block: Leave BDS.backing_{file,format} constant

Parts of the block layer treat BDS.backing_file as if it were whatever
the image header says (i.e., if it is a relative path, it is relative to
the overlay), other parts treat it like a cache for
bs->backing->bs->filename (relative paths are relative to the CWD).
Considering bs->backing->bs->filename exists, let us make it mean the
former.

Among other things, this now allows the user to specify a base when
using qemu-img to commit an image file in a directory that is not the
CWD (assuming, everything uses relative filenames).

Before this patch:

$ ./qemu-img create -f qcow2 foo/bot.qcow2 1M
$ ./qemu-img create -f qcow2 -b bot.qcow2 foo/mid.qcow2
$ ./qemu-img create -f qcow2 -b mid.qcow2 foo/top.qcow2
$ ./qemu-img commit -b mid.qcow2 foo/top.qcow2
qemu-img: Did not find 'mid.qcow2' in the backing chain of 'foo/top.qcow2'
$ ./qemu-img commit -b foo/mid.qcow2 foo/top.qcow2
qemu-img: Did not find 'foo/mid.qcow2' in the backing chain of 'foo/top.qcow2'
$ ./qemu-img commit -b $PWD/foo/mid.qcow2 foo/top.qcow2
qemu-img: Did not find '[...]/foo/mid.qcow2' in the backing chain of 'foo/top.qcow2'

After this patch:

$ ./qemu-img commit -b mid.qcow2 foo/top.qcow2
Image committed.
$ ./qemu-img commit -b foo/mid.qcow2 foo/top.qcow2
qemu-img: Did not find 'foo/mid.qcow2' in the backing chain of 'foo/top.qcow2'
$ ./qemu-img commit -b $PWD/foo/mid.qcow2 foo/top.qcow2
Image committed.

With this change, bdrv_find_backing_image() must look at whether the
user has overridden a BDS's backing file.  If so, it can no longer use
bs->backing_file, but must instead compare the given filename against
the backing node's filename directly.

Note that this changes the QAPI output for a node's backing_file.  We
had very inconsistent output there (sometimes what the image header
said, sometimes the actual filename of the backing image).  This
inconsistent output was effectively useless, so we have to decide one
way or the other.  Considering that bs->backing_file usually at runtime
contained the path to the image relative to qemu's CWD (or absolute),
this patch changes QAPI's backing_file to always report the
bs->backing->bs->filename from now on.  If you want to receive the image
header information, you have to refer to full-backing-filename.

This necessitates a change to iotest 228.  The interesting information
it really wanted is the image header, and it can get that now, but it
has to use full-backing-filename instead of backing_file.  Because of
this patch's changes to bs->backing_file's behavior, we also need some
reference output changes.

Along with the changes to bs->backing_file, stop updating
BDS.backing_format in bdrv_backing_attach() as well.  This way,
ImageInfo's backing-filename and backing-filename-format fields will
represent what the image header says and nothing else.

iotest 245 changes in behavior: With the backing node no longer
overriding the parent node's backing_file string, you can now omit the
@backing option when reopening a node with neither a default nor a
current backing file even if it used to have a backing node at some
point.

273 also changes: The base image is opened without a format layer, so
ImageInfo.backing-filename-format used to report "file" for the base
image's overlay after blockdev-snapshot.  However, the image header
never says "file" anywhere, so it now reports $IMGFMT.

Signed-off-by: Max Reitz <mreitz@redhat.com>

block.c

@@ -1155,10 +1155,6 @@ static void bdrv_backing_attach(BdrvChild *c)
     bdrv_refresh_filename(backing_hd);
 
     parent->open_flags &= ~BDRV_O_NO_BACKING;
-    pstrcpy(parent->backing_file, sizeof(parent->backing_file),
-            backing_hd->filename);
-    pstrcpy(parent->backing_format, sizeof(parent->backing_format),
-            backing_hd->drv ? backing_hd->drv->format_name : "");
 
     bdrv_op_block_all(backing_hd, parent->backing_blocker);
     /* Otherwise we won't be able to commit or stream */
@@ -5673,6 +5669,7 @@ BlockDriverState *bdrv_find_backing_image(BlockDriverState *bs,
     char *backing_file_full = NULL;
     char *filename_tmp = NULL;
     int is_protocol = 0;
+    bool filenames_refreshed = false;
     BlockDriverState *curr_bs = NULL;
     BlockDriverState *retval = NULL;
     BlockDriverState *bs_below;
@@ -5698,9 +5695,31 @@ BlockDriverState *bdrv_find_backing_image(BlockDriverState *bs,
     {
         bs_below = bdrv_backing_chain_next(curr_bs);
 
