[8.x] [Deprecation] Refine Transform Destination Index message (#122192) (#122524) (#122648)

When we detect that a Transform writes to the index and the index is
incompatible with the next version, change the message, detail, and
URL to help the user take the necessary steps to migrate the destination
index.

Author: prwhelan

x-pack/plugin/deprecation/src/main/java/org/elasticsearch/xpack/deprecation/IndexDeprecationChecker.java

@@ -13,6 +13,7 @@
 import org.elasticsearch.common.TriFunction;
 import org.elasticsearch.common.time.DateFormatter;
 import org.elasticsearch.common.time.LegacyFormatNames;
+import org.elasticsearch.core.Strings;
 import org.elasticsearch.index.IndexModule;
 import org.elasticsearch.index.IndexSettings;
 import org.elasticsearch.index.IndexVersion;
@@ -97,25 +98,39 @@ private DeprecationIssue oldIndicesCheck(
         IndexVersion currentCompatibilityVersion = indexMetadata.getCompatibilityVersion();
         // We intentionally exclude indices that are in data streams because they will be picked up by DataStreamDeprecationChecks
         if (DeprecatedIndexPredicate.reindexRequired(indexMetadata, false) && isNotDataStreamIndex(indexMetadata, clusterState)) {
-            return new DeprecationIssue(
-                DeprecationIssue.Level.CRITICAL,
-                "Old index with a compatibility version < 8.0",
-                "https://www.elastic.co/guide/en/elasticsearch/reference/current/migrating-8.0.html#breaking-changes-8.0",
-                "This index has version: " + currentCompatibilityVersion.toReleaseVersion(),
-                false,
-                meta(indexMetadata, indexToTransformIds)
-            );
+            var transforms = transformIdsForIndex(indexMetadata, indexToTransformIds);
+            if (transforms.isEmpty() == false) {
+                return new DeprecationIssue(
+                    DeprecationIssue.Level.CRITICAL,
+                    "One or more Transforms write to this index with a compatibility version < 8.0",
+                    "https://www.elastic.co/guide/en/elasticsearch/reference/current/migrating-9.0.html"
+                        + "#breaking_90_transform_destination_index",
+                    Strings.format(
+                        "This index was created in version [%s] and requires action before upgrading to 9.0. The following transforms are "
+                            + "configured to write to this index: [%s]. Refer to the migration guide to learn more about how to prepare "
+                            + "transforms destination indices for your upgrade.",
+                        currentCompatibilityVersion.toReleaseVersion(),
+                        String.join(", ", transforms)
+                    ),
+                    false,
+                    Map.of("reindex_required", true, "transform_ids", transforms)
+                );
+            } else {
+                return new DeprecationIssue(
+                    DeprecationIssue.Level.CRITICAL,
+                    "Old index with a compatibility version < 8.0",
+                    "https://www.elastic.co/guide/en/elasticsearch/reference/current/migrating-9.0.html",
+                    "This index has version: " + currentCompatibilityVersion.toReleaseVersion(),
+                    false,
+                    Map.of("reindex_required", true)
+                );
+            }
         }
         return null;
     }
 
-    private Map<String, Object> meta(IndexMetadata indexMetadata, Map<String, List<String>> indexToTransformIds) {
-        var transforms = indexToTransformIds.getOrDefault(indexMetadata.getIndex().getName(), List.of());
-        if (transforms.isEmpty()) {
-            return Map.of("reindex_required", true);
-        } else {
-            return Map.of("reindex_required", true, "transform_ids", transforms);
-        }
+    private List<String> transformIdsForIndex(IndexMetadata indexMetadata, Map<String, List<String>> indexToTransformIds) {
+        return indexToTransformIds.getOrDefault(indexMetadata.getIndex().getName(), List.of());
     }
 
     private DeprecationIssue ignoredOldIndicesCheck(
@@ -126,16 +141,35 @@ private DeprecationIssue ignoredOldIndicesCheck(
         IndexVersion currentCompatibilityVersion = indexMetadata.getCompatibilityVersion();
         // We intentionally exclude indices that are in data streams because they will be picked up by DataStreamDeprecationChecks
         if (DeprecatedIndexPredicate.reindexRequired(indexMetadata, true) && isNotDataStreamIndex(indexMetadata, clusterState)) {
-            return new DeprecationIssue(
-                DeprecationIssue.Level.WARNING,
-                "Old index with a compatibility version < 8.0 Has Been Ignored",
-                "https://www.elastic.co/guide/en/elasticsearch/reference/current/migrating-8.0.html#breaking-changes-8.0",
-                "This read-only index has version: "
-                    + currentCompatibilityVersion.toReleaseVersion()
-                    + " and will be supported as read-only in 9.0",
-                false,
-                meta(indexMetadata, indexToTransformIds)
-            );
+            var transforms = transformIdsForIndex(indexMetadata, indexToTransformIds);
+            if (transforms.isEmpty() == false) {
+                return new DeprecationIssue(
+                    DeprecationIssue.Level.WARNING,
+                    "One or more Transforms write to this old index with a compatibility version < 8.0",
+                    "https://www.elastic.co/guide/en/elasticsearch/reference/current/migrating-9.0.html"
+                        + "#breaking_90_transform_destination_index",
+                    Strings.format(
+                        "This index was created in version [%s] and will be supported as a read-only index in 9.0. The following "
+                            + "transforms are no longer able to write to this index: [%s]. Refer to the migration guide to learn more "
+                            + "about how to handle your transforms destination indices.",
+                        currentCompatibilityVersion.toReleaseVersion(),
+                        String.join(", ", transforms)
+                    ),
+                    false,
+                    Map.of("reindex_required", true, "transform_ids", transforms)
+                );
+            } else {
+                return new DeprecationIssue(
+                    DeprecationIssue.Level.WARNING,
+                    "Old index with a compatibility version < 8.0 has been ignored",
+                    "https://www.elastic.co/guide/en/elasticsearch/reference/current/breaking-changes-9.0.html",
+                    "This read-only index has version: "
+                        + currentCompatibilityVersion.toReleaseVersion()
+                        + " and will be supported as read-only in 9.0",
+                    false,
+                    Map.of("reindex_required", true)
+                );
+            }
         }
         return null;
     }