[PATCH 3/3] libxl: libxl__device_from_disk should retrieve backend from xenstore

[Wei Liu <wei.liu2@citrix.com>]

... if backend is not set by caller.

Also change the function to use "goto" idiom while I was there.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
---
 tools/libxl/libxl.c | 21 ++++++++++++++++++---
 1 file changed, 18 insertions(+), 3 deletions(-)

diff --git a/tools/libxl/libxl.c b/tools/libxl/libxl.c
index 01a2d11..56c2e09 100644
--- a/tools/libxl/libxl.c
+++ b/tools/libxl/libxl.c
@@ -2285,12 +2285,24 @@ int libxl__device_from_disk(libxl__gc *gc, uint32_t domid,
 {
     libxl_ctx *ctx = libxl__gc_owner(gc);
     int devid;
+    int rc;
 
     devid = libxl__device_disk_dev_number(disk->vdev, NULL, NULL);
     if (devid==-1) {
         LIBXL__LOG(ctx, LIBXL__LOG_ERROR, "Invalid or unsupported"
                " virtual disk identifier %s", disk->vdev);
-        return ERROR_INVAL;
+        rc = ERROR_INVAL;
+        goto out;
+    }
+
+    if (disk->backend == LIBXL_DISK_BACKEND_UNKNOWN) {
+        char *be_path;
+
+        be_path = GCSPRINTF("/local/domain/%u/backend/vbd/%u/%d",
+                            disk->backend_domid,
+                            domid, devid);
+        rc = libxl__disk_backend_from_xs_be(gc, be_path, disk);
+        if (rc) goto out;
     }
 
     device->backend_domid = disk->backend_domid;
@@ -2309,14 +2321,17 @@ int libxl__device_from_disk(libxl__gc *gc, uint32_t domid,
         default:
             LIBXL__LOG(ctx, LIBXL__LOG_ERROR, "unrecognized disk backend type: %d\n",
                        disk->backend);
-            return ERROR_INVAL;
+            rc = ERROR_INVAL;
+            goto out;
     }
 
     device->domid = domid;
     device->devid = devid;
     device->kind  = LIBXL__DEVICE_KIND_VBD;
 
-    return 0;
+    rc = 0;
+out:
+    return rc;
 }
 
 /* Specific function called directly only by local disk attach,
-- 
1.9.1