nspawn: make sure our container PID 1 keeps logging to the original stderr as long...

If we log to the pty that is configured as stdin/stdout/stderr of the
container too early we risk filling it up in full before we start
processing the pty from the parent process, resulting in deadlocks.
Let's hence keep a copy of the original tty we were started on before
setting up stdin/stdout/stderr, so that we can log to it, and keep using
it as long as we can.

Since the kernel's pty internal buffer is pretty small this actually
triggered deadlocks when we debug logged at lot from nspawn's child
processes, see: https://github.com/systemd/systemd/pull/9024#issuecomment-390403674

With this change we won't use the pty at all, only the actual payload we
start will, and hence we won't deadlock on it, ever.

diff --git a/src/basic/log.c b/src/basic/log.c
index ed02a4b906c4f4fdb9213a3eaa016066c3950d8d..6fe3a8fb138eb0f5d4fc5d7c0a544ee8ec21cc95 100644 (file)
--- a/src/basic/log.c
+++ b/src/basic/log.c
@@ -1354,3 +1354,20 @@ int log_emergency_level(void) {
 
         return getpid_cached() == 1 ? LOG_EMERG : LOG_ERR;
 }
+
+int log_dup_console(void) {
+        int copy;
+
+        /* Duplicate the fd we use for fd logging if it's < 3 and use the copy from now on. This call is useful
+         * whenever we want to continue logging through the original fd, but want to rearrange stderr. */
+
+        if (console_fd >= 3)
+                return 0;
+
+        copy = fcntl(console_fd, F_DUPFD_CLOEXEC, 3);
+        if (copy < 0)
+                return -errno;
+
+        console_fd = copy;
+        return 0;
+}

diff --git a/src/basic/log.h b/src/basic/log.h
index a2d6253687d44bb741c768a8c8ef39c5ea031115..be4412007de6d0e456efb5f464c085abc24c7eba 100644 (file)
--- a/src/basic/log.h
+++ b/src/basic/log.h
@@ -293,6 +293,8 @@ void log_set_open_when_needed(bool b);
  * stderr, the console or kmsg */
 void log_set_prohibit_ipc(bool b);
 
+int log_dup_console(void);
+
 int log_syntax_internal(
                 const char *unit,
                 int level,