chat: Improve signal handling

This improves the way that signals are handled in chat.

First, signal handlers should not be calling functions which are not
async-signal-safe; doing so incurs the possibility of deadlock.  Thus
we can't call fatal() in signal handlers; instead we set 'fatalsig',
which functions both as a flag and as an indication of which signal
occurred, and check that at various points (basically after any
operation which might block) using the new function checksigs().

Secondly, using sigaction rather than signal() means that we can
control whether calls such as read() get restarted after a signal, and
whether the signal disposition gets reset when the signal is
delivered.  That simplifies sigalrm(); we no longer need to
re-register the handler, and we don't need the kludge of setting stdin
to non-blocking mode in order to get the read() in get_char() to
return.

This also removes a #ifdef ultrix since ultrix is no longer supported.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>

1 files changed, 53 insertions, 48 deletions

diff --git a/chat/chat.c b/chat/chat.c
index 64d96c2..f625a8a 100644
--- a/chat/chat.c
+++ b/chat/chat.c
@@ -198,7 +198,7 @@ SIGTYPE sigalrm (int signo);
 SIGTYPE sigint (int signo);
 SIGTYPE sigterm (int signo);
 SIGTYPE sighup (int signo);
-void unalarm (void);
+void checksigs(void);
 void init (void);
 void set_tty_parameters (void);
 int  echo_stderr (int);
@@ -215,7 +215,6 @@ char *character (int c);
 void chat_expect (register char *s);
 char *clean (register char *s, int sending);
 void break_sequence (void);
-void terminate (int status);
 void pack_array (char **array, int end);
 char *expect_strtok (char *, char *);
 int vfmtmsg (char *, int, const char *, va_list);	/* vsprintf++ */
@@ -347,16 +346,12 @@ main(int argc, char **argv)
 	report_fp = stderr;
 
     if (to_log) {
-#ifdef ultrix
-	openlog("chat", LOG_PID);
-#else
 	openlog("chat", LOG_PID | LOG_NDELAY, LOG_LOCAL2);
 
 	if (verbose)
 	    setlogmask(LOG_UPTO(LOG_INFO));
 	else
 	    setlogmask(LOG_UPTO(LOG_WARNING));
-#endif
     }
 
     init();
@@ -373,6 +368,7 @@ main(int argc, char **argv)
 
 	    if ((arg = ARG(argc, argv)) != NULL)
 		chat_send(arg);
+	    checksigs();
 	}
     }
 
@@ -444,8 +440,10 @@ void do_file (char *chat_file)
 	    else
 		chat_expect (arg);
 	    sendflg = !sendflg;
+	    checksigs();
 	}
     }
+    checksigs();
     fclose (cfp);
 }
 
@@ -499,59 +497,68 @@ void fatal(int code, const char *fmt, ...)
 }
 
 int alarmed = 0;
+int alarmsig = 0;
 
 SIGTYPE sigalrm(int signo)
 {
     int flags;
 
     alarm(1);
-    alarmed = 1;		/* Reset alarm to avoid race window */
-    signal(SIGALRM, sigalrm);	/* that can cause hanging in read() */
-
-    if ((flags = fcntl(0, F_GETFL, 0)) == -1)
-	fatal(2, "Can't get file mode flags on stdin: %m");
-
-    if (fcntl(0, F_SETFL, flags | O_NONBLOCK) == -1)
-	fatal(2, "Can't set file mode flags on stdin: %m");
-
-    if (verbose)
-	msgf("alarm");
+    alarmed = 1;
+    alarmsig = 1;
 }
 
-void unalarm(void)
-{
-    int flags;
-
-    if ((flags = fcntl(0, F_GETFL, 0)) == -1)
-	fatal(2, "Can't get file mode flags on stdin: %m");
-
-    if (fcntl(0, F_SETFL, flags & ~O_NONBLOCK) == -1)
-	fatal(2, "Can't set file mode flags on stdin: %m");
-}
+const char *fatalsig = NULL;
 
 SIGTYPE sigint(int signo)
 {
-    fatal(2, "SIGINT");
+    fatalsig = "SIGINT";
 }
 
