[PATCH] libxl: Fix bug in libxl_cdrom_insert, make more robust against bad xenstore data

[George Dunlap <george.dunlap@eu.citrix.com>]

# HG changeset patch
# User George Dunlap <george.dunlap@eu.citrix.com>
# Date 1353518844 0
# Node ID a4f707f6049a4a8152d2886f1b1d49f9e70ef5eb
# Parent  ae6fb202b233af815466055d9f1a635802a50855
libxl: Fix bug in libxl_cdrom_insert, make more robust against bad xenstore data

libxl_cdrom_insert was failing to initialize the backend type,
resulting in the wrong default backend.  The result was not only that
the CD was not inserted properly, but also that some improper xenstore
entries were created, causing further block commands to fail.

This patch fixes the bug by setting the disk backend type based on the
type of the existing device.

It also makes the system more robust by:
* Checking to see that it has got a valid path before proceeding to
write a partial xenstore entry
* Handling non-existent nodes in the backend xenstore entries more
gracefully

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>

diff --git a/tools/libxl/libxl.c b/tools/libxl/libxl.c
--- a/tools/libxl/libxl.c
+++ b/tools/libxl/libxl.c
@@ -2201,14 +2201,14 @@ static void libxl__device_disk_from_xs_b
         disk->removable = 0;
 
     tmp = libxl__xs_read(gc, XBT_NULL, libxl__sprintf(gc, "%s/mode", be_path));
-    if (!strcmp(tmp, "w"))
+    if (tmp && !strcmp(tmp, "w"))
         disk->readwrite = 1;
     else
         disk->readwrite = 0;
 
     tmp = libxl__xs_read(gc, XBT_NULL,
                          libxl__sprintf(gc, "%s/device-type", be_path));
-    disk->is_cdrom = !strcmp(tmp, "cdrom");
+    disk->is_cdrom = tmp && !strcmp(tmp, "cdrom");
 
     disk->format = LIBXL_DISK_FORMAT_UNKNOWN;
 }
@@ -2353,6 +2353,7 @@ int libxl_cdrom_insert(libxl_ctx *ctx, u
 
     libxl__device device;
     const char * path;
+    char * tmp;
 
     flexarray_t *insert = NULL;
 
@@ -2383,8 +2384,11 @@ int libxl_cdrom_insert(libxl_ctx *ctx, u
     disks = libxl_device_disk_list(ctx, domid, &num);
     for (i = 0; i < num; i++) {
         if (disks[i].is_cdrom && !strcmp(disk->vdev, disks[i].vdev))
-            /* found */
+        {
+            /* Found.  Set backend type appropriately. */
+            disk->backend=disks[i].backend;
             break;
+        }
     }
     if (i == num) {
         LIBXL__LOG(ctx, LIBXL__LOG_ERROR, "Virtual device not found");
@@ -2410,6 +2414,17 @@ int libxl_cdrom_insert(libxl_ctx *ctx, u
 
     path = libxl__device_backend_path(gc, &device);
 
+    /* Sanity check: make sure the backend exists before writing here */
+    tmp = libxl__xs_read(gc, XBT_NULL, libxl__sprintf(gc, "%s/frontend", path));
+    if (!tmp)
+    {
+        LIBXL__LOG(ctx, LIBXL__LOG_ERROR, "Internal error: %s does not exist",
+            libxl__sprintf(gc, "%s/frontend", path));
+        rc = ERROR_FAIL;
+        goto out;
+    }
+
+
     insert = flexarray_make(gc, 4, 1);
 
     flexarray_append_pair(insert, "type",