thread.c: Add log message before calling pthread_cancel.

Add a log message *before* calling pthread_cancel, so
if a deadlock occurs and this is possibly suspected to
be related to pthread_cancel usage, that can be confirmed
more easily from the logs (see LBBS-86).

bbs/thread.c

@@ -253,6 +253,14 @@ int bbs_pthread_cancel_kill(pthread_t thread)
 {
 	int res;
 
+	/* Log a message before calling pthread_cancel, since this is dangerous.
+	 * In the unlikely event that a thread is cancelled while in a critical section,
+	 * deadlock could ensue. I've suspected this might have been responsible for
+	 * some issues in the past but few clues have been available to such activity,
+	 * so this log message is an important clue if that happens (see [LBBS-86]). */
+	bbs_debug(1, "Attempting to cancel thread %lu\n", (unsigned long) thread);
+
+#undef pthread_cancel
 	res = pthread_cancel(thread);
 	if (res) {
 		if (res == ESRCH) {

include/bbs.h

@@ -89,6 +89,8 @@
 #define ctime(a) Do_not_use_ctime__use_ctime_r
 #define ptsname(fd) Do_not_use_ptsname__use_ptsname_r
 #define strncpy(dest, src, size) Do_not_use_strncpy__use_safe_strncpy
+/* Prohibit direct usage of dangerous functions */
+#define pthread_cancel(t) Do_not_use_pthread_cancel__use_bbs_pthread_cancel_kill
 
 #ifndef BBS_MAIN_FILE
 /* Allow printf only in bbs.c */