 SIGTYPE sigterm(int signo)
 {
-    fatal(2, "SIGTERM");
+    fatalsig = "SIGTERM";
 }
 
 SIGTYPE sighup(int signo)
 {
-    fatal(2, "SIGHUP");
+    fatalsig = "SIGHUP";
+}
+
+void checksigs(void)
+{
+    int err;
+    const char *signame;
+
+    if (fatalsig) {
+	signame = fatalsig;
+	fatalsig = NULL;
+	alarmsig = 0;
+	fatal(2, signame);
+    }
+    if (alarmsig && verbose) {
+	err = errno;
+	msgf("alarm");
+	errno = err;
+	alarmsig = 0;
+    }
 }
 
 void init(void)
 {
-    signal(SIGINT, sigint);
-    signal(SIGTERM, sigterm);
-    signal(SIGHUP, sighup);
+    struct sigaction sa;
+
+    memset(&sa, 0, sizeof(sa));
+    sa.sa_handler = sigint;
+    sigaction(SIGINT, &sa, NULL);
+    sa.sa_handler = sigterm;
+    sigaction(SIGTERM, &sa, NULL);
+    sa.sa_handler = sighup;
+    sigaction(SIGHUP, &sa, NULL);
 
     set_tty_parameters();
-    signal(SIGALRM, sigalrm);
+    sa.sa_handler = sigalrm;
+    sigaction(SIGALRM, &sa, NULL);
     alarm(0);
     alarmed = 0;
 }
@@ -945,6 +952,7 @@ void chat_expect (char *s)
 	    break;
 
 	chat_send (reply);
+	checksigs();
     }
 
 /*
@@ -987,6 +995,7 @@ int chat_send (register char *s)
 {
     char file_data[STR_LEN];
     int len, ret = 0;
+    struct sigaction sa;
 
     if (say_next) {
 	say_next = 0;
@@ -999,10 +1008,13 @@ int chat_send (register char *s)
 
     if (hup_next) {
         hup_next = 0;
+	memset(&sa, 0, sizeof(sa));
+
 	if (strcmp(s, "OFF") == 0)
-           signal(SIGHUP, SIG_IGN);
+	    sa.sa_handler = SIG_IGN;
         else
-           signal(SIGHUP, sighup);
+	    sa.sa_handler = sighup;
+	sigaction(SIGHUP, &sa, NULL);
         return 0;
     }
 
@@ -1179,6 +1191,7 @@ int get_char(void)
     char c;
 
     status = read(0, &c, 1);
+    checksigs();
 
     switch (status) {
     case 1:
@@ -1188,12 +1201,6 @@ int get_char(void)
 	msgf("warning: read() on stdin returned %d", status);
 
     case -1:
-	if ((status = fcntl(0, F_GETFL, 0)) == -1)
-	    fatal(2, "Can't get file mode flags on stdin: %m");
-
-	if (fcntl(0, F_SETFL, status & ~O_NONBLOCK) == -1)
-	    fatal(2, "Can't set file mode flags on stdin: %m");
-	
 	return (-1);
     }
 }
@@ -1204,8 +1211,10 @@ int put_char(int c)
     char ch = c;
 
     usleep(10000);		/* inter-character typing delay (?) */
+    checksigs();
 
     status = write(1, &ch, 1);
+    checksigs();
 
     switch (status) {
     case 1:
@@ -1215,12 +1224,6 @@ int put_char(int c)
 	msgf("warning: write() on stdout returned %d", status);
 	
     case -1:
-	if ((status = fcntl(0, F_GETFL, 0)) == -1)
-	    fatal(2, "Can't get file mode flags on stdin, %m");
-
-	if (fcntl(0, F_SETFL, status & ~O_NONBLOCK) == -1)
-	    fatal(2, "Can't set file mode flags on stdin: %m");
-	
 	return (-1);
     }
 }
@@ -1284,6 +1287,7 @@ int put_string(register char *s)
 		return 0;
 	    break;
 	}
+	checksigs();
     }
 
     alarm(0);
@@ -1320,6 +1324,7 @@ int echo_stderr(int n)
 	need_lf = 1;
 	break;
     }
+    checksigs();
     return ret;
 }