pbio/protocol: add macro for status report size

The status report may grow in the future, so add a macro to help catch
places in the code that need to be updated if the size changes.

Author: dlech

lib/pbio/include/pbio/protocol.h

@@ -341,6 +341,9 @@ typedef enum {
  */
 #define PBIO_PYBRICKS_STATUS_FLAG(status) (1 << status)
 
+/** Size of status report event message in bytes. */
+#define PBIO_PYBRICKS_EVENT_STATUS_REPORT_SIZE 6
+
 uint32_t pbio_pybricks_event_status_report(uint8_t *buf, uint32_t flags, pbio_pybricks_user_program_id_t program_id);
 
 /**

lib/pbio/src/protocol/pybricks.c

@@ -13,7 +13,9 @@ _Static_assert(NUM_PBIO_PYBRICKS_STATUS <= sizeof(uint32_t) * 8,
     "oh no, we added too many status flags");
 
 /**
- * Writes Pybricks status report command to @p buf
+ * Writes Pybricks status report command to @p buf.
+ *
+ * The buffer must be at least ::PBIO_PYBRICKS_EVENT_STATUS_REPORT_SIZE bytes.
  *
  * @param [in]  buf         The buffer to hold the binary data.
  * @param [in]  flags       The status flags.
@@ -24,7 +26,7 @@ uint32_t pbio_pybricks_event_status_report(uint8_t *buf, uint32_t flags, pbio_py
     buf[0] = PBIO_PYBRICKS_EVENT_STATUS_REPORT;
     pbio_set_uint32_le(&buf[1], flags);
     buf[5] = program_id;
-    return 6;
+    return PBIO_PYBRICKS_EVENT_STATUS_REPORT_SIZE;
 }
 
 /**