qemu-img: make "info" backing file output correct and easier to use

qemu-img info should use the same logic as qemu when printing the
backing file path, or debugging becomes quite tricky.  We can also
simplify the output in case the backing file has an absolute path
or a protocol.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block.c

@@ -270,6 +270,15 @@ void path_combine(char *dest, int dest_size,
     }
 }
 
+void bdrv_get_full_backing_filename(BlockDriverState *bs, char *dest, size_t sz)
+{
+    if (bs->backing_file[0] == '\0' || path_has_protocol(bs->backing_file)) {
+        pstrcpy(dest, sz, bs->backing_file);
+    } else {
+        path_combine(dest, sz, bs->filename, bs->backing_file);
+    }
+}
+
 void bdrv_register(BlockDriver *bdrv)
 {
     /* Block drivers without coroutine functions need emulation */
@@ -796,14 +805,8 @@ int bdrv_open(BlockDriverState *bs, const char *filename, int flags,
         BlockDriver *back_drv = NULL;
 
         bs->backing_hd = bdrv_new("");
-
-        if (path_has_protocol(bs->backing_file)) {
-            pstrcpy(backing_filename, sizeof(backing_filename),
-                    bs->backing_file);
-        } else {
-            path_combine(backing_filename, sizeof(backing_filename),
-                         filename, bs->backing_file);
-        }
+        bdrv_get_full_backing_filename(bs, backing_filename,
+                                       sizeof(backing_filename));
 
         if (bs->backing_format[0] != '\0') {
             back_drv = bdrv_find_format(bs->backing_format);

block.h

@@ -303,6 +303,8 @@ int bdrv_get_info(BlockDriverState *bs, BlockDriverInfo *bdi);
 const char *bdrv_get_encrypted_filename(BlockDriverState *bs);
 void bdrv_get_backing_filename(BlockDriverState *bs,
                                char *filename, int filename_size);
+void bdrv_get_full_backing_filename(BlockDriverState *bs,
+                                    char *dest, size_t sz);
 int bdrv_can_snapshot(BlockDriverState *bs);
 int bdrv_is_snapshot(BlockDriverState *bs);
 BlockDriverState *bdrv_snapshots(void);

qemu-img.c

@@ -1138,11 +1138,13 @@ static int img_info(int argc, char **argv)
     }
     bdrv_get_backing_filename(bs, backing_filename, sizeof(backing_filename));
     if (backing_filename[0] != '\0') {
-        path_combine(backing_filename2, sizeof(backing_filename2),
-                     filename, backing_filename);
-        printf("backing file: %s (actual path: %s)\n",
-               backing_filename,
-               backing_filename2);
+        bdrv_get_full_backing_filename(bs, backing_filename2,
+                                       sizeof(backing_filename2));
+        printf("backing file: %s", backing_filename);
+        if (strcmp(backing_filename, backing_filename2) != 0) {
+            printf(" (actual path: %s)", backing_filename2);
+        }
+        putchar('\n');
     }
     dump_snapshots(bs);
     bdrv_delete(bs);