Fix system data stream restore warning

docs/changelog/125881.yaml

@@ -0,0 +1,5 @@
+pr: 125881
+summary: Fixes a invalid warning from being issued when restoring a system data stream from a snapshot.
+area: "Data streams"
+type: bug
+issues: []

server/src/main/java/org/elasticsearch/snapshots/RestoreService.java

@@ -539,9 +539,19 @@ private void validateDataStreamTemplatesExistAndWarnIfMissing(
         Set<String> templatePatterns = streams.filter(cit -> cit.getDataStreamTemplate() != null)
             .flatMap(cit -> cit.indexPatterns().stream())
             .collect(Collectors.toSet());
+        warnIfIndexTemplateMissing(dataStreamsToRestore, templatePatterns, snapshotInfo);
+    }
 
-        for (String name : dataStreamsToRestore.keySet()) {
-            if (templatePatterns.stream().noneMatch(pattern -> Regex.simpleMatch(pattern, name))) {
+    // Visible for testing
+    static void warnIfIndexTemplateMissing(
+        Map<String, DataStream> dataStreamsToRestore,
+        Set<String> templatePatterns,
+        SnapshotInfo snapshotInfo
+    ) {
+        for (var entry : dataStreamsToRestore.entrySet()) {
+            String name = entry.getKey();
+            DataStream dataStream = entry.getValue();
+            if (dataStream.isSystem() == false && templatePatterns.stream().noneMatch(pattern -> Regex.simpleMatch(pattern, name))) {
                 String warningMessage = format(
                     "Snapshot [%s] contains data stream [%s] but custer does not have a matching index template. This will cause"
                         + " rollover to fail until a matching index template is created",

server/src/test/java/org/elasticsearch/snapshots/RestoreServiceTests.java

@@ -36,6 +36,7 @@
 import java.util.Set;
 import java.util.concurrent.atomic.AtomicBoolean;
 
+import static org.elasticsearch.core.Strings.format;
 import static org.hamcrest.Matchers.empty;
 import static org.hamcrest.Matchers.equalTo;
 import static org.mockito.ArgumentMatchers.any;
@@ -48,6 +49,50 @@
 
 public class RestoreServiceTests extends ESTestCase {
 
+    /**
+     * Test that {@link RestoreService#warnIfIndexTemplateMissing(Map, Set, SnapshotInfo)} does not warn for system
+     * datastreams.
+     */
+    public void testWarnIfIndexTemplateMissingSkipsSystemDataStreams() throws Exception {
+        String dataStreamName = ".test-system-data-stream";
+        String backingIndexName = DataStream.getDefaultBackingIndexName(dataStreamName, 1);
+        List<Index> indices = List.of(new Index(backingIndexName, randomUUID()));
+
+        var dataStream = DataStream.builder(dataStreamName, indices).setSystem(true).setHidden(true).build();
+        var dataStreamsToRestore = Map.of(dataStreamName, dataStream);
+        var templatePatterns = Set.of("matches_none");
+        var snapshotInfo = createSnapshotInfo(new Snapshot("repository", new SnapshotId("name", "uuid")), Boolean.FALSE);
+
+        RestoreService.warnIfIndexTemplateMissing(dataStreamsToRestore, templatePatterns, snapshotInfo);
+
+        ensureNoWarnings();
+    }
+
+    /**
+     * Test that {@link RestoreService#warnIfIndexTemplateMissing(Map, Set, SnapshotInfo)} warns for non-system datastreams.
+     */
+    public void testWarnIfIndexTemplateMissing() throws Exception {
+        String dataStreamName = ".test-system-data-stream";
+        String backingIndexName = DataStream.getDefaultBackingIndexName(dataStreamName, 1);
+        List<Index> indices = List.of(new Index(backingIndexName, randomUUID()));
+
+        var dataStream = DataStream.builder(dataStreamName, indices).build();
+        var dataStreamsToRestore = Map.of(dataStreamName, dataStream);
+        var templatePatterns = Set.of("matches_none");
+        var snapshotInfo = createSnapshotInfo(new Snapshot("repository", new SnapshotId("name", "uuid")), Boolean.FALSE);
+
+        RestoreService.warnIfIndexTemplateMissing(dataStreamsToRestore, templatePatterns, snapshotInfo);
+
+        assertWarnings(
+            format(
+                "Snapshot [%s] contains data stream [%s] but custer does not have a matching index template. This will cause"
+                    + " rollover to fail until a matching index template is created",
+                snapshotInfo.snapshotId(),
+                dataStreamName
+            )
+        );
+    }
+
     public void testUpdateDataStream() {
         long now = System.currentTimeMillis();
         String dataStreamName = "data-stream-1";