daemonize: Flush stdio on exit

In spite of a comment suggesting otherwise, os._exit() does not flush
buffered output from file descriptors before exiting the process like
os.exit() does. This means that any un-flushed output is lost in the
daemon process, in particular the traceback from any thrown exceptions,
making debugging exceptions in the daemon quite difficult.

The solution is to flush stdout and stderr before exiting.

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

1 files changed, 7 insertions, 2 deletions

diff --git a/lib/bb/daemonize.py b/lib/bb/daemonize.py
index 8300d1d0f..bf1679346 100644
--- a/lib/bb/daemonize.py
+++ b/lib/bb/daemonize.py
@@ -49,8 +49,8 @@ def createDaemon(function, logfile):
             # exit() or _exit()?
             # _exit is like exit(), but it doesn't call any functions registered
             # with atexit (and on_exit) or any registered signal handlers.  It also
-            # closes any open file descriptors.  Using exit() may cause all stdio
-            # streams to be flushed twice and any temporary files may be unexpectedly
+            # closes any open file descriptors, but doesn't flush any buffered output.
+            # Using exit() may cause all any temporary files to be unexpectedly
             # removed.  It's therefore recommended that child branches of a fork()
             # and the parent branch(es) of a daemon use _exit().
             os._exit(0)
@@ -79,4 +79,9 @@ def createDaemon(function, logfile):
         traceback.print_exc()
     finally:
         bb.event.print_ui_queue()
+        # os._exit() doesn't flush open files like os.exit() does. Manually flush
+        # stdout and stderr so that any logging output will be seen, particularly
+        # exception tracebacks.
+        sys.stdout.flush()
+        sys.stderr.flush()
         os._exit(0)