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        | 8 ++++++++
 lib/notmuch-private.h | 6 ++++++
 lib/notmuch.h         | 8 ++++++++
 lib/string-list.c     | 6 ++++++
 4 files changed, 28 insertions(+)

diff --git a/lib/message.cc b/lib/message.cc
index b330dcce..f3d1e159 100644
--- a/lib/message.cc
+++ b/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)
diff --git a/lib/notmuch-private.h b/lib/notmuch-private.h
index 4534fb52..bcd22bee 100644
--- a/lib/notmuch-private.h
+++ b/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'.
diff --git a/lib/notmuch.h b/lib/notmuch.h
index e1745444..f6ed3990 100644
--- a/lib/notmuch.h
+++ b/lib/notmuch.h
@@ -1342,6 +1342,14 @@ 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'.
  *
  * The returned filename is an absolute filename, (the initial
diff --git a/lib/string-list.c b/lib/string-list.c
index 43ebe499..9c3ae7ef 100644
--- a/lib/string-list.c
+++ b/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)
-- 
2.11.0