Add lvm_vg_get_seqno, updating lvm.h and unit test.

Adding the ability to get the seqno is important for an application to
determine if something has changed in a VG.  Otherwise, the only way to
know is to open the VG with write permission and hold the handle.

lib/metadata/metadata-exported.h

@@ -705,6 +705,7 @@ uint32_t pv_mda_count(const pv_t *pv);
 uint64_t lv_size(const lv_t *lv);
 
 int vg_missing_pv_count(const vg_t *vg);
+uint32_t vg_seqno(const vg_t *vg);
 uint32_t vg_status(const vg_t *vg);
 uint64_t vg_size(const vg_t *vg);
 uint64_t vg_free(const vg_t *vg);

lib/metadata/metadata.c

@@ -3424,6 +3424,11 @@ uint32_t pv_mda_count(const pv_t *pv)
 	return info ? dm_list_size(&info->mdas) : UINT64_C(0);
 }
 
+uint32_t vg_seqno(const vg_t *vg)
+{
+	return vg->seqno;
+}
+
 uint32_t vg_status(const vg_t *vg)
 {
 	return vg->status;

liblvm/.exported_symbols

@@ -6,6 +6,7 @@ lvm_config_override
 lvm_pv_get_name
 lvm_pv_get_uuid
 lvm_pv_get_mda_count
+lvm_vg_get_seqno
 lvm_vg_get_name
 lvm_vg_get_uuid
 lvm_vg_get_size

liblvm/lvm.h

@@ -49,6 +49,17 @@
  * A volume group handle may be obtained with read or write permission.
  * Any attempt to change a property of a pv_t, vg_t, or lv_t without
  * obtaining write permission on the vg_t will fail with EPERM.
+ *
+ * An application first opening a VG read-only, then later wanting to change
+ * a property of an object must first close the VG and re-open with write
+ * permission. Currently liblvm provides no mechanism to determine whether
+ * the VG has changed on-disk in between these operations - this is the
+ * application's responsiblity. One way the application can ensure the VG
+ * has not changed is to save the "vg_seqno" field after opening the VG with
+ * READ permission. If the application later needs to modify the VG, it can
+ * close the VG and re-open with WRITE permission. It should then check
+ * whether the original "vg_seqno" obtained with READ permission matches
+ * the new one obtained with WRITE permission.
  */
 
 /**
@@ -469,6 +480,21 @@ int lvm_vg_reduce(vg_t *vg, const char *device);
  */
 int lvm_vg_set_extent_size(vg_t *vg, uint32_t new_size);
 
+/**
+ * Get the current metadata sequence number of a volume group.
+ *
+ * The metadata sequence number is incrented for each metadata change.
+ * Applications may use the sequence number to determine if any LVM objects
+ * have changed from a prior query.
+ *
+ * \param   vg
+ * VG handle obtained from lvm_vg_create or lvm_vg_open.
+ *
+ * \return
+ * Metadata sequence number.
+ */
+uint64_t lvm_vg_get_seqno(const vg_t *vg);
+
 /**
  * Get the current name of a volume group.
  *

liblvm/lvm_vg.c

@@ -223,6 +223,11 @@ struct dm_list *lvm_vg_list_lvs(vg_t *vg)
 	return list;
 }
 
+uint64_t lvm_vg_get_seqno(const vg_t *vg)
+{
+	return vg_seqno(vg);
+}
+
 /* FIXME: invalid handle? return INTMAX? */
 uint64_t lvm_vg_get_size(const vg_t *vg)
 {

test/api/test.c

@@ -345,10 +345,11 @@ static void _vg_close(char **argv, int argc)
 
 static void _show_one_vg(vg_t *vg)
 {
-	printf("%s (%s): size=%"PRIu64", free=%"PRIu64", #pv=%"PRIu64"\n",
+	printf("%s (%s): sz=%"PRIu64", free=%"PRIu64", #pv=%"PRIu64
+		", seq#=%"PRIu64"\n",
 		lvm_vg_get_name(vg), lvm_vg_get_uuid(vg),
 		lvm_vg_get_size(vg), lvm_vg_get_free_size(vg),
-		lvm_vg_get_pv_count(vg));
+		lvm_vg_get_pv_count(vg), lvm_vg_get_seqno(vg));
 }
 
 static void _list_open_vgs(void)