[PATCH 4/4] terminal: restore settings on SIGTSTP

From: Phillip Wood <phillip.wood123@gmail.com>
Date: Mar 4, 2022, 13:11 UTC
Message-ID: <20220304131126.8293-5-phillip.wood123@gmail.com>
In-Reply-To: <20220304131126.8293-1-phillip.wood123@gmail.com>

From: Phillip Wood <phillip.wood@dunelm.org.uk>

If the user suspends git while it is waiting for a keypress reset the terminal before stopping and restore the settings when git resumes. If the user tries to resume in the background print an error message (taking care to use async safe functions) before stopping again. Ideally we would reprint the prompt for the user when git resumes but this patch just restarts the read().

The signal handler is established with sigaction() rather than using sigchain_push() as this allows us to control the signal mask when the handler is invoked and ensure SA_RESTART is used to restart the read() when resuming.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
---
 compat/terminal.c | 124 ++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 120 insertions(+), 4 deletions(-)

diff --git a/compat/terminal.c b/compat/terminal.c
index 5d516ff546..79ab54c2f8 100644
--- a/compat/terminal.c
+++ b/compat/terminal.c
@@ -1,4 +1,4 @@
-#include "git-compat-util.h"
+#include "cache.h"
 #include "compat/terminal.h"
 #include "sigchain.h"
 #include "strbuf.h"
@@ -23,6 +23,89 @@ static void restore_term_on_signal(int sig)
 static int term_fd = -1;
 static struct termios old_term;
 
+static char *background_resume_msg;
+static char *restore_error_msg;
+static volatile sig_atomic_t ttou_received;
+
+static void write_msg(const char *msg)
+{
+	write_in_full(2, msg, strlen(msg));
+	write_in_full(2, "\n", 1);
+}
+
+static void print_background_resume_msg(int signo)
+{
+	int saved_errno = errno;
+	sigset_t mask;
+	struct sigaction old_sa;
+	struct sigaction sa = { .sa_handler = SIG_DFL };
+
+	ttou_received = 1;
+	write_msg(background_resume_msg);
+	sigaction(signo, &sa, &old_sa);
+	raise(signo);
+	sigemptyset(&mask);
+	sigaddset(&mask, signo);
+	sigprocmask(SIG_UNBLOCK, &mask, NULL);
+	/* Stopped here */
+	sigprocmask(SIG_BLOCK, &mask, NULL);
+	sigaction(signo, &old_sa, NULL);
+	errno = saved_errno;
+}
+
+static void restore_terminal_on_suspend(int signo)
+{
+	int saved_errno = errno;
+	int res;
+	struct termios t;
+	sigset_t mask;
+	struct sigaction old_sa;
+	struct sigaction sa = { .sa_handler = SIG_DFL };
+	int can_restore = 1;
+
+	if (tcgetattr(term_fd, &t) < 0)
+		can_restore = 0;
+
+	if (tcsetattr(term_fd, TCSAFLUSH, &old_term) < 0)
+		write_msg(restore_error_msg);
+
+	sigaction(signo, &sa, &old_sa);
+	raise(signo);
+	sigemptyset(&mask);
+	sigaddset(&mask, signo);
+	sigprocmask(SIG_UNBLOCK, &mask, NULL);
+	/* Stopped here */
+	sigprocmask(SIG_BLOCK, &mask, NULL);
+	sigaction(signo, &old_sa, NULL);
+	if (!can_restore) {
+		write_msg(restore_error_msg);
+		goto out;
+	}
+	/*
+	 * If we resume in the background then we receive SIGTTOU when calling
+	 * tcsetattr() below. Set up a handler to print an error message in that
+	 * case.
+	 */
+	sigemptyset(&mask);
+	sigaddset(&mask, SIGTTOU);
+	sa.sa_mask = old_sa.sa_mask;
+	sa.sa_handler = print_background_resume_msg;
+	sa.sa_flags = SA_RESTART;
+	sigaction(SIGTTOU, &sa, &old_sa);
+ again:
+	ttou_received = 0;
+	sigprocmask(SIG_UNBLOCK, &mask, NULL);
+	res = tcsetattr(term_fd, TCSAFLUSH, &t);
+	sigprocmask(SIG_BLOCK, &mask, NULL);
+	if (ttou_received)
+		goto again;
+	else if (res < 0)
+		write_msg(restore_error_msg);
+	sigaction(SIGTTOU, &old_sa, NULL);
+ out:
+	errno = saved_errno;
+}
+
 void restore_term(void)
 {
 	if (term_fd < 0)
@@ -32,10 +115,19 @@ void restore_term(void)
 	close(term_fd);
 	term_fd = -1;
 	sigchain_pop_common();
+	if (restore_error_msg) {
+		signal(SIGTTIN, SIG_DFL);
+		signal(SIGTTOU, SIG_DFL);
+		signal(SIGTSTP, SIG_DFL);
+		FREE_AND_NULL(restore_error_msg);
+		FREE_AND_NULL(background_resume_msg);
+	}
 }
 
 int save_term(unsigned flags)
 {
+	struct sigaction sa;
+
 	if (term_fd < 0)
 		term_fd = (flags & SAVE_TERM_STDIN) ? 0
 						    : open("/dev/tty", O_RDWR);
@@ -44,6 +136,26 @@ int save_term(unsigned flags)
 	if (tcgetattr(term_fd, &old_term) < 0)
 		return -1;
 	sigchain_push_common(restore_term_on_signal);
+	/*
+	 * If job control is disabled then the shell will have set the
+	 * disposition of SIGTSTP to SIG_IGN.
+	 */
+	sigaction(SIGTSTP, NULL, &sa);
+	if (sa.sa_handler == SIG_IGN)
+		return 0;
+
+	/* avoid calling gettext() from signal handler */
+	background_resume_msg = xstrdup(_("error: cannot resume in the background"));
+	restore_error_msg = xstrdup(_("error: cannot restore terminal settings"));
+	sa.sa_handler = restore_terminal_on_suspend;
+	sa.sa_flags = SA_RESTART;
+	sigemptyset(&sa.sa_mask);
+	sigaddset(&sa.sa_mask, SIGTSTP);
+	sigaddset(&sa.sa_mask, SIGTTIN);
+	sigaddset(&sa.sa_mask, SIGTTOU);
+	sigaction(SIGTSTP, &sa, NULL);
+	sigaction(SIGTTIN, &sa, NULL);
+	sigaction(SIGTTOU, &sa, NULL);
 
 	return 0;
 }
@@ -93,6 +205,7 @@ static int getchar_with_timeout(int timeout)
 	fd_set readfds;
 	int res;
 
+ again:
 	if (timeout >= 0) {
 		tv.tv_sec = timeout / 1000;
 		tv.tv_usec = (timeout % 1000) * 1000;
@@ -102,9 +215,12 @@ static int getchar_with_timeout(int timeout)
 	FD_ZERO(&readfds);
 	FD_SET(0, &readfds);
 	res = select(1, &readfds, NULL, NULL, tvp);
-	if (res < 0)
-		return EOF;
-
+	if (res < 0) {
+		if (errno == EINTR)
+			goto again;
+		else
+			return EOF;
+	}
 	return getchar();
 }

-- 
2.35.1