terminal: pop signal handler when terminal is restored

When disable_bits() changes the terminal attributes it uses
sigchain_push_common() to restore the terminal if a signal is received
before restore_term() is called. However there is no corresponding
call to sigchain_pop_common() when the settings are restored so the
signal handler is left on the sigchain stack. This leaves the stack
unbalanced so code such as

sigchain_push_common(my_handler);
...
read_key_without_echo(...);
...
sigchain_pop_common();

pops the handler pushed by disable_bits() rather than the one it
intended to. Additionally "git add -p" changes the terminal settings
every time it reads a key press so the stack can grow significantly.

In order to fix this save_term() now sets up the signal handler so
restore_term() can unconditionally call sigchain_pop_common(). There
are no callers of save_term() outside of terminal.c as the only
external caller was removed by e3f7e01b50 ("Revert "editor: save and
reset terminal after calling EDITOR"", 2021-11-22). Any future callers
of save_term() should benefit from having the signal handler set up
for them.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/terminal.c

@@ -11,7 +11,7 @@
 static void restore_term_on_signal(int sig)
 {
 	restore_term();
-	sigchain_pop(sig);
+	/* restore_term calls sigchain_pop_common */
 	raise(sig);
 }
 
@@ -31,14 +31,20 @@ void restore_term(void)
 	tcsetattr(term_fd, TCSAFLUSH, &old_term);
 	close(term_fd);
 	term_fd = -1;
+	sigchain_pop_common();
 }
 
 int save_term(int full_duplex)
 {
 	if (term_fd < 0)
 		term_fd = open("/dev/tty", O_RDWR);
+	if (term_fd < 0)
+		return -1;
+	if (tcgetattr(term_fd, &old_term) < 0)
+		return -1;
+	sigchain_push_common(restore_term_on_signal);
 
-	return (term_fd < 0) ? -1 : tcgetattr(term_fd, &old_term);
+	return 0;
 }
 
 static int disable_bits(tcflag_t bits)
@@ -49,12 +55,12 @@ static int disable_bits(tcflag_t bits)
 		goto error;
 
 	t = old_term;
-	sigchain_push_common(restore_term_on_signal);
 
 	t.c_lflag &= ~bits;
 	if (!tcsetattr(term_fd, TCSAFLUSH, &t))
 		return 0;
 
+	sigchain_pop_common();
 error:
 	close(term_fd);
 	term_fd = -1;
@@ -100,6 +106,8 @@ void restore_term(void)
 		return;
 	}
 
+	sigchain_pop_common();
+
 	if (hconin == INVALID_HANDLE_VALUE)
 		return;
 
@@ -134,6 +142,7 @@ int save_term(int full_duplex)
 
 	GetConsoleMode(hconin, &cmode_in);
 	use_stty = 0;
+	sigchain_push_common(restore_term_on_signal);
 	return 0;
 error:
 	CloseHandle(hconin);
@@ -177,10 +186,10 @@ static int disable_bits(DWORD bits)
 	if (save_term(0) < 0)
 		return -1;
 
-	sigchain_push_common(restore_term_on_signal);
 	if (!SetConsoleMode(hconin, cmode_in & ~bits)) {
 		CloseHandle(hconin);
 		hconin = INVALID_HANDLE_VALUE;
+		sigchain_pop_common();
 		return -1;
 	}
 

compat/terminal.h

@@ -1,7 +1,15 @@
 #ifndef COMPAT_TERMINAL_H
 #define COMPAT_TERMINAL_H
 
+/*
+ * Save the terminal attributes so they can be restored later by a
+ * call to restore_term(). Note that every successful call to
+ * save_term() must be matched by a call to restore_term() even if the
+ * attributes have not been changed. Returns 0 on success, -1 on
+ * failure.
+ */
 int save_term(int full_duplex);
+/* Restore the terminal attributes that were saved with save_term() */
 void restore_term(void);
 
 char *git_terminal_prompt(const char *prompt, int echo);