-        /* If either of the filename paths is actually a protocol, then
-         * compare unmodified paths; otherwise make paths relative */
-        if (is_protocol || path_has_protocol(curr_bs->backing_file)) {
+        if (bdrv_backing_overridden(curr_bs)) {
+            /*
+             * If the backing file was overridden, we can only compare
+             * directly against the backing node's filename.
+             */
+
+            if (!filenames_refreshed) {
+                /*
+                 * This will automatically refresh all of the
+                 * filenames in the rest of the backing chain, so we
+                 * only need to do this once.
+                 */
+                bdrv_refresh_filename(bs_below);
+                filenames_refreshed = true;
+            }
+
+            if (strcmp(backing_file, bs_below->filename) == 0) {
+                retval = bs_below;
+                break;
+            }
+        } else if (is_protocol || path_has_protocol(curr_bs->backing_file)) {
+            /*
+             * If either of the filename paths is actually a protocol, then
+             * compare unmodified paths; otherwise make paths relative.
+             */
             char *backing_file_full_ret;
 
             if (strcmp(backing_file, curr_bs->backing_file) == 0) {
@@ -6820,7 +6839,7 @@ static bool append_strong_runtime_options(QDict *d, BlockDriverState *bs)
 /* Note: This function may return false positives; it may return true
  * even if opening the backing file specified by bs's image header
  * would result in exactly bs->backing. */
-static bool bdrv_backing_overridden(BlockDriverState *bs)
+bool bdrv_backing_overridden(BlockDriverState *bs)
 {
     if (bs->backing) {
         return strcmp(bs->auto_backing_file,

block/qapi.c

@@ -47,7 +47,7 @@ BlockDeviceInfo *bdrv_block_device_info(BlockBackend *blk,
                                         Error **errp)
 {
     ImageInfo **p_image_info;
-    BlockDriverState *bs0;
+    BlockDriverState *bs0, *backing;
     BlockDeviceInfo *info;
 
     if (!bs->drv) {
@@ -76,9 +76,10 @@ BlockDeviceInfo *bdrv_block_device_info(BlockBackend *blk,
         info->node_name = g_strdup(bs->node_name);
     }
 
-    if (bs->backing_file[0]) {
+    backing = bdrv_cow_bs(bs);
+    if (backing) {
         info->has_backing_file = true;
-        info->backing_file = g_strdup(bs->backing_file);
+        info->backing_file = g_strdup(backing->filename);
     }
 
     if (!QLIST_EMPTY(&bs->dirty_bitmaps)) {
@@ -314,6 +315,7 @@ void bdrv_query_image_info(BlockDriverState *bs,
     backing_filename = bs->backing_file;
     if (backing_filename[0] != '\0') {
         char *backing_filename2;
+
         info->backing_filename = g_strdup(backing_filename);
         info->has_backing_filename = true;
         backing_filename2 = bdrv_get_full_backing_filename(bs, NULL);

include/block/block_int.h

@@ -847,11 +847,20 @@ struct BlockDriverState {
     bool walking_aio_notifiers; /* to make removal during iteration safe */
 
     char filename[PATH_MAX];
-    char backing_file[PATH_MAX]; /* if non zero, the image is a diff of
-                                    this file image */
-    /* The backing filename indicated by the image header; if we ever
-     * open this file, then this is replaced by the resulting BDS's
-     * filename (i.e. after a bdrv_refresh_filename() run). */
+    /*
+     * If not empty, this image is a diff in relation to backing_file.
+     * Note that this is the name given in the image header and
+     * therefore may or may not be equal to .backing->bs->filename.
+     * If this field contains a relative path, it is to be resolved
+     * relatively to the overlay's location.
+     */
+    char backing_file[PATH_MAX];
+    /*
+     * The backing filename indicated by the image header.  Contrary
+     * to backing_file, if we ever open this file, auto_backing_file
+     * is replaced by the resulting BDS's filename (i.e. after a
+     * bdrv_refresh_filename() run).
+     */
     char auto_backing_file[PATH_MAX];
     char backing_format[16]; /* if non-zero and backing_file exists */
 
