajor(LegacyESVersion.fromString("7.3.0")), is(1));
+        assertThat(LegacyESVersion.fromString("7.3.0").compareMajor(Version.V_2_0_0), is(-1));
     }
 
     public void testMin() {

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

@@ -245,6 +245,26 @@ public void testOpenSearchMinIndexCompatVersion() {
         assertEquals(expected.revision, actual.revision);
     }
 
+    /** test first version of opensearch compatibility that does not support legacy versions */
+    public void testOpenSearchPreLegacyRemoval() {
+        Version opensearchVersion = Version.fromString("3.0.0");
+        int opensearchMajor = opensearchVersion.major;
+        List<Version> candidates = VersionUtils.allOpenSearchVersions();
+        Version expectedMinIndexCompat = VersionUtils.getFirstVersionOfMajor(candidates, opensearchMajor - 1);
+        Version actualMinIndexCompat = opensearchVersion.minimumIndexCompatibilityVersion();
+
+        Version expectedMinCompat = VersionUtils.lastFirstReleasedMinorFromMajor(VersionUtils.allOpenSearchVersions(), opensearchMajor - 1);
+        Version actualMinCompat = opensearchVersion.minimumCompatibilityVersion();
+        // since some legacy versions still support build (alpha, beta, RC) we check major minor revision only
+        assertEquals(expectedMinIndexCompat.major, actualMinIndexCompat.major);
+        assertEquals(expectedMinIndexCompat.minor, actualMinIndexCompat.minor);
+        assertEquals(expectedMinIndexCompat.revision, actualMinIndexCompat.revision);
+
+        assertEquals(expectedMinCompat.major, actualMinCompat.major);
+        assertEquals(expectedMinCompat.minor, actualMinCompat.minor);
+        assertEquals(expectedMinCompat.revision, actualMinCompat.revision);
+    }
+
     public void testToString() {
         assertEquals("2.0.0-beta1", Version.fromString("2.0.0-beta1").toString());
         assertEquals("5.0.0-alpha1", Version.fromId(5000001).toString());