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[PATCH v4 4/4] terminal: restore settings on SIGTSTP

From
PWPhillip Wood <phillip.wood123@gmail.com>
Date
Mar 16, 2022, 18:54 UTC
Message-ID
<20220316185405.29551-5-phillip.wood123@gmail.com>
In-Reply-To
<20220316185405.29551-1-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 | 132 ++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 129 insertions(+), 3 deletions(-)
diff --git a/compat/terminal.c b/compat/terminal.c
index 2ae8a6f13e..7db330c52d 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"
@@ -24,6 +24,102 @@ static volatile sig_atomic_t term_fd_needs_closing;
 static int term_fd = -1;
 static struct termios old_term;
 
+static const char *background_resume_msg;
+static const char *restore_error_msg;
+static volatile sig_atomic_t ttou_received;
+
+/* async safe error function for use by signal handlers. */
+static void write_err(const char *msg)
+{
+	write_in_full(2, "error: ", strlen("error: "));
+	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_err(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_err(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_err(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_err(restore_error_msg);
+	sigaction(SIGTTOU, &old_sa, NULL);
+ out:
+	errno = saved_errno;
+}
+
+static void reset_job_signals(void)
+{
+	if (restore_error_msg) {
+		signal(SIGTTIN, SIG_DFL);
+		signal(SIGTTOU, SIG_DFL);
+		signal(SIGTSTP, SIG_DFL);
+		restore_error_msg = NULL;
+		background_resume_msg = NULL;
+	}
+}
+
 static void close_term_fd(void)
 {
 	if (term_fd_needs_closing)
@@ -40,10 +136,13 @@ void restore_term(void)
 	tcsetattr(term_fd, TCSAFLUSH, &old_term);
 	close_term_fd();
 	sigchain_pop_common();
+	reset_job_signals();
 }
 
 int save_term(enum save_term_flags flags)
 {
+	struct sigaction sa;
+
 	if (term_fd < 0)
 		term_fd = ((flags & SAVE_TERM_STDIN)
 			   ? 0
@@ -56,6 +155,26 @@ int save_term(enum save_term_flags flags)
 		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 = _("cannot resume in the background, please use 'fg' to resume");
+	restore_error_msg = _("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;
 }
@@ -78,6 +197,7 @@ static int disable_bits(enum save_term_flags flags, tcflag_t bits)
 		return 0;
 
 	sigchain_pop_common();
+	reset_job_signals();
 	close_term_fd();
 	return -1;
 }
@@ -102,6 +222,7 @@ 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;
@@ -111,9 +232,14 @@ static int getchar_with_timeout(int timeout)
 	FD_ZERO(&readfds);
 	FD_SET(0, &readfds);
 	res = select(1, &readfds, NULL, NULL, tvp);
-	if (res <= 0)
+	if (!res)
 		return EOF;
-
+	if (res < 0) {
+		if (errno == EINTR)
+			goto again;
+		else
+			return EOF;
+	}
 	return getchar();
 }
 

-- 
2.35.1
Previous: Phillip Wood
Message 53 of 53 in “builtin add -p: hopefully final readkey fixes”
  1. 0/4 builtin add -p: hopefully final readkey fixesPhillip Wood, Mar 4, 2022
  2. 1/4 terminal: use flags for save_term()Phillip Wood, Mar 4, 2022
  3. Ramsay JonesMar 4, 2022
  4. Phillip WoodMar 7, 2022
  5. Ramsay JonesMar 7, 2022
  6. Phillip WoodMar 8, 2022
  7. Ævar Arnfjörð BjarmasonMar 5, 2022
  8. Phillip WoodMar 7, 2022
  9. Ævar Arnfjörð BjarmasonMar 7, 2022
  10. 3/4 terminal: work around macos poll() bugPhillip Wood, Mar 4, 2022
  11. 2/4 terminal: don't assume stdin is /dev/ttyPhillip Wood, Mar 4, 2022
  12. Ramsay JonesMar 4, 2022
  13. 4/4 terminal: restore settings on SIGTSTPPhillip Wood, Mar 4, 2022
  14. Ævar Arnfjörð BjarmasonMar 5, 2022
  15. Phillip WoodMar 7, 2022
  16. Ævar Arnfjörð BjarmasonMar 7, 2022
  17. Phillip WoodMar 7, 2022
  18. Ævar Arnfjörð BjarmasonMar 7, 2022
  19. Phillip WoodMar 8, 2022
  20. Johannes SchindelinMar 9, 2022
  21. Phillip WoodMar 10, 2022
  22. 0/4 builtin add -p: hopefully final readkey fixesPhillip Wood, Mar 9, 2022
  23. 1/4 terminal: use flags for save_term()Phillip Wood, Mar 9, 2022
  24. Carlo ArenasMar 11, 2022
  25. Phillip WoodMar 14, 2022
  26. 2/4 terminal: don't assume stdin is /dev/ttyPhillip Wood, Mar 9, 2022
  27. 3/4 terminal: work around macos poll() bugPhillip Wood, Mar 9, 2022
  28. Ævar Arnfjörð BjarmasonMar 10, 2022
  29. Phillip WoodMar 10, 2022
  30. Junio C HamanoMar 10, 2022
  31. 4/4 terminal: restore settings on SIGTSTPPhillip Wood, Mar 9, 2022
  32. Junio C HamanoMar 9, 2022
  33. Junio C HamanoMar 9, 2022
  34. Phillip WoodMar 10, 2022
  35. Phillip WoodMar 10, 2022
  36. Junio C HamanoMar 10, 2022
  37. Johannes SchindelinMar 10, 2022
  38. Phillip WoodMar 10, 2022
  39. 0/4 builtin add -p: hopefully final readkey fixesPhillip Wood, Mar 15, 2022
  40. 2/4 terminal: don't assume stdin is /dev/ttyPhillip Wood, Mar 15, 2022
  41. Junio C HamanoMar 15, 2022
  42. rsbecker@nexbridge.comMar 15, 2022
  43. Junio C HamanoMar 15, 2022
  44. rsbecker@nexbridge.comMar 15, 2022
  45. 1/4 terminal: use flags for save_term()Phillip Wood, Mar 15, 2022
  46. 4/4 terminal: restore settings on SIGTSTPPhillip Wood, Mar 15, 2022
  47. Junio C HamanoMar 15, 2022
  48. 3/4 terminal: work around macos poll() bugPhillip Wood, Mar 15, 2022
  49. 0/4 builtin add -p: hopefully final readkey fixesPhillip Wood, Mar 16, 2022
  50. 1/4 terminal: use flags for save_term()Phillip Wood, Mar 16, 2022
  51. 2/4 terminal: don't assume stdin is /dev/ttyPhillip Wood, Mar 16, 2022
  52. 3/4 terminal: work around macos poll() bugPhillip Wood, Mar 16, 2022
  53. 4/4 terminal: restore settings on SIGTSTPPhillip Wood, Mar 16, 2022

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