[PATCH] sandbox: also restore terminal settings when killed by SIGINT

[Rasmus Villemoes]

Hitting Ctrl-C is a documented way to exit the sandbox, but it is not
actually equivalent to the reset command. The latter, since it follows
normal process exit, takes care to reset terminal settings and
restoring the O_NONBLOCK behaviour of stdin (and, in a terminal, that
is usually the same file description as stdout and stderr, i.e. some
/dev/pts/NN).

Failure to restore (remove) O_NONBLOCK from stdout/stderr can cause
very surprising and hard to debug problems back in the terminal. For
example, I had "make -j8" consistently failing without much
information about just exactly what went wrong, but sometimes I did
get a "echo: write error". I was at first afraid my disk was getting
bad, but then a simple "dmesg" _also_ failed with write error - so it
was writing to the terminal that was buggered. And both "make -j8" and
dmesg in another terminal window worked just fine.

So install a SIGINT handler so that if the chosen terminal
mode (cooked or raw-with-sigs) means Ctrl-C sends a SIGINT, we will
still call os_fd_restore(), then reraise the signal and die as usual
from SIGINT.

Before:

$ grep flags /proc/$$/fdinfo/1
flags:  0102002
$ ./u-boot
# hit Ctrl-C
$ grep flags /proc/$$/fdinfo/1
flags:  0106002

After:

$ grep flags /proc/$$/fdinfo/1
flags:  0102002
$ ./u-boot
# hit Ctrl-C
$ grep flags /proc/$$/fdinfo/1
flags:  0102002

Signed-off-by: Rasmus Villemoes <rasmus.villemoes at prevas.dk>
---
 arch/sandbox/cpu/os.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/sandbox/cpu/os.c b/arch/sandbox/cpu/os.c
index f7c73e3a0b..e7ec892bdf 100644
--- a/arch/sandbox/cpu/os.c
+++ b/arch/sandbox/cpu/os.c
@@ -8,6 +8,7 @@
 #include <fcntl.h>
 #include <getopt.h>
 #include <setjmp.h>
+#include <signal.h>
 #include <stdio.h>
 #include <stdint.h>
 #include <stdlib.h>
@@ -175,6 +176,13 @@ void os_fd_restore(void)
 	}
 }
 
+static void os_sigint_handler(int sig)
+{
+	os_fd_restore();
+	signal(SIGINT, SIG_DFL);
+	raise(SIGINT);
+}
+
 /* Put tty into raw mode so <tab> and <ctrl+c> work */
 void os_tty_raw(int fd, bool allow_sigs)
 {
@@ -205,6 +213,7 @@ void os_tty_raw(int fd, bool allow_sigs)
 
 	term_setup = true;
 	atexit(os_fd_restore);
+	signal(SIGINT, os_sigint_handler);
 }
 
 void *os_malloc(size_t length)
-- 
2.23.0