lvmlockd: fix metadata validation when rescanning VG

When rescanning a VG from disk, the metadata read from
each PV was compared as a sanity check.  The comparison
is done by exporting the vg metadata from each dev to
a config tree, and then comparing the config trees.
The function to create the config tree inserts
extraneous information along with the actual VG metadata.
This extra info includes creation_time.  The config
trees for two devs can easily be created one second
apart in which case the different creation_times would
cause the metadata comparison to fail.  The fix is to
exclude the extraneous info from the metadata comparison.

1 files changed, 30 insertions, 1 deletions

diff --git a/lib/cache/lvmetad.c b/lib/cache/lvmetad.c
index b3cd1bf22..727d3d399 100644
--- a/lib/cache/lvmetad.c
+++ b/lib/cache/lvmetad.c
@@ -1918,15 +1918,44 @@ scan_more:
 			vgmeta_ret = vgmeta;
 			save_dev = devl->dev;
 		} else {
-			if (compare_config(vgmeta_ret->root, vgmeta->root)) {
+			struct dm_config_node *meta1 = vgmeta_ret->root;
+			struct dm_config_node *meta2 = vgmeta->root;
+			struct dm_config_node *sib1 = meta1->sib;
+			struct dm_config_node *sib2 = meta2->sib;
+
+			/*
+			 * Do not compare the extraneous data that
+			 * export_vg_to_config_tree() inserts next to the
+			 * actual VG metadata.  This includes creation_time
+			 * which may not match since it is generated separately
+			 * for each call to create the config tree.
+			 *
+			 * We're saving the sibling pointer and restoring it
+			 * after the compare because we're unsure if anything
+			 * later might want it.
+			 *
+			 * FIXME: make it clearer what we're doing here, e.g.
+			 * pass a parameter to export_vg_to_config_tree()
+			 * telling it to skip the extraneous data, or something.
+			 * It's very non-obvious that setting sib=NULL does that.
+			 */
+			meta1->sib = NULL;
+			meta2->sib = NULL;
+
+			if (compare_config(meta1, meta2)) {
 				log_error("VG %s metadata comparison failed for device %s vs %s",
 					  vg->name, dev_name(devl->dev), save_dev ? dev_name(save_dev) : "none");
 				_log_debug_inequality(vg->name, vgmeta_ret->root, vgmeta->root);
+
+				meta1->sib = sib1;
+				meta2->sib = sib2;
 				dm_config_destroy(vgmeta);
 				dm_config_destroy(vgmeta_ret);
 				release_vg(baton.vg);
 				return NULL;
 			}
+			meta1->sib = sib1;
+			meta2->sib = sib2;
 			dm_config_destroy(vgmeta);
 		}