slave: user-notifications: don't block in write(3)

The main process is blocking and waiting for us to close the error
pipe (in exec(3), via O_CLOEXEC).

Thus, pts data will *not* be processed until we've exec:d the shell.

Because of this we must not write too much and block in write(3). Do
this by setting O_NONBLOCK on the pts fd (*after* dup:ing it to
stdin/stderr/sdout) and watch for EAGAIN EWOULDBLOCK.

slave.c

@@ -93,6 +93,16 @@ emit_one_notification(int fd, const struct user_notification *notif)
         write(fd, notif->text, strlen(notif->text)) < 0 ||
         write(fd, postfix, strlen(postfix)) < 0)
     {
+        if (errno == EWOULDBLOCK || errno == EAGAIN) {
+            /*
+             * The main process is blocking and waiting for us to
+             * close the error pipe. Thus, pts data will *not* be
+             * processed until we've exec:d. This means we cannot
+             * write anymore once the kernel buffer is full. Don't
+             * treat this as a fatal error.
+             */
+            return true;
+        }
         return false;
     }
 
@@ -186,6 +196,14 @@ slave_exec(int ptmx, char *argv[], int err_fd, bool login_shell,
         goto err;
     }
 
+    {
+        int flags = fcntl(pts, F_GETFL);
+        if (flags < 0)
+            goto err;
+        if (fcntl(pts, F_SETFL, flags | O_NONBLOCK) < 0)
+            goto err;
+    }
+
     if (!emit_notifications(pts, notifications))
         goto err;