@@ -1053,6 +1062,8 @@ BlockDriver *bdrv_probe_all(const uint8_t *buf, int buf_size,
 void bdrv_parse_filename_strip_prefix(const char *filename, const char *prefix,
                                       QDict *options);
 
+bool bdrv_backing_overridden(BlockDriverState *bs);
+
 
 /**
  * bdrv_add_before_write_notifier:

tests/qemu-iotests/228

@@ -36,7 +36,7 @@ def log_node_info(node):
 
     log('bs->filename: ' + node['image']['filename'],
         filters=[filter_testfiles, filter_imgfmt])
-    log('bs->backing_file: ' + node['backing_file'],
+    log('bs->backing_file: ' + node['image']['full-backing-filename'],
         filters=[filter_testfiles, filter_imgfmt])
 
     if 'backing-image' in node['image']:
@@ -73,8 +73,8 @@ with iotests.FilePath('base.img') as base_img_path, \
                 },
                 filters=[filter_qmp_testfiles, filter_qmp_imgfmt])
 
-    # Filename should be plain, and the backing filename should not
-    # contain the "file:" prefix
+    # Filename should be plain, and the backing node filename should
+    # not contain the "file:" prefix
     log_node_info(vm.node_info('node0'))
 
     vm.qmp_log('blockdev-del', node_name='node0')

tests/qemu-iotests/228.out

@@ -4,7 +4,7 @@
 {"return": {}}
 
 bs->filename: TEST_DIR/PID-top.img
-bs->backing_file: TEST_DIR/PID-base.img
+bs->backing_file: file:TEST_DIR/PID-base.img
 bs->backing->bs->filename: TEST_DIR/PID-base.img
 
 {"execute": "blockdev-del", "arguments": {"node-name": "node0"}}
@@ -41,7 +41,7 @@ bs->backing->bs->filename: TEST_DIR/PID-base.img
 {"return": {}}
 
 bs->filename: TEST_DIR/PID-top.img
-bs->backing_file: TEST_DIR/PID-base.img
+bs->backing_file: file:TEST_DIR/PID-base.img
 bs->backing->bs->filename: TEST_DIR/PID-base.img
 
 {"execute": "blockdev-del", "arguments": {"node-name": "node0"}}
@@ -55,7 +55,7 @@ bs->backing->bs->filename: TEST_DIR/PID-base.img
 {"return": {}}
 
 bs->filename: json:{"backing": {"driver": "null-co"}, "driver": "IMGFMT", "file": {"driver": "file", "filename": "TEST_DIR/PID-top.img"}}
-bs->backing_file: null-co://
+bs->backing_file: TEST_DIR/PID-base.img
 bs->backing->bs->filename: null-co://
 
 {"execute": "blockdev-del", "arguments": {"node-name": "node0"}}

tests/qemu-iotests/245

@@ -725,7 +725,9 @@ class TestBlockdevReopen(iotests.QMPTestCase):
 
         # Detach hd2 from hd0.
         self.reopen(opts, {'backing': None})
-        self.reopen(opts, {}, "backing is missing for 'hd0'")
+
+        # Without a backing file, we can omit 'backing' again
+        self.reopen(opts)
 
         # Remove both hd0 and hd2
         result = self.vm.qmp('blockdev-del', conv_keys = True, node_name = 'hd0')

tests/qemu-iotests/273.out

@@ -32,7 +32,7 @@ Testing: -blockdev file,node-name=base,filename=TEST_DIR/t.IMGFMT.base -blockdev
                         "actual-size": SIZE,
                         "dirty-flag": false
                     },
-                    "backing-filename-format": "file",
+                    "backing-filename-format": "IMGFMT",
                     "virtual-size": 67108864,
                     "filename": "TEST_DIR/t.IMGFMT.mid",
                     "cluster-size": 65536,
@@ -112,7 +112,7 @@ Testing: -blockdev file,node-name=base,filename=TEST_DIR/t.IMGFMT.base -blockdev
                     "actual-size": SIZE,
                     "dirty-flag": false
                 },
-                "backing-filename-format": "file",
+                "backing-filename-format": "IMGFMT",
                 "virtual-size": 67108864,
                 "filename": "TEST_DIR/t.IMGFMT.mid",
                 "cluster-size": 65536,