Add exception type response header for 5xx errors

server/src/main/java/org/elasticsearch/ElasticsearchException.java

@@ -118,6 +118,7 @@ public class ElasticsearchException extends RuntimeException implements ToXConte
     private static final String ROOT_CAUSE = "root_cause";
 
     static final String TIMED_OUT_HEADER = "X-Timed-Out";
+    static final String EXCEPTION_TYPE_HEADER = "X-Elasticsearch-Exception";
 
     private static final Map<Integer, CheckedFunction<StreamInput, ? extends ElasticsearchException, IOException>> ID_TO_SUPPLIER;
     private static final Map<Class<? extends ElasticsearchException>, ElasticsearchExceptionHandle> CLASS_TO_ELASTICSEARCH_EXCEPTION_HANDLE;
@@ -130,7 +131,7 @@ public class ElasticsearchException extends RuntimeException implements ToXConte
     @SuppressWarnings("this-escape")
     public ElasticsearchException(Throwable cause) {
         super(cause);
-        maybePutTimeoutHeader();
+        addErrorHeaders();
     }
 
     /**
@@ -145,7 +146,7 @@ public ElasticsearchException(Throwable cause) {
     @SuppressWarnings("this-escape")
     public ElasticsearchException(String msg, Object... args) {
         super(LoggerMessageFormat.format(msg, args));
-        maybePutTimeoutHeader();
+        addErrorHeaders();
     }
 
     /**
@@ -162,7 +163,7 @@ public ElasticsearchException(String msg, Object... args) {
     @SuppressWarnings("this-escape")
     public ElasticsearchException(String msg, Throwable cause, Object... args) {
         super(LoggerMessageFormat.format(msg, args), cause);
-        maybePutTimeoutHeader();
+        addErrorHeaders();
     }
 
     @SuppressWarnings("this-escape")
@@ -173,11 +174,17 @@ public ElasticsearchException(StreamInput in) throws IOException {
         metadata.putAll(in.readMapOfLists(StreamInput::readString));
     }
 
-    private void maybePutTimeoutHeader() {
+    private void addErrorHeaders() {
         if (isTimeout()) {
             // see https://www.rfc-editor.org/rfc/rfc8941.html#section-4.1.9 for booleans in structured headers
             headers.put(TIMED_OUT_HEADER, List.of("?1"));
         }
+        // TODO: cache unwrapping the cause? we do this in several places...
+        Throwable cause = unwrapCause();
+        RestStatus status = ExceptionsHelper.status(cause);
+        if (status.getStatus() >= 500) {
+            headers.put(EXCEPTION_TYPE_HEADER, List.of(cause.getClass().getSimpleName()));
+        }
     }
 
     /**

server/src/test/java/org/elasticsearch/ElasticsearchExceptionTests.java

@@ -1553,19 +1553,24 @@ private void testExceptionLoop(Exception rootException) throws IOException {
         assertArrayEquals(ser.getStackTrace(), rootException.getStackTrace());
     }
 
-    static class ExceptionSubclass extends ElasticsearchException {
+    static class TimeoutSubclass extends ElasticsearchException {
+        TimeoutSubclass(String message) {
+            super(message);
+        }
+
         @Override
         public boolean isTimeout() {
             return true;
         }
 
-        ExceptionSubclass(String message) {
-            super(message);
+        @Override
+        public RestStatus status() {
+            return RestStatus.BAD_REQUEST;
         }
     }
 
-    public void testTimeout() throws IOException {
-        var e = new ExceptionSubclass("some timeout");
+    public void testTimeoutHeader() throws IOException {
+        var e = new TimeoutSubclass("some timeout");
         assertThat(e.getHeaderKeys(), hasItem(ElasticsearchException.TIMED_OUT_HEADER));
 
         XContentBuilder builder = XContentFactory.jsonBuilder();
@@ -1574,7 +1579,7 @@ public void testTimeout() throws IOException {
         builder.endObject();
         String expected = """
             {
-              "type": "exception_subclass",
+              "type": "timeout_subclass",
               "reason": "some timeout",
               "timed_out": true,
               "header": {
@@ -1583,4 +1588,61 @@ public void testTimeout() throws IOException {
             }""";
         assertEquals(XContentHelper.stripWhitespace(expected), Strings.toString(builder));
     }
+
+    static class Exception5xx extends ElasticsearchException {
+        Exception5xx(String message) {
+            super(message);
+        }
+
+        @Override
+        public RestStatus status() {
+            return RestStatus.INTERNAL_SERVER_ERROR;
+        }
+    }
+
+    static class Exception4xx extends ElasticsearchException {
+        Exception4xx(String message) {
+            super(message);
+        }
+
+        @Override
+        public RestStatus status() {
+            return RestStatus.BAD_REQUEST;
+        }
+    }
+
+    public void testExceptionTypeHeader() throws IOException {
+        var e = new Exception5xx("some exception");
+        assertThat(e.getHeaderKeys(), hasItem(ElasticsearchException.EXCEPTION_TYPE_HEADER));
+
+        XContentBuilder builder = XContentFactory.jsonBuilder();
+        builder.startObject();
+        e.toXContent(builder, ToXContent.EMPTY_PARAMS);
+        builder.endObject();
+        String expected = """
+            {
+              "type": "exception5xx",
+              "reason": "some exception",
+              "header": {
+                "X-Elasticsearch-Exception": "Exception5xx"
+              }
+            }""";
+        assertEquals(XContentHelper.stripWhitespace(expected), Strings.toString(builder));
+    }
+
+    public void testNoExceptionTypeHeaderOn4xx() throws IOException {
+        var e = new Exception4xx("some exception");
+        assertThat(e.getHeaderKeys(), not(hasItem(ElasticsearchException.EXCEPTION_TYPE_HEADER)));
+
+        XContentBuilder builder = XContentFactory.jsonBuilder();
+        builder.startObject();
+        e.toXContent(builder, ToXContent.EMPTY_PARAMS);
+        builder.endObject();
+        String expected = """
+            {
+              "type": "exception4xx",
+              "reason": "some exception"
+            }""";
+        assertEquals(XContentHelper.stripWhitespace(expected), Strings.toString(builder));
+    }
 }

server/src/test/java/org/elasticsearch/ExceptionSerializationTests.java

@@ -148,7 +148,7 @@ public void testExceptionRegistration() throws IOException, URISyntaxException {
             CancellableThreadsTests.CustomException.class,
             RestResponseTests.WithHeadersException.class,
             AbstractClientHeadersTestCase.InternalException.class,
-            ElasticsearchExceptionTests.ExceptionSubclass.class
+            ElasticsearchExceptionTests.TimeoutSubclass.class
         );
         FileVisitor<Path> visitor = new FileVisitor<Path>() {
             private Path pkgPrefix = PathUtils.get(path).getParent();