terminal-util: when resetting terminals, don't wait for carrier

In case of non-CLOCAL lines (i.e. those with carrier detect configured)
we shouldnt wait for a carrier if all we try to do is reset the TTY.
Hence, whenever we open such a TTY pass O_NONBLOCK.

Note that we continue to open ttys we intend to write to without
O_ONBLOCK, we only add it in cases we invoke ioctl()s or other terminal
operations without reading or writing to the device.

Fixes #835.

Cherry-picked from: 0a8b555
Resolves: #1266745

src/shared/util.c

@@ -1713,7 +1713,7 @@ bool fstype_is_network(const char *fstype) {
 int chvt(int vt) {
         _cleanup_close_ int fd;
 
-        fd = open_terminal("/dev/tty0", O_RDWR|O_NOCTTY|O_CLOEXEC);
+        fd = open_terminal("/dev/tty0", O_RDWR|O_NOCTTY|O_CLOEXEC|O_NONBLOCK);
         if (fd < 0)
                 return -errno;
 
@@ -1953,7 +1953,11 @@ int reset_terminal_fd(int fd, bool switch_to_text) {
 int reset_terminal(const char *name) {
         _cleanup_close_ int fd = -1;
 
-        fd = open_terminal(name, O_RDWR|O_NOCTTY|O_CLOEXEC);
+        /* We open the terminal with O_NONBLOCK here, to ensure we
+         * don't block on carrier if this is a terminal with carrier
+         * configured. */
+
+        fd = open_terminal(name, O_RDWR|O_NOCTTY|O_CLOEXEC|O_NONBLOCK);
         if (fd < 0)
                 return fd;
 
@@ -2204,7 +2208,7 @@ int release_terminal(void) {
         struct sigaction sa_old;
         int r = 0;
 
-        fd = open("/dev/tty", O_RDWR|O_NOCTTY|O_NDELAY|O_CLOEXEC);
+        fd = open("/dev/tty", O_RDWR|O_NOCTTY|O_CLOEXEC|O_NONBLOCK);
         if (fd < 0)
                 return -errno;
 
@@ -4405,7 +4409,7 @@ int terminal_vhangup_fd(int fd) {
 int terminal_vhangup(const char *name) {
         _cleanup_close_ int fd;
 
-        fd = open_terminal(name, O_RDWR|O_NOCTTY|O_CLOEXEC);
+        fd = open_terminal(name, O_RDWR|O_NOCTTY|O_CLOEXEC|O_NONBLOCK);
         if (fd < 0)
                 return fd;
 
@@ -4452,7 +4456,7 @@ int vt_disallocate(const char *name) {
                 return -EINVAL;
 
         /* Try to deallocate */
-        fd = open_terminal("/dev/tty0", O_RDWR|O_NOCTTY|O_CLOEXEC);
+        fd = open_terminal("/dev/tty0", O_RDWR|O_NOCTTY|O_CLOEXEC|O_NONBLOCK);
         if (fd < 0)
                 return fd;