Avoid changing tsid creating strategy for an index

This avoids that the tsid and routing is calculated differently on the same index.
Doing so would have the following consequences:
* De-duplication would not work, for example, when a client re-transmits a batch as the id (which is based on the tsid) will be different.
* When replaying a translog operation, the re-computed tsid and id will differ  from the id stored in the translog. This would lead to a failure of the index operation.

Author: felixbarny

modules/data-streams/src/main/java/org/elasticsearch/datastreams/DataStreamIndexSettingsProvider.java

@@ -172,11 +172,13 @@ public void onUpdateMappings(IndexMetadata indexMetadata, DocumentMapper documen
         boolean hasChanges = indexDimensions.size() != newIndexDimensions.size()
             && new HashSet<>(indexDimensions).equals(new HashSet<>(newIndexDimensions)) == false;
         if (matchesAllDimensions == false) {
-            // If the new dimensions don't match all potential dimension fields, we need to unset index.dimensions
-            // and set index.routing_path instead.
-            // This can happen if a new dynamic template with time_series_dimension: true is added to an existing index.
-            additionalSettings.putList(INDEX_DIMENSIONS.getKey(), List.of());
-            additionalSettings.putList(INDEX_ROUTING_PATH.getKey(), newIndexDimensions);
+            throw new IllegalArgumentException(
+                "Cannot add dimension fields via dynamic templates or mappings for an index with "
+                    + INDEX_DIMENSIONS.getKey()
+                    + ". "
+                    + "Please change the index template and roll over the data stream "
+                    + "instead of modifying the mappings of the backing indices."
+            );
         } else if (hasChanges) {
             additionalSettings.putList(INDEX_DIMENSIONS.getKey(), newIndexDimensions);
         }

modules/data-streams/src/test/java/org/elasticsearch/datastreams/DataStreamIndexSettingsProviderTests.java

@@ -956,10 +956,18 @@ public void testAddDynamicTemplate() throws Exception {
             }
             """;
         // we don't support index.dimensions with dynamic templates so we'll unset index.dimensions
-        Settings result = onUpdateMappings(null, "labels.*", mapping);
-        assertThat(result.size(), equalTo(2));
-        assertThat(IndexMetadata.INDEX_DIMENSIONS.get(result), empty());
-        assertThat(IndexMetadata.INDEX_ROUTING_PATH.get(result), containsInAnyOrder("labels.*"));
+        IllegalArgumentException exception = assertThrows(
+            IllegalArgumentException.class,
+            () -> onUpdateMappings(null, "labels.*", mapping)
+        );
+        assertThat(
+            exception.getMessage(),
+            equalTo(
+                "Cannot add dimension fields via dynamic templates or mappings for an index with index.dimensions. "
+                    + "Please change the index template and roll over the data stream "
+                    + "instead of modifying the mappings of the backing indices."
+            )
+        );
     }
 
     private Settings generateTsdbSettings(String mapping, Instant now) throws IOException {