[dm-devel] [PATCH] libmultipath/discovery: read sysfs files uncached

[Sebastian Riemer]

The libudev function udev_device_get_sysattr_value() reads sysfs
attributes cached. This is useless for checking a device state.
There we want to see if it changes.

Unfortunately, libudev doesn't provide an uncached variant. This
is why we have to reimplement the functionality and some libudev
internal functions here.

Cc: Christophe Varoqui <christophe.varoqui at opensvc.com>
Cc: Bart Van Assche <bvanassche at acm.org>
Cc: Hannes Reinecke <hare at suse.de>
Signed-off-by: Sebastian Riemer <sebastian.riemer at profitbricks.com>
---
 libmultipath/discovery.c |   78 +++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 77 insertions(+), 1 deletion(-)

diff --git a/libmultipath/discovery.c b/libmultipath/discovery.c
index 0b5fd1d..1cb1f15 100644
--- a/libmultipath/discovery.c
+++ b/libmultipath/discovery.c
@@ -132,6 +132,82 @@ path_discovery (vector pathvec, struct config * conf, int flag)
 	return r;
 }
 
+/* helpers for get_udev_device_sysattr_value() */
+static size_t
+strpcpy(char **dest, size_t size, const char *src)
+{
+	size_t len;
+
+	len = strlen(src);
+	if (len >= size) {
+		if (size > 1)
+			*dest = mempcpy(*dest, src, size-1);
+		size = 0;
+	} else {
+		if (len > 0) {
+			*dest = mempcpy(*dest, src, len);
+			size -= len;
+		}
+	}
+	*dest[0] = '\0';
+	return size;
+}
+
+static size_t
+strscpyl(char *dest, size_t size, const char *src, ...)
+{
+	va_list va;
+	char *s;
+
+	va_start(va, src);
+	s = dest;
+	do {
+		size = strpcpy(&s, size, src);
+		src = va_arg(va, char *);
+	} while (src != NULL);
+	va_end(va);
+
+	return size;
+}
+
+static void
+util_remove_trailing_chars(char *path, char c)
+{
+	size_t len;
+
+	if (path == NULL)
+		return;
+	len = strlen(path);
+	while (len > 0 && path[len-1] == c)
+		path[--len] = '\0';
+}
+
+/* like udev_device_get_sysattr_value() but uncached */
+static const char *
+get_udev_device_sysattr_value(struct udev_device *udev,
+			      const char *sysattr)
+{
+	const char *val = NULL;
+	char path[1024];
+	char value[4096];
+	int fd;
+	ssize_t size;
+
+	strscpyl(path, sizeof(path), udev_device_get_syspath(udev),
+		 "/", sysattr, NULL);
+	fd = open(path, O_RDONLY|O_CLOEXEC);
+	if (fd >= 0) {
+		size = read(fd, value, sizeof(value));
+		close(fd);
+		if (size >= 0 && size < sizeof(value)) {
+			value[size] = '\0';
+			util_remove_trailing_chars(value, '\n');
+			val = value;
+		}
+	}
+	return val;
+}
+
 #define declare_sysfs_get_str(fname)					\
 extern int								\
 sysfs_get_##fname (struct udev_device * udev, char * buff, size_t len)	\
@@ -141,7 +217,7 @@ sysfs_get_##fname (struct udev_device * udev, char * buff, size_t len)	\
 									\
 	devname = udev_device_get_sysname(udev);			\
 									\
-	attr = udev_device_get_sysattr_value(udev, #fname);		\
+	attr = get_udev_device_sysattr_value(udev, #fname);		\
 	if (!attr) {							\
 		condlog(3, "%s: attribute %s not found in sysfs",	\
 			devname, #fname);				\
-- 
1.7.10.4