Add lvm_vg_open() to open an existing VG for reading or writing.

lvm_vg_open() calls internal vg_read() function which is the entry point
for reading an existing VG.  In addition to the mode, we include a 'flags'
parameter for future extensions.

Signed-off-by: Dave Wysochanski <dwysocha@redhat.com>

liblvm/.exported_symbols

@@ -5,6 +5,7 @@ lvm_vg_create
 lvm_vg_extend
 lvm_vg_set_extent_size
 lvm_vg_write
+lvm_vg_open
 lvm_vg_close
 lvm_vg_remove
 lvm_errno

liblvm/lvm.h

@@ -190,4 +190,27 @@ int lvm_vg_remove(vg_t *vg);
  */
 int lvm_vg_close(vg_t *vg);
 
+/**
+ * Open an existing VG.
+ *
+ * Open a VG for reading or writing.
+ *
+ * \param   libh
+ *          Handle obtained from lvm_create.
+ *
+ * \param   vgname
+ *          Name of the VG to open.
+ *
+ * \param   mode
+ *          Open mode - either "r" (read) or "w" (read/write).
+ *          Any other character results in an error with EINVAL set.
+ *
+ * \param   flags
+ *          Open flags - currently ignored.
+ *
+ * \return  non-NULL VG handle (success) or NULL (failure).
+ */
+vg_t *lvm_vg_open(lvm_t libh, const char *vgname, const char *mode,
+		  uint32_t flags);
+
 #endif /* _LIB_LVM_H */

liblvm/lvm_vg.c

@@ -12,6 +12,9 @@
  * Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
  */
 
+#include <errno.h>
+#include <string.h>
+
 #include "lib.h"
 #include "lvm.h"
 #include "toolcontext.h"
@@ -80,3 +83,26 @@ int lvm_vg_remove(vg_t *vg)
 bad:
 	return 0;
 }
+
+vg_t *lvm_vg_open(lvm_t libh, const char *vgname, const char *mode,
+		  uint32_t flags)
+{
+	uint32_t internal_flags = 0;
+	vg_t *vg;
+
+	if (!strncmp(mode, "w", 1))
+		internal_flags |= READ_FOR_UPDATE;
+	else if (strncmp(mode, "r", 1)) {
+		log_errno(EINVAL, "Invalid VG open mode");
+		return NULL;
+	}
+
+	vg = vg_read((struct cmd_context *)libh, vgname, NULL, internal_flags);
+	if (vg_read_error(vg)) {
+		/* FIXME: use log_errno either here in inside vg_read */
+		vg_release(vg);
+		return NULL;
+	}
+
+	return vg;
+}