Fix test issue

Author: jbaiera

server/src/main/java/org/elasticsearch/ingest/IngestDocument.java

@@ -1349,26 +1349,39 @@ private static String[] processPathParts(String fullPath, String[] pathParts, In
                 throw new IllegalArgumentException("path [" + fullPath + "] is not valid");
             }
             return switch (accessPattern) {
-                case CLASSIC -> pathParts;
+                case CLASSIC -> validateClassicFields(fullPath, pathParts);
                 case FLEXIBLE -> parseFlexibleFields(fullPath, pathParts);
             };
         }
 
+        /**
+         * Parses path syntax that is specific to the {@link IngestPipelineFieldAccessPattern#CLASSIC} ingest doc access pattern. Supports
+         * syntax like context aware array access.
+         * @param fullPath The un-split path to use for error messages
+         * @param pathParts The tokenized field path to parse
+         * @return An array of Strings
+         */
+        private static String[] validateClassicFields(String fullPath, String[] pathParts) {
+            for (String pathPart : pathParts) {
+                if (pathPart.isEmpty()) {
+                    throw new IllegalArgumentException("path [" + fullPath + "] is not valid");
+                }
+            }
+            return pathParts;
+        }
+
         /**
          * Parses path syntax that is specific to the {@link IngestPipelineFieldAccessPattern#FLEXIBLE} ingest doc access pattern. Supports
          * syntax like square bracket array access, which is the only way to index arrays in flexible mode.
          * @param fullPath The un-split path to use for error messages
          * @param pathParts The tokenized field path to parse
-         * @return An array of Elements
+         * @return An array of Strings
          */
         private static String[] parseFlexibleFields(String fullPath, String[] pathParts) {
-            boolean invalidPath = Arrays.stream(pathParts).anyMatch(pathPart -> {
-                int openBracket = pathPart.indexOf('[');
-                int closedBracket = pathPart.indexOf(']');
-                return openBracket != -1 && closedBracket != -1;
-            });
-            if (invalidPath) {
-                throw new IllegalArgumentException("path [" + fullPath + "] is not valid");
+            for (String pathPart : pathParts) {
+                if (pathPart.isEmpty() || pathPart.indexOf('[') >= 0 || pathPart.indexOf(']') >= 0) {
+                    throw new IllegalArgumentException("path [" + fullPath + "] is not valid");
+                }
             }
             return pathParts;
         }

server/src/test/java/org/elasticsearch/ingest/IngestDocumentTests.java

@@ -216,8 +216,8 @@ public void testPathParsingLogic() throws Exception {
             assertPathValid(doc, "a..");
             assertPathValid(doc, "a...");
             // Empty field names are not allowed in the beginning or middle of the path though
-            assertPathInvalid(doc, ".a.b", "fieldName cannot be empty");
-            assertPathInvalid(doc, "a..b", "fieldName cannot be empty");
+            assertPathInvalid(doc, ".a.b", "path [.a.b] is not valid");
+            assertPathInvalid(doc, "a..b", "path [a..b] is not valid");
         });
 
         doWithAccessPattern(CLASSIC, (doc) -> {