Fix the device cache to cope reasonably safely with device name changes.

This should be a rare occurrence so the aim is to recover if it's
straightforward to do so, otherwise just to abort the operation.
If people *knowingly* change device names, they should always run vgscan
afterwards.

A few bytes of memory gets leaked inside a pool each time an alias
has to be discarded - it's not worth restructuring the code to reuse it.

More of LVM2 needs updating to pass device objects (or uuids) about
instead of pathnames so that resolution of pathname->object only happens
once per operation.

dev_cache_get() should now always return the *current* device at the path given

dev_name_confirmed() replaces dev_name() whenever it's important to
know that name for the device is still current (ie when opening it).
If the cache doesn't know a current name, the function fails.

dev_open() guarantees that the file descriptor returned is for the dev_t
of the device structure it was passed.

lib/device/dev-cache.c

@@ -19,6 +19,7 @@
 #include <unistd.h>
 #include <sys/param.h>
 #include <dirent.h>
+#include <linux/kdev_t.h>
 
 /*
  * FIXME: really need to seperate names from the devices since
@@ -303,10 +304,56 @@ int dev_cache_add_dir(const char *path)
 	return 1;
 }
 
+/* Check cached device name is still valid before returning it */
+/* This should be a rare occurrence */
+/* FIXME Make rest of code pass/cache struct device instead of dev_name */
+const char *dev_name_confirmed(struct device *dev)
+{
+	struct stat buf;
+	char *name;
+	int r;
+
+	while ((r = stat(name = list_item(dev->aliases.n, 
+					   struct str_list)->str, &buf)) || 
+	       (buf.st_rdev != dev->dev)) {
+		if (r < 0)
+			log_sys_error("stat", name);
+		log_error("Path %s no longer valid for device(%d,%d)",
+			  name, (int) MAJOR(dev->dev), (int) MINOR(dev->dev));
+
+		/* Remove the incorrect hash entry */
+		hash_remove(_cache.names, name);
+
+		/* Leave list alone if there isn't an alternative name */
+		/* so dev_name will always find something to return. */
+		/* Otherwise add the name to the correct device. */
+		if (list_size(&dev->aliases) > 1) {
+			list_del(dev->aliases.n);
+			if (!r)
+				_insert(name, 0);
+			continue;
+		}
+
+		log_error("Aborting - please provide new pathname for what "
+			  "used to be %s", name);
+		return NULL;
+	}
+
+	return dev_name(dev);
+}
+
+
 struct device *dev_cache_get(const char *name, struct dev_filter *f)
 {
+	struct stat buf;
 	struct device *d = (struct device *) hash_lookup(_cache.names, name);
 
+	/* If the entry's wrong, remove it */
+	if (d && (stat(name, &buf) || (buf.st_rdev != d->dev))) {
+		hash_remove(_cache.names, name);
+		d = NULL;
+	}
+
 	if (!d) {
 		_insert(name, 0);
 		d = (struct device *) hash_lookup(_cache.names, name);
@@ -353,5 +400,3 @@ struct device *dev_iter_get(struct dev_iter *iter)
 	return NULL;
 }
 
-
-

lib/device/dev-io.c

@@ -65,7 +65,12 @@ int dev_get_sectsize(struct device *dev, uint32_t *size)
 int dev_open(struct device *dev, int flags)
 {
 	struct stat buf;
-	const char *name = dev_name(dev);
+	const char *name = dev_name_confirmed(dev);
+
+	if (!name) {
+		stack;
+		return 0;
+	}
 
 	if (dev->fd >= 0) {
 		log_error("Device '%s' has already been opened", name);

lib/device/device.h

@@ -47,6 +47,8 @@ static inline const char *dev_name(struct device *dev) {
 	return list_item(dev->aliases.n, struct str_list)->str;
 }
 
+/* Return a valid device name from the alias list; NULL otherwise */
+const char *dev_name_confirmed(struct device *dev);
 
 static inline int is_lvm_partition(const char *name) {
 	return 1;