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>

diff --git a/compat/terminal.c b/compat/terminal.c
index fb8c70a625118e96dd93b791d83bbfedd71dad1f..11288cfe5c96680862c101441ffdcb4fd8755e1b 100644 (file)
--- a/compat/terminal.c
+++ b/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;
        }
 

diff --git a/compat/terminal.h b/compat/terminal.h
index e1770c575b2c5c501d3fbf3bcbe3873d288a1c92..0fb9fa147c4ff0cb4ee509ecae1d4bb5c7cc9050 100644 (file)
--- a/compat/terminal.h
+++ b/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);