lib: add notmuch_message_count_files

This operation is relatively inexpensive, as the needed metadata is
already computed by our lazy metadata fetching. The goal is to support
better UI for messages with multipile files.

lib/message.cc

@@ -946,6 +946,14 @@ notmuch_message_get_filenames (notmuch_message_t *message)
     return _notmuch_filenames_create (message, message->filename_list);
 }
 
+int
+notmuch_message_count_files (notmuch_message_t *message)
+{
+    _notmuch_message_ensure_filename_list (message);
+
+    return _notmuch_string_list_length (message->filename_list);
+}
+
 notmuch_bool_t
 notmuch_message_get_flag (notmuch_message_t *message,
 			  notmuch_message_flag_t flag)

lib/notmuch-private.h

@@ -550,6 +550,12 @@ typedef struct _notmuch_string_list {
 notmuch_string_list_t *
 _notmuch_string_list_create (const void *ctx);
 
+/*
+ * return the number of strings in 'list'
+ */
+int
+_notmuch_string_list_length (notmuch_string_list_t *list);
+
 /* Add 'string' to 'list'.
  *
  * The list will create its own talloced copy of 'string'.

lib/notmuch.h

@@ -1341,6 +1341,14 @@ notmuch_message_get_thread_id (notmuch_message_t *message);
 notmuch_messages_t *
 notmuch_message_get_replies (notmuch_message_t *message);
 
+/**
+ * Get the total number of files associated with a message.
+ * @returns Non-negative integer
+ * @since libnotmuch 5.0 (notmuch 0.25)
+ */
+int
+notmuch_message_count_files (notmuch_message_t *message);
+
 /**
  * Get a filename for the email corresponding to 'message'.
  *

lib/string-list.c

@@ -42,6 +42,12 @@ _notmuch_string_list_create (const void *ctx)
     return list;
 }
 
+int
+_notmuch_string_list_length (notmuch_string_list_t *list)
+{
+    return list->length;
+}
+
 void
 _notmuch_string_list_append (notmuch_string_list_t *list,
 			     const char *string)