ptyfwd: before deciding that a pty is fully drained, ask the kernel again

Apparently there's no guarantee that EPOLLIN is immediately propagated
from a pty slave to the master when data is written to it, hence it's
not sufficient to check EPOLLIN to decide whether the pty device is
drained.

Let's fix this by asking the kernel directly through SIOCINQ + SIOCOUTQ,
if there's anything buffered left.

Fixes: #7531

src/shared/ptyfwd.c

@@ -171,6 +171,30 @@ static bool ignore_vhangup(PTYForward *f) {
         return false;
 }
 
+static bool drained(PTYForward *f) {
+        int q = 0;
+
+        assert(f);
+
+        if (f->out_buffer_full > 0)
+                return false;
+
+        if (f->master_readable)
+                return false;
+
+        if (ioctl(f->master, TIOCINQ, &q) < 0)
+                log_debug_errno(errno, "TIOCINQ failed on master: %m");
+        else if (q > 0)
+                return false;
+
+        if (ioctl(f->master, TIOCOUTQ, &q) < 0)
+                log_debug_errno(errno, "TIOCOUTQ failed on master: %m");
+        else if (q > 0)
+                return false;
+
+        return true;
+}
+
 static int shovel(PTYForward *f) {
         ssize_t k;
 
@@ -306,7 +330,7 @@ static int shovel(PTYForward *f) {
 
         /* If we were asked to drain, and there's nothing more to handle from the master, then call the callback
          * too. */
-        if (f->drain && f->out_buffer_full == 0 && !f->master_readable)
+        if (f->drain && drained(f))
                 return pty_forward_done(f, 0);
 
         return 0;
@@ -547,6 +571,5 @@ bool pty_forward_drain(PTYForward *f) {
          */
 
         f->drain = true;
-
-        return f->out_buffer_full == 0 && !f->master_readable;
+        return drained(f);
 }