Add notmuch_message_get_thread_ids function

Along with all of the notmuch_thread_ids_t iterator functions.
Using a consistent idiom seems better here rather than returning
a comma-separated string and forcing the user to parse it.

message.cc

@@ -32,6 +32,11 @@ struct _notmuch_tags {
     Xapian::TermIterator iterator_end;
 };
 
+struct _notmuch_thread_ids {
+    char *current;
+    char *next;
+};
+
 #define ARRAY_SIZE(arr) (sizeof (arr) / sizeof (arr[0]))
 
 /* These prefix values are specifically chosen to be compatible
@@ -167,6 +172,25 @@ notmuch_message_get_tags (notmuch_message_t *message)
     return tags;
 }
 
+notmuch_thread_ids_t *
+notmuch_message_get_thread_ids (notmuch_message_t *message)
+{
+    notmuch_thread_ids_t *thread_ids;
+    const char *id_str;
+
+    thread_ids = talloc (message, notmuch_thread_ids_t);
+    if (unlikely (thread_ids == NULL))
+	return NULL;
+
+    id_str = message->doc.get_value (NOTMUCH_VALUE_THREAD).c_str ();
+    thread_ids->next = talloc_strdup (message, id_str);
+
+    /* Initialize thread_ids->current and terminate first ID. */
+    notmuch_thread_ids_advance (thread_ids);
+
+    return thread_ids;
+}
+
 void
 notmuch_message_destroy (notmuch_message_t *message)
 {
@@ -205,3 +229,30 @@ notmuch_tags_destroy (notmuch_tags_t *tags)
 {
     talloc_free (tags);
 }
+
+notmuch_bool_t
+notmuch_thread_ids_has_more (notmuch_thread_ids_t *thread_ids)
+{
+    if (thread_ids->current == NULL || *thread_ids->current == '\0')
+	return FALSE;
+    else
+	return TRUE;
+}
+
+const char *
+notmuch_thread_ids_get (notmuch_thread_ids_t *thread_ids)
+{
+    return thread_ids->current;
+}
+
+void
+notmuch_thread_ids_advance (notmuch_thread_ids_t *thread_ids)
+{
+    thread_ids->current = strsep (&thread_ids->next, ",");
+}
+
+void
+notmuch_thread_ids_destroy (notmuch_thread_ids_t *thread_ids)
+{
+    talloc_free (thread_ids);
+}

notmuch.h

@@ -67,6 +67,7 @@ typedef struct _notmuch_query notmuch_query_t;
 typedef struct _notmuch_results notmuch_results_t;
 typedef struct _notmuch_message notmuch_message_t;
 typedef struct _notmuch_tags notmuch_tags_t;
+typedef struct _notmuch_thread_ids notmuch_thread_ids_t;
 
 /* Create a new, empty notmuch database located at 'path'.
  *
@@ -304,24 +305,59 @@ notmuch_message_get_message_id (notmuch_message_t *message);
  *     notmuch_tags_t *tags;
  *     const char *tag;
  *
- *     message = notmuch_results_get (results);
+ *     message = notmuch_database_find_message (database, message_id);
  *
  *     for (tags = notmuch_message_get_tags (message);
  *          notmuch_tags_has_more (tags);
  *          notmuch_result_advance (tags))
  *     {
- *         tag = notmuch_tags_get_string (tags);
+ *         tag = notmuch_tags_get (tags);
  *         ....
  *     }
  *
- * Note: If you are finished with a message before its containing
- * query, you can call notmuch_message_destroy to clean up some memory
- * sooner. If you don't call it, all the memory will still be
- * reclaimed when the query is destroyed.
+ *     notmuch_message_destroy (message);
+ *
+ * Note that there's no explicit destructor needed for the
+ * notmuch_tags_t object. (For consistency, we do provide a
+ * notmuch_tags_destroy function, but there's no good reason to call
+ * it if the message is about to be destroyed).
  */
 notmuch_tags_t *
 notmuch_message_get_tags (notmuch_message_t *message);
 
+/* Get the thread IDs for 'message', returning a notmuch_thread_ids_t
+ * object which can be used to iterate over all thread IDs.
+ *
+ * The thread_ids object is owned by the message and as such, will
+ * only be valid for as long as the message is valid, (which is until
+ * the query from which it derived is destroyed).
+ *
+ * Typical usage might be:
+ *
+ *     notmuch_message_t *message;
+ *     notmuch_thread_ids_t *thread_ids;
+ *     const char *thread_id;
+ *
+ *     message = notmuch_database_find_message (database, message_id);
+ *
+ *     for (thread_ids = notmuch_message_get_thread_ids (message);
+ *          notmuch_thread_ids_has_more (thread_ids);
+ *          notmuch_thread_ids_advance (thread_ids))
+ *     {
+ *         thread_id = notmuch_thread_ids_get (thread_ids);
+ *         ....
+ *     }
+ *
+ *     notmuch_message_destroy (message);
+ *
+ * Note that there's no explicit destructor needed for the
+ * notmuch_thread_ids_t object. (For consistency, we do provide a
+ * notmuch_thread_ids_destroy function, but there's no good reason to
+ * call it if the message is about to be destroyed).
+ */
+notmuch_thread_ids_t *
+notmuch_message_get_thread_ids (notmuch_message_t *message);
+
 /* Destroy a notmuch_message_t object.
  *
  * It can be useful to call this function in the case of a single
@@ -373,6 +409,46 @@ notmuch_tags_advance (notmuch_tags_t *results);
 void
 notmuch_tags_destroy (notmuch_tags_t *tags);
 
+/* Does the given notmuch_thread_ids_t object contain any more thread IDs.
+ *
+ * When this function returns TRUE, notmuch_thread_ids_get will return a
+ * valid string. Whereas when this function returns FALSE,
+ * notmuch_thread_ids_get will return NULL.
+ *
+ * See the documentation of notmuch_message_get_thread_ids for example code
+ * showing how to iterate over a notmuch_thread_ids_t object.
+ */
+notmuch_bool_t
+notmuch_thread_ids_has_more (notmuch_thread_ids_t *thread_ids);
+
+/* Get the current thread ID from 'thread_ids' as a string.
+ *
+ * Note: The returned string belongs to 'thread_ids' and has a lifetime
+ * identical to it (and the query to which it utlimately belongs).
+ *
+ * See the documentation of notmuch_message_get_thread_ids for example code
+ * showing how to iterate over a notmuch_thread_ids_t object.
+ */
+const char *
+notmuch_thread_ids_get (notmuch_thread_ids_t *thread_ids);
+
+/* Advance the 'thread_ids' iterator to the next tag.
+ *
+ * See the documentation of notmuch_message_get_thread_ids for example code
+ * showing how to iterate over a notmuch_thread_ids_t object.
+ */
+void
+notmuch_thread_ids_advance (notmuch_thread_ids_t *thread_ids);
+
+/* Destroy a notmuch_thread_ids_t object.
+ *
+ * It's not strictly necessary to call this function. All memory from
+ * the notmuch_thread_ids_t object will be reclaimed when the containg
+ * message or query objects are destroyed.
+ */
+void
+notmuch_thread_ids_destroy (notmuch_thread_ids_t *thread_ids);
+
 NOTMUCH_END_DECLS
 
 #endif