plugins: pidfile: tricky zebra doesn't close its pid file

This patch fixes a few really hard problems wrt PID files:

 1. Listening for IN_CREATE events means we get notified immediately by
    the kernel when someone calls open()/fopen() on a PID file.  Reading
    the contents returns 0, thank you atoi() ... so we drop IN_CREATE
    and instead look for IN_CLOSE_WRITE, there fixed it!  Not quite ...

 2. Some programs, like Zebra, and other Quagga/Frr daemons, don't
    close() their PID files after creation.  Instead they ftruncate()
    and keep them open, and locked.  Presumably to get a mechanism to
    detect already running instances -- messes life up a bit for the
    rest of us though.  So we need to read files after IN_MODIFY too.

 3. We also want to track IN_DELETE so we can deassert conditions when
    services exit gracefully and clean up their PID files

The rest fo the commit is debug instrumentation changes.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

plugins/pidfile.c

@@ -58,7 +58,7 @@ static int pidfile_add_path(struct iwatch *iw, char *path)
 		}
 	}
 
-	return iwatch_add(iw, path, IN_ONLYDIR);
+	return iwatch_add(iw, path, IN_ONLYDIR | IN_CLOSE_WRITE);
 }
 
 static void pidfile_update_conds(char *dir, char *name, uint32_t mask)
@@ -68,11 +68,11 @@ static void pidfile_update_conds(char *dir, char *name, uint32_t mask)
 	svc_t *svc;
 
 	paste(fn, sizeof(fn), dir, name);
-	_d("path: %s, mask: %08x", fn, mask);
-
 	if (fnmatch("*\\.pid", fn, 0) && fnmatch("*/pid", fn, 0))
 		return;
 
+	_d("path: %s, mask: %08x", fn, mask);
+
 	svc = svc_find_by_pidfile(fn);
 	if (!svc) {
 		_d("No matching svc for %s", fn);
@@ -82,7 +82,7 @@ static void pidfile_update_conds(char *dir, char *name, uint32_t mask)
 	_d("Found svc %s for %s with pid %d", svc->name, fn, svc->pid);
 
 	mkcond(svc, cond, sizeof(cond));
-	if (mask & (IN_CREATE | IN_ATTRIB | IN_MODIFY | IN_MOVED_TO)) {
+	if (mask & (IN_CLOSE_WRITE | IN_ATTRIB | IN_MODIFY | IN_MOVED_TO)) {
 		svc_started(svc);
 		if (svc_is_forking(svc)) {
 			pid_t pid;
@@ -174,8 +174,6 @@ static void pidfile_callback(void *arg, int fd, int events)
 		if (!ev->mask)
 			continue;
 
-		_d("name %s, event: 0x%08x", ev->name, ev->mask);
-
 		/* Find base path for this event */
 		iwp = iwatch_find_by_wd(&iw_pidfile, ev->wd);
 		if (!iwp)
@@ -186,12 +184,7 @@ static void pidfile_callback(void *arg, int fd, int events)
 			continue;
 		}
 
-		if (ev->mask & IN_DELETE) {
+		if (ev->mask & (IN_CLOSE_WRITE | IN_DELETE | IN_ATTRIB | IN_MODIFY | IN_MOVED_TO))
-			_d("pidfile %s/%s removed ...", iwp->path, ev->name);
-			continue;
-		}
-
-		if (ev->mask & (IN_CREATE | IN_ATTRIB | IN_MODIFY | IN_MOVED_TO))
 			pidfile_update_conds(iwp->path, ev->name, ev->mask);
 	}
 }

src/svc.c

@@ -415,8 +415,10 @@ svc_t *svc_find_by_pidfile(char *fn)
 	pid_t pid;
 
 	pid = pid_file_read(fn);
-	if (pid == -1)
+	if (pid <= 0) {
+		_d("pid_file_read(%s) => %d", fn, pid);
 		return NULL;
+	}
 
 	for (svc = svc_iterator(&iter, 1); svc; svc = svc_iterator(&iter, 0)) {
 		if (svc->pid != pid)