Reallocate default-initialized strings once on deserialization

rosidl_typesupport_microxrcedds_c/CMakeLists.txt

@@ -19,8 +19,10 @@ project(rosidl_typesupport_microxrcedds_c LANGUAGES C)
 find_package(ament_cmake REQUIRED)
 find_package(microcdr REQUIRED)
 find_package(ament_cmake_python REQUIRED)
+find_package(rcutils REQUIRED)
 find_package(rosidl_runtime_c REQUIRED)
 
+ament_export_dependencies(rcutils)
 ament_export_dependencies(rmw)
 ament_export_dependencies(rosidl_cmake)
 ament_export_dependencies(rosidl_runtime_c)
@@ -47,6 +49,7 @@ target_include_directories(${PROJECT_NAME}
   )
 
 target_link_libraries(${PROJECT_NAME}
+  rcutils::rcutils
   rosidl_runtime_c::rosidl_runtime_c
   )
 ament_export_libraries(${PROJECT_NAME})

rosidl_typesupport_microxrcedds_c/include/rosidl_typesupport_microxrcedds_c/message_type_support.h

@@ -16,9 +16,17 @@
 #define ROSIDL_TYPESUPPORT_MICROXRCEDDS_C__MESSAGE_TYPE_SUPPORT_H_
 
 #include "rosidl_runtime_c/message_type_support_struct.h"
+#include "rosidl_runtime_c/string.h"
 
+#include <rcutils/allocator.h>
 #include <ucdr/microcdr.h>
 
+// Bytes (null terminator included) reserved for a received string whose buffer is still in the
+// state left by rosidl_runtime_c__String__init() (capacity 1)
+#ifndef ROSIDL_TYPESUPPORT_MICROXRCEDDS_C_STRING_RESERVATION
+#define ROSIDL_TYPESUPPORT_MICROXRCEDDS_C_STRING_RESERVATION 50
+#endif
+
 typedef struct message_type_support_callbacks_t
 {
   const char * message_namespace_;
@@ -48,4 +56,53 @@ typedef struct message_type_support_callbacks_t
 
 } message_type_support_callbacks_t;
 
+/**
+ * Deserializes a string into the memory already owned by `str`.
+ *
+ * If the received string does not fit, it is skipped, `str` is left empty and false is
+ * returned, so the message is rejected.
+ *
+ * If `str` is still in the state left by rosidl_runtime_c__String__init() (capacity 1), its
+ * buffer is reallocated once with the rcutils default allocator to
+ * ROSIDL_TYPESUPPORT_MICROXRCEDDS_C_STRING_RESERVATION bytes.
+ * Buffers provided by the user always have a larger capacity, so they are never reallocated.
+ */
+static inline bool rosidl_typesupport_microxrcedds_c__deserialize_string(
+  ucdrBuffer * cdr,
+  rosidl_runtime_c__String * str)
+{
+  const size_t capacity = str->capacity;
+  uint32_t length = 0;  // CDR length, null terminator included
+
+  // When the string does not fit, micro-CDR only consumes its length and sets the error flag
+  bool rv = ucdr_deserialize_sequence_char(cdr, str->data, capacity, &length);
+
+  if (!rv && length > capacity && 1 == capacity &&
+    length <= ROSIDL_TYPESUPPORT_MICROXRCEDDS_C_STRING_RESERVATION)
+  {
+    rcutils_allocator_t allocator = rcutils_get_default_allocator();
+    char * data = (char *)allocator.reallocate(
+      str->data, ROSIDL_TYPESUPPORT_MICROXRCEDDS_C_STRING_RESERVATION, allocator.state);
+    if (NULL != data) {
+      str->data = data;
+      str->capacity = ROSIDL_TYPESUPPORT_MICROXRCEDDS_C_STRING_RESERVATION;
+      cdr->error = false;
+      rv = ucdr_deserialize_array_char(cdr, str->data, length);
+    }
+  }
+
+  if (rv) {
+    str->size = (length == 0) ? 0 : length - 1;
+  } else if (length > str->capacity) {
+    // Skip the string, keeping the buffer aligned for the next member
+    cdr->error = false;
+    cdr->last_data_size = 1;
+    str->size = 0;
+    ucdr_align_to(cdr, sizeof(char));
+    ucdr_advance_buffer(cdr, length);
+  }
+
+  return rv;
+}
+
 #endif  // ROSIDL_TYPESUPPORT_MICROXRCEDDS_C__MESSAGE_TYPE_SUPPORT_H_

rosidl_typesupport_microxrcedds_c/package.xml

@@ -22,6 +22,7 @@
   <!-- ament_export_dependencies(rosidl_cmake) needs it findable downstream -->
   <build_export_depend>rosidl_cmake</build_export_depend>
 
+  <depend>rcutils</depend>
   <depend>rmw</depend>
 
   <exec_depend>rosidl_parser</exec_depend>

rosidl_typesupport_microxrcedds_c/resource/msg__type_support_c.c.em

@@ -353,40 +353,15 @@ static bool _@(message.structure.namespaced_type.name)__cdr_deserialize(
     ros_message->@(member.name).size = size;
 
     for (size_t i = 0; rv && i < size; i++) {
-      size_t capacity = ros_message->@(member.name).data[i].capacity;
-      uint32_t string_size;
-      char * data = ros_message->@(member.name).data[i].data;
-      rv = ucdr_deserialize_sequence_char(cdr, data, capacity, &string_size);
-      if (rv) {
-        ros_message->@(member.name).data[i].size = (string_size == 0) ? 0 : string_size - 1;
-      } else if(string_size > capacity){
-        cdr->error = false;
-        cdr->last_data_size = 1;
-        ros_message->@(member.name).data[i].size = 0;
-        ucdr_align_to(cdr, sizeof(char));
-        ucdr_advance_buffer(cdr, string_size);
-      }
+      rv = rosidl_typesupport_microxrcedds_c__deserialize_string(cdr, &ros_message->@(member.name).data[i]);
     }
 @[      end if]@
 @[    end if]@
   }
 @[  elif isinstance(member.type, BasicType)]@
   rv = ucdr_deserialize_@(get_suffix(member.type.typename))(cdr, &ros_message->@(member.name));
 @[  elif isinstance(member.type, AbstractString)]@
-  {
-    size_t capacity = ros_message->@(member.name).capacity;
-    uint32_t string_size;
-    rv = ucdr_deserialize_sequence_char(cdr, ros_message->@(member.name).data, capacity, &string_size);
-    if (rv) {
-      ros_message->@(member.name).size = (string_size == 0) ? 0 : string_size - 1;
-    } else if(string_size > capacity){
-      cdr->error = false;
-      cdr->last_data_size = 1;
-      ros_message->@(member.name).size = 0;
-      ucdr_align_to(cdr, sizeof(char));
-      ucdr_advance_buffer(cdr, string_size);
-    }
-  }
+  rv = rosidl_typesupport_microxrcedds_c__deserialize_string(cdr, &ros_message->@(member.name));
 @[  elif isinstance(member.type, AbstractWString)]@
   // Micro CDR does not support WString type.
 @[  elif isinstance(member.type, NamespacedType)]@

test/c/CMakeLists.txt

@@ -36,6 +36,7 @@ if(BUILD_TESTING)
   ament_add_gtest(${TEST_NAME}
     src/test_typesupport.cpp
     src/cross_serialization_tests.cpp
+    src/test_string_reservation.cpp
     )
 
   target_link_libraries(${TEST_NAME}