Fix a memory leak introduced in #6418 (#6778) (#6779)

The root of the issue is that when serializing a singular filter, it is being treated as a unary filter before it is definitively established whether it is a unary filter or a field filter. The leak is caused by always serializing the unary filter's field path field for equality and non-equality filters -- if the filter's value turns out not to be NaN or null, the serialization code switches the filter's type to a field filter without clearing the partially-initialized unary filter. `pb_release` would not free the `unary_filter.field.field_path` member variable because it would consider the object not to be a unary filter.

Also a small refactoring to make the function easier to digest.

Co-authored-by: Konstantin Varlamov <[email protected]>

Author: paulb777

Firestore/CHANGELOG.md

@@ -5,6 +5,8 @@
   setting. Timestamp fields that read from a `FIRDocumentSnapshot` now always
   return `FIRTimestamp` objects. Use `FIRTimestamp.dateValue` to convert to
   `NSDate` if required.
+- [fixed] Fixed a memory leak introduced in 1.18.0 that may manifest when
+  serializing queries containing equality or non-equality comparisons.
 
 # v1.19.0
 - [changed] Internal improvements for future C++ and Unity support. Includes a

Firestore/core/src/remote/serializer.cc

@@ -1102,30 +1102,34 @@ google_firestore_v1_StructuredQuery_Filter Serializer::EncodeSingularFilter(
     const FieldFilter& filter) const {
   google_firestore_v1_StructuredQuery_Filter result{};
 
-  if (filter.op() == Filter::Operator::Equal ||
-      filter.op() == Filter::Operator::NotEqual) {
+  bool is_unary = (filter.op() == Filter::Operator::Equal ||
+                   filter.op() == Filter::Operator::NotEqual) &&
+                  (filter.value().is_nan() || filter.value().is_null());
+  if (is_unary) {
     result.which_filter_type =
         google_firestore_v1_StructuredQuery_Filter_unary_filter_tag;
     result.unary_filter.which_operand_type =
         google_firestore_v1_StructuredQuery_UnaryFilter_field_tag;
     result.unary_filter.field.field_path = EncodeFieldPath(filter.field());
 
+    bool is_equality = filter.op() == Filter::Operator::Equal;
     if (filter.value().is_nan()) {
-      auto op =
-          filter.op() == Filter::Operator::Equal
+      result.unary_filter.op =
+          is_equality
               ? google_firestore_v1_StructuredQuery_UnaryFilter_Operator_IS_NAN
               : google_firestore_v1_StructuredQuery_UnaryFilter_Operator_IS_NOT_NAN;  // NOLINT
-      result.unary_filter.op = op;
-      return result;
 
     } else if (filter.value().is_null()) {
-      auto op =
-          filter.op() == Filter::Operator::Equal
+      result.unary_filter.op =
+          is_equality
               ? google_firestore_v1_StructuredQuery_UnaryFilter_Operator_IS_NULL
               : google_firestore_v1_StructuredQuery_UnaryFilter_Operator_IS_NOT_NULL;  // NOLINT
-      result.unary_filter.op = op;
-      return result;
+
+    } else {
+      HARD_FAIL("Expected a unary filter");
     }
+
+    return result;
   }
 
   result.which_filter_type =