Make sure non-aborting signal handlers save and restore errno

If an async signal interrupts some function, we can have this
anti-pattern:

    /* in normal code */
    result = some_syscall (); /* fails, e.g. errno = EINVAL */

        /* interrupted by async signal handler */
        write (...); /* fails, e.g. errno = ENOBUFS */

    /* back to normal code */
    if (errno == EINVAL) /* problem! it should be but it isn't */

The solution is for signal handlers to save and restore errno.

This is unnecessary for signal handlers that can't touch errno (like
the one in dbus-launch that just sets a flag), and for signal handlers
that never return (like the one in test-utils-glib for timeouts).

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=103010
Signed-off-by: Simon McVittie <smcv@collabora.com>
Reviewed-by: Philip Withnall <withnall@endlessm.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/bus/main.c b/bus/main.c
index 0ede03f7..b3fcddd0 100644
--- a/bus/main.c
+++ b/bus/main.c
@@ -67,6 +67,10 @@ typedef enum
 static void
 signal_handler (int sig)
 {
+  /* Signal handlers that might set errno must save and restore the errno
+   * that the interrupted function might have been relying on. */
+  int saved_errno = errno;
+
   switch (sig)
     {
     case SIGHUP:
@@ -134,6 +138,8 @@ signal_handler (int sig)
        * signal, but keep -Wswitch-default happy */
       break;
     }
+
+  errno = saved_errno;
 }
 #endif /* DBUS_UNIX */