[api] Use FixedVector for ExecuteServiceRequest/Argument arrays to eliminate reallocations

script/api_protobuf/api_protobuf.py

@@ -1879,6 +1879,9 @@ def build_message_type(
         )
         public_content.append("#endif")
 
+    # Collect fixed_vector fields for custom decode generation
+    fixed_vector_fields = []
+
     for field in desc.field:
         # Skip deprecated fields completely
         if field.options.deprecated:
@@ -1910,6 +1913,14 @@ def build_message_type(
                 f"since we cannot trust or control the number of items received from clients."
             )
 
+        # Collect fixed_vector repeated fields for custom decode generation
+        if (
+            needs_decode
+            and field.label == 3
+            and get_field_opt(field, pb.fixed_vector, False)
+        ):
+            fixed_vector_fields.append((field.name, field.number))
+
         ti = create_field_type_info(field, needs_decode, needs_encode)
 
         # Skip field declarations for fields that are in the base class
@@ -2018,6 +2029,22 @@ def build_message_type(
         prot = "bool decode_64bit(uint32_t field_id, Proto64Bit value) override;"
         protected_content.insert(0, prot)
 
+    # Generate custom decode() override for messages with FixedVector fields
+    if fixed_vector_fields:
+        # Generate the decode() implementation in cpp
+        o = f"void {desc.name}::decode(const uint8_t *buffer, size_t length) {{\n"
+        # Count and init each FixedVector field
+        for field_name, field_number in fixed_vector_fields:
+            o += f"  uint32_t count_{field_name} = ProtoDecodableMessage::count_repeated_field(buffer, length, {field_number});\n"
+            o += f"  this->{field_name}.init(count_{field_name});\n"
+        # Call parent decode to populate the fields
+        o += "  ProtoDecodableMessage::decode(buffer, length);\n"
+        o += "}\n"
+        cpp += o
+        # Generate the decode() declaration in header (public method)
+        prot = "void decode(const uint8_t *buffer, size_t length) override;"
+        public_content.append(prot)
+
     # Only generate encode method if this message needs encoding and has fields
     if needs_encode and encode:
         o = f"void {desc.name}::encode(ProtoWriteBuffer buffer) const {{"