getty-generator: verify ttys before we make use of them

The ttyS[0-3] devices are weird. They may be enumerated, but when one
actually tries to open and use them they return EIO, because they don't
actually exist. Because they may be enumerated they may be specified on
the kernel command line as console=. And some people do that as default.
As response to that we'll spawn a getty on the tty that will quickly
fail, and we retry a couple of time before giving up. That is quite
noisy.

With this new change we will validate all serial terminals configured
with console= on the kernel cmdline before adding gettys on them, and
remove the invalid ones. THis should remove the noise later on.

This should make Eric Paris happy!

src/getty-generator/getty-generator.c

@@ -22,6 +22,7 @@
 #include <string.h>
 #include <errno.h>
 #include <unistd.h>
+#include <fcntl.h>
 
 #include "log.h"
 #include "util.h"
@@ -92,6 +93,30 @@ static int add_container_getty(const char *tty) {
         return add_symlink("container-getty@.service", n);
 }
 
+static int verify_tty(const char *name) {
+        _cleanup_close_ int fd = -1;
+        const char *p;
+
+        /* Some TTYs are weird and have been enumerated but don't work
+         * when you try to use them, such as classic ttyS0 and
+         * friends. Let's check that and open the device and run
+         * isatty() on it. */
+
+        p = strappenda("/dev/", name);
+
+        /* O_NONBLOCK is essential here, to make sure we don't wait
+         * for DCD */
+        fd = open(p, O_RDWR|O_NONBLOCK|O_NOCTTY|O_CLOEXEC|O_NOFOLLOW);
+        if (fd < 0)
+                return -errno;
+
+        errno = 0;
+        if (isatty(fd) <= 0)
+                return errno ? -errno : -EIO;
+
+        return 0;
+}
+
 int main(int argc, char *argv[]) {
 
         static const char virtualization_consoles[] =
@@ -180,6 +205,9 @@ int main(int argc, char *argv[]) {
                         if (isempty(tty) || tty_is_vc(tty))
                                 continue;
 
+                        if (verify_tty(tty) < 0)
+                                continue;
+
                         /* We assume that gettys on virtual terminals are
                          * started via manual configuration and do this magic
                          * only for non-VC terminals. */