Tighten version compatibility test

This commit tightens the version compatibility test, generalizing it
beyond a hard-coded 7.0.0.

Author: jasontedor

core/src/test/java/org/elasticsearch/VersionTests.java

@@ -36,6 +36,7 @@
 import java.util.Locale;
 import java.util.Map;
 import java.util.Set;
+import java.util.stream.Collectors;
 
 import static org.elasticsearch.Version.V_5_3_0;
 import static org.elasticsearch.Version.V_6_0_0_beta1;
@@ -340,11 +341,38 @@ public void testIsCompatible() {
         assertFalse(isCompatible(Version.fromId(2000099), Version.V_5_0_0));
         assertFalse(isCompatible(Version.fromString("6.0.0"), Version.fromString("7.0.0")));
         assertFalse(isCompatible(Version.fromString("6.0.0-alpha1"), Version.fromString("7.0.0")));
-        final boolean buildSnapshot = Booleans.parseBoolean(System.getProperty("build.snapshot", "true"));
+
+        final Version currentMajorVersion = Version.fromId(Version.CURRENT.major * 1000000 + 99);
+        final Version currentOrNextMajorVersion;
+        if (Version.CURRENT.after(currentMajorVersion)) {
+            currentOrNextMajorVersion = Version.fromId((Version.CURRENT.major + 1) * 1000000 + 99);
+        } else {
+            assert Version.CURRENT.minor == 0;
+            currentOrNextMajorVersion = Version.CURRENT;
+        }
+        final List<Version> previousMajorVersions =
+                VersionUtils
+                        .allVersions()
+                        .stream()
+                        .filter(v -> v.before(currentOrNextMajorVersion))
+                        .collect(Collectors.toList());
+        final Version lastReleasedVersionFromPreviousMajor = previousMajorVersions.get(previousMajorVersions.size() - 1);
+        final Version previousMinorVersion = VersionUtils.getPreviousMinorVersion();
+        assert lastReleasedVersionFromPreviousMajor.major == previousMinorVersion.major;
+
+        boolean isCompatible = previousMinorVersion.minor == lastReleasedVersionFromPreviousMajor.minor;
+
+        final String message = String.format(
+                Locale.ROOT,
+                "[%s] should %s be compatible with [%s]",
+                previousMinorVersion,
+                isCompatible ? "" : " not",
+                currentOrNextMajorVersion);
         assertThat(
-                "[" + VersionUtils.getPreviousMinorVersion() + "] should" + (!buildSnapshot ? " not" : "") + " be compatible with 7.0.0",
-                isCompatible(VersionUtils.getPreviousMinorVersion(), Version.fromString("7.0.0")),
-                equalTo(!buildSnapshot));
+                message,
+                isCompatible(VersionUtils.getPreviousMinorVersion(), currentOrNextMajorVersion),
+                equalTo(isCompatible));
+
         assertFalse(isCompatible(Version.V_5_0_0, Version.fromString("6.0.0")));
         assertFalse(isCompatible(Version.V_5_0_0, Version.fromString("7.0.0")));