logging: Change stderr + file target to use non-blocking write

So far, we used blocking, buffered fwrite() to write to stderr
and file targets.  This causes problems if there are [slow] consumers
causing delays, such as gnome-terminal (when the program is started
interactively) or systemd/journald (where we observe 64..128ms blocks on
stderr).

This patch introduces stderr/file based logging via write_queue
and osmo_select_main(), i.e. switch from glibc-buffered, blocking
to internally buffered, non-blocking writes.

* when osmo_stderr_target is created via application.c, we create it
  in blocking stream mode for backwards compatibility, particularly
  for [smaller] programs that don't use osmo_select_main()

* when the VTY code encounters 'log stderr' or 'log file FILENAME',
  we switch that respective target to non-blocking write-queue mode,
  as this means the application is in fact using osmo_select_main()

* The config file can now state 'log stderr blocking-io' or
  'log file FILENAME blocking-io' to explicitly enforce using blocking
  stream based I/O

* The application can at any time use API functions to switch either way

Closes: OS#4311
Change-Id: Ia58fd78535c41b3da3aeb7733aadc785ace610da

1 files changed, 25 insertions, 7 deletions

diff --git a/src/vty/logging_vty.c b/src/vty/logging_vty.c
index 6a7a8f44..76067466 100644
--- a/src/vty/logging_vty.c
+++ b/src/vty/logging_vty.c
@@ -819,8 +819,9 @@ DEFUN(cfg_log_gsmtap, cfg_log_gsmtap_cmd,
 }
 
 DEFUN(cfg_log_stderr, cfg_log_stderr_cmd,
-	"log stderr",
-	LOG_STR "Logging via STDERR of the process\n")
+	"log stderr [blocking-io]",
+	LOG_STR "Logging via STDERR of the process\n"
+	"Use blocking, synchronous I/O\n")
 {
 	struct log_target *tgt;
 
@@ -836,6 +837,11 @@ DEFUN(cfg_log_stderr, cfg_log_stderr_cmd,
 		log_add_target(tgt);
 	}
 
+	if (argc > 0 && !strcmp(argv[0], "blocking-io"))
+		log_target_file_switch_to_stream(tgt);
+	else
+		log_target_file_switch_to_wqueue(tgt);
+
 	vty->index = tgt;
 	vty->node = CFG_LOG_NODE;
 
@@ -862,8 +868,9 @@ DEFUN(cfg_no_log_stderr, cfg_no_log_stderr_cmd,
 }
 
 DEFUN(cfg_log_file, cfg_log_file_cmd,
-	"log file .FILENAME",
-	LOG_STR "Logging to text file\n" "Filename\n")
+	"log file FILENAME [blocking-io]",
+	LOG_STR "Logging to text file\n" "Filename\n"
+	"Use blocking, synchronous I/O\n")
 {
 	const char *fname = argv[0];
 	struct log_target *tgt;
@@ -880,6 +887,11 @@ DEFUN(cfg_log_file, cfg_log_file_cmd,
 		log_add_target(tgt);
 	}
 
+	if (argc > 1 && !strcmp(argv[1], "blocking-io"))
+		log_target_file_switch_to_stream(tgt);
+	else
+		log_target_file_switch_to_wqueue(tgt);
+
 	vty->index = tgt;
 	vty->node = CFG_LOG_NODE;
 
@@ -888,7 +900,7 @@ DEFUN(cfg_log_file, cfg_log_file_cmd,
 
 
 DEFUN(cfg_no_log_file, cfg_no_log_file_cmd,
-	"no log file .FILENAME",
+	"no log file FILENAME",
 	NO_STR LOG_STR "Logging to text file\n" "Filename\n")
 {
 	const char *fname = argv[0];
@@ -963,7 +975,10 @@ static int config_write_log_single(struct vty *vty, struct log_target *tgt)
 		return 1;
 		break;
 	case LOG_TGT_TYPE_STDERR:
-		vty_out(vty, "log stderr%s", VTY_NEWLINE);
+		if (tgt->tgt_file.wqueue)
+			vty_out(vty, "log stderr%s", VTY_NEWLINE);
+		else
+			vty_out(vty, "log stderr blocking-io%s", VTY_NEWLINE);
 		break;
 	case LOG_TGT_TYPE_SYSLOG:
 #ifdef HAVE_SYSLOG_H
@@ -974,7 +989,10 @@ static int config_write_log_single(struct vty *vty, struct log_target *tgt)
 #endif
 		break;
 	case LOG_TGT_TYPE_FILE:
-		vty_out(vty, "log file %s%s", tgt->tgt_file.fname, VTY_NEWLINE);
+		if (tgt->tgt_file.wqueue)
+			vty_out(vty, "log file %s%s", tgt->tgt_file.fname, VTY_NEWLINE);
+		else
+			vty_out(vty, "log file %s blocking-io%s", tgt->tgt_file.fname, VTY_NEWLINE);
 		break;
 	case LOG_TGT_TYPE_STRRB:
 		vty_out(vty, "log alarms %zu%s",