lib/thread: add macro for debug printing of threading

This is analogous to DEBUG_DATABASE_SANITY, and is intended to help
debugging and to help users submit bug reports.

lib/notmuch-private.h

@@ -56,6 +56,7 @@ NOTMUCH_BEGIN_DECLS
 
 #ifdef DEBUG
 # define DEBUG_DATABASE_SANITY 1
+# define DEBUG_THREADING 1
 # define DEBUG_QUERY 1
 #endif
 

lib/thread.cc

@@ -24,6 +24,12 @@
 #include <gmime/gmime.h>
 #include <glib.h> /* GHashTable */
 
+#ifdef DEBUG_THREADING
+#define THREAD_DEBUG(format, ...) fprintf(stderr, format " (%s).\n", ##__VA_ARGS__, __location__)
+#else
+#define THREAD_DEBUG(format, ...) do {} while (0) /* ignored */
+#endif
+
 #define EMPTY_STRING(s) ((s)[0] == '\0')
 
 struct _notmuch_thread {