[OpenWrt-Devel] [PATCH V2 procd] service: start apps with LD_PRELOAD & lib for line buffering

[Rafał Miłecki]

Using pipe automatically switches service to block buffering which kind
of breaks our logging. We won't get anything from FD until the buffer
gets filled fully or the service exits. This makes log messages appear
with an unwanted delay.
This adds a tiny libsetlbf.so that switches stdout to line buffering and
uses this lib for every logging-enabled service started by procd.

Signed-off-by: Rafał Miłecki <zajec5 at gmail.com>
---
V2: Use strncat for safety
    Use line buffering instead of no buffering
---
 CMakeLists.txt     |  7 +++++++
 service/instance.c | 19 ++++++++++++++++++-
 service/setlbf.c   |  6 ++++++
 3 files changed, 31 insertions(+), 1 deletion(-)
 create mode 100644 service/setlbf.c

diff --git a/CMakeLists.txt b/CMakeLists.txt
index dfa9413..6af17a3 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -10,6 +10,13 @@ IF(APPLE)
   LINK_DIRECTORIES(/opt/local/lib)
 ENDIF()
 
+
+ADD_LIBRARY(setlbf SHARED service/setlbf.c)
+INSTALL(TARGETS setlbf
+	LIBRARY DESTINATION lib
+)
+
+
 SET(SOURCES procd.c signal.c watchdog.c state.c	inittab.c rcS.c	ubus.c system.c
 	service/service.c service/instance.c service/validate.c service/trigger.c service/watch.c
 	plug/coldplug.c plug/hotplug.c utils/utils.c)
diff --git a/service/instance.c b/service/instance.c
index 35b2def..d0d4251 100644
--- a/service/instance.c
+++ b/service/instance.c
@@ -224,6 +224,8 @@ instance_run(struct service_instance *in, int _stdout, int _stderr)
 	struct blobmsg_list_node *var;
 	struct blob_attr *cur;
 	char **argv;
+	char ld_preload[64] = {}; /* Has to be big enough for all cases */
+	size_t ld_preload_space = sizeof(ld_preload) - 1;
 	int argc = 1; /* NULL terminated */
 	int rem, _stdin;
 
@@ -238,8 +240,23 @@ instance_run(struct service_instance *in, int _stdout, int _stderr)
 
 	if (!in->trace && !in->has_jail && in->seccomp) {
 		setenv("SECCOMP_FILE", in->seccomp, 1);
-		setenv("LD_PRELOAD", "/lib/libpreload-seccomp.so", 1);
+		if (ld_preload[0] && ld_preload_space > 0) {
+			strcat(ld_preload, ":");
+			ld_preload_space--;
+		}
+		strncat(ld_preload, "/lib/libpreload-seccomp.so", ld_preload_space);
+		ld_preload_space = sizeof(ld_preload) - strlen(ld_preload) - 1;
+	}
+	if (_stdout >= 0) {
+		if (ld_preload[0] && ld_preload_space > 0) {
+			strcat(ld_preload, ":");
+			ld_preload_space--;
+		}
+		strncat(ld_preload, "/lib/libsetlbf.so", ld_preload_space);
+		ld_preload_space = sizeof(ld_preload) - strlen(ld_preload) - 1;
 	}
+	if (ld_preload[0])
+		setenv("LD_PRELOAD", ld_preload, 1);
 
 	blobmsg_list_for_each(&in->limits, var)
 		instance_limits(blobmsg_name(var->data), blobmsg_data(var->data));
diff --git a/service/setlbf.c b/service/setlbf.c
new file mode 100644
index 0000000..df00366
--- /dev/null
+++ b/service/setlbf.c
@@ -0,0 +1,6 @@
+#include <stdio.h>
+
+static void __attribute__((constructor)) setlbf(void)
+{
+	setlinebuf(stdout);
+}
-- 
1.8.4.5
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