[fg] track master proc for suspend via pids, not term fds, for fork_on_start

When ui.sessions.fork_on_start, we fork a process that has a different
fdout than get_output_handle() (which has hardcoded fdout of 1), so it
will never be considered term->master, yet this is the process we want
to wake up in SIGTSTP handler.

Since we cannot rely on term->master to determine if we are the master
process, we instead move master_pid to be set explicitly at the places
where we have information about whether our process is a master or a
slave: first on start, then once the interlink determination has been
made.

master_pid has to be set in both parent and child, because both will get
suspended and need to know which one needs to resume in background with
SIGCONT (the master).  We can't inherit from the parent because it's
unknown at the time of fork.

Previously, master_pid worked correctly with fork_on_start=0,
-no-connect or -dump, but not with fork_on_start=1.

See #287 for background.

src/main/interlink.c

@@ -518,13 +518,15 @@ int
 init_interlink(void)
 {
 	int fd = connect_to_af_unix();
-	int pid;
 
 	if (fd != -1 || remote_session_flags) return fd;
 
 	parse_options_again();
 
 	if (get_opt_bool("ui.sessions.fork_on_start", NULL)) {
+
+		pid_t pid;
+
 		pid = fork();
 
 		if (pid == -1) return -1;
@@ -535,11 +537,18 @@ init_interlink(void)
 			for (i = 1; i <= (MAX_BIND_TRIES+2); ++i) {
 				fd = connect_to_af_unix();
 
-				if (fd != -1) return fd;
+				if (fd != -1) {
+					master_pid = pid;
+					return fd;
+				}
 				elinks_usleep(BIND_TRIES_DELAY * i);
 			}
 			return -1;
 		}
+		/* child */
+#ifdef HAVE_GETPID
+		master_pid = getpid();
+#endif
 		close_terminal_pipes();
 	}
 	bind_to_af_unix();

src/main/main.c

@@ -60,6 +60,8 @@
 
 struct program program;
 
+pid_t master_pid = 0;
+
 static int ac;
 static char **av;
 static int init_b = 0;
@@ -193,6 +195,9 @@ init(void)
 	    || get_cmd_opt_bool("source")
 	    || (fd = init_interlink()) == -1) {
 
+#ifdef HAVE_GETPID
+		master_pid = getpid();
+#endif
 		parse_options_again();
 		init_b = 1;
 		init_modules(builtin_modules);

src/main/main.h

@@ -23,6 +23,7 @@ struct program {
 };
 
 extern struct program program;
+extern pid_t master_pid;
 
 void shrink_memory(int);
 void parse_options_again(void);

src/osdep/signals.c

@@ -77,7 +77,7 @@ sig_tstp(struct terminal *term)
 
 	block_itrm();
 #if defined (SIGCONT) && defined(SIGTTOU)
-	if (master_pid) {
+	if (pid == master_pid) {
 		pid_t newpid = fork();
 		if (!newpid) {
 			int r;

src/terminal/terminal.c

@@ -54,8 +54,6 @@ INIT_LIST_OF(struct terminal, terminals);
 struct hash *temporary_files;
 static void check_if_no_terminal(void);
 
-pid_t master_pid = 0;
-
 void
 clean_temporary_files(void)
 {
@@ -191,12 +189,6 @@ init_term(int fdin, int fdout)
 	term->fdout = fdout;
 	term->master = (term->fdout == get_output_handle());
 
-#ifdef HAVE_GETPID
-	if (term->master) {
-		master_pid = getpid();
-	}
-#endif
-
 	term->blocked = -1;
 
 	get_terminal_name(name + 9);