fix(es-compat): reject empty index_filter to match ES behavior

- Remove empty object {} handling in parse_index_filter_to_query_ast
- ES rejects empty index_filter with 400, now QW does too
- Add tag_fields config and tag-based index_filter test
- Update unit and integration tests accordingly

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
Signed-off-by: ruo <ruoliu.dev@gmail.com>

Author: 2 people

quickwit/quickwit-serve/src/elasticsearch_api/model/field_capability.rs

@@ -178,14 +178,14 @@ pub fn convert_to_es_field_capabilities_response(
 
 /// Parses an Elasticsearch index_filter JSON value into a Quickwit QueryAst.
 ///
-/// Returns `Ok(None)` if the index_filter is null or empty.
+/// Returns `Ok(None)` if the index_filter is null.
 /// Returns `Ok(Some(QueryAst))` if the index_filter is valid.
-/// Returns `Err` if the index_filter is invalid or cannot be converted.
+/// Returns `Err` if the index_filter is invalid or cannot be converted (including empty object).
 #[allow(clippy::result_large_err)]
 pub fn parse_index_filter_to_query_ast(
     index_filter: serde_json::Value,
 ) -> Result<Option<QueryAst>, ElasticsearchError> {
-    if index_filter.is_null() || index_filter == serde_json::Value::Object(Default::default()) {
+    if index_filter.is_null() {
         return Ok(None);
     }
 
@@ -378,8 +378,9 @@ mod tests {
 
     #[test]
     fn test_parse_index_filter_to_query_ast_empty_object() {
-        let result = parse_index_filter_to_query_ast(json!({})).unwrap();
-        assert!(result.is_none());
+        // Empty object {} should return error to match ES behavior
+        let result = parse_index_filter_to_query_ast(json!({}));
+        assert!(result.is_err());
     }
 
     #[test]

quickwit/rest-api-tests/scenarii/es_field_capabilities/0001-field-capabilities.yaml

@@ -437,14 +437,25 @@ json:
       field: "value"
 status_code: 400
 ---
-# Test _field_caps API with empty index_filter (should work like no filter)
+# Test _field_caps API with empty index_filter (should return 400 like ES)
 method: [POST]
 engines:
   - quickwit
   - elasticsearch
 endpoint: fieldcaps/_field_caps?fields=name
 json:
   index_filter: {}
+status_code: 400
+---
+# Test _field_caps API with index_filter using tag field for split pruning (QW-only)
+method: [POST]
+engines:
+  - quickwit
+endpoint: fieldcaps/_field_caps?fields=name
+json:
+  index_filter:
+    term:
+      tags: "nice"
 expected:
   indices:
   - fieldcaps

quickwit/rest-api-tests/scenarii/es_field_capabilities/_setup.quickwit.yaml

@@ -22,6 +22,7 @@ json:
       tokenizer: default
       fast: true
     timestamp_field: date
+    tag_fields: ["tags"]
     field_mappings:
       - name: date
         type: datetime
@@ -32,6 +33,10 @@ json:
       - name: host
         type: ip
         fast: true
+      - name: tags
+        type: array<text>
+        tokenizer: raw
+        fast: true
 ---
 # Create index
 method: POST