add protection for existsAlias with no index and no alias

Author: javanna

client/rest-high-level/src/main/java/org/elasticsearch/client/Request.java

@@ -477,6 +477,9 @@ static Request existsAlias(GetAliasesRequest getAliasesRequest) {
         Params params = Params.builder();
         params.withIndicesOptions(getAliasesRequest.indicesOptions());
         params.withLocal(getAliasesRequest.local());
+        if (getAliasesRequest.indices().length == 0 && getAliasesRequest.aliases().length == 0) {
+            throw new IllegalArgumentException("existsAlias requires at least an alias or an index");
+        }
         String endpoint = endpoint(getAliasesRequest.indices(), "_alias", getAliasesRequest.aliases());
         return new Request("HEAD", endpoint, params.getParams(), null);
     }

client/rest-high-level/src/test/java/org/elasticsearch/client/RequestTests.java

@@ -988,7 +988,8 @@ public void testExistsAlias() {
         GetAliasesRequest getAliasesRequest = new GetAliasesRequest();
         String[] indices = randomIndicesNames(0, 5);
         getAliasesRequest.indices(indices);
-        String[] aliases = randomIndicesNames(0, 5);
+        //the HEAD endpoint requires at least an alias or an index
+        String[] aliases = randomIndicesNames(indices.length == 0 ? 1 : 0, 5);
         getAliasesRequest.aliases(aliases);
         Map<String, String> expectedParams = new HashMap<>();
         if (randomBoolean()) {
@@ -1015,6 +1016,12 @@ public void testExistsAlias() {
         assertNull(request.getEntity());
     }
 
+    public void testExistsAliasNoAliasNoIndex() {
+        GetAliasesRequest getAliasesRequest = new GetAliasesRequest();
+        IllegalArgumentException iae = expectThrows(IllegalArgumentException.class, () -> Request.existsAlias(getAliasesRequest));
+        assertEquals("existsAlias requires at least an alias or an index", iae.getMessage());
+    }
+
     private static void assertToXContentBody(ToXContent expectedBody, HttpEntity actualEntity) throws IOException {
         BytesReference expectedBytes = XContentHelper.toXContent(expectedBody, REQUEST_BODY_CONTENT_TYPE, false);
         assertEquals(XContentType.JSON.mediaTypeWithoutParameters(), actualEntity.getContentType().getValue());