[Qemu-devel] [PATCH v6 for 2.1 10/10] block: add QAPI command to allow live backing file change

[Jeff Cody <jcody@redhat.com>]

This allows a user to make a live change to the backing file recorded in
an open image.

The image file to modify can be specified 2 ways:

1) image filename
2) image node-name

Note: this does not cause the backing file itself to be reopened; it
merely changes the backing filename in the image file structure, and
in internal BDS structures.

It is the responsibility of the user to pass a filename string that
can be resolved when the image chain is reopened, and the filename
string is not validated.

A good analogy for this command is that it is a live version of
'qemu-img rebase -u', with respect to changing the backing file string.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Jeff Cody <jcody@redhat.com>
---
 blockdev.c           | 102 +++++++++++++++++++++++++++++++++++++++++++++++++++
 qapi/block-core.json |  60 ++++++++++++++++++++++++++++++
 qmp-commands.hx      |  74 +++++++++++++++++++++++++++++++++++++
 3 files changed, 236 insertions(+)

diff --git a/blockdev.c b/blockdev.c
index 42c7b3f..273e52a 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -2414,6 +2414,108 @@ void qmp_block_job_complete(const char *device, Error **errp)
     block_job_complete(job, errp);
 }
 
+void qmp_change_backing_file(const char *device,
+                             bool has_image, const char *image,
+                             bool has_image_node_name,
+                             const char *image_node_name,
+                             const char *backing_file,
+                             Error **errp)
+{
+    BlockDriverState *bs = NULL;
+    BlockDriverState *image_bs = NULL;
+    Error *local_err = NULL;
+    bool ro;
+    int open_flags;
+    int ret;
+
+    /* validate argument combinations */
+    if (has_image && has_image_node_name) {
+        error_setg(errp, "'image' and 'image-node-name' "
+                         "are mutually exclusive");
+        return;
+    }
+
+    /* find the top layer BDS of the chain */
+    bs = bdrv_find(device);
+    if (!bs) {
+        error_set(errp, QERR_DEVICE_NOT_FOUND, device);
+        return;
+    }
+
+    if (has_image_node_name) {
+        image_bs = bdrv_lookup_bs(NULL, image_node_name, &local_err);
+        if (local_err) {
+            error_propagate(errp, local_err);
+            return;
+        }
+    }
+
+    if (has_image) {
+        if (!strcmp(bs->filename, image)) {
+            image_bs = bs;
+        } else {
+            image_bs = bdrv_find_backing_image(bs, image);
+        }
+    }
+
+    if (!has_image && !has_image_node_name) {
+        image_bs = bs;
+    }
+
+    if (!image_bs) {
+        error_setg(errp, "image file not found");
+        return;
+    }
+
+    if (bdrv_find_base(image_bs) == image_bs) {
+        error_setg(errp, "not allowing backing file change on an image "
+                         "without a backing file");
+        return;
+    }
+
+    /* even though we are not necessarily operating on bs, we need it to
+     * determine if block ops are currently prohibited on the chain */
+    if (bdrv_op_is_blocked(bs, BLOCK_OP_TYPE_CHANGE, errp)) {
+        return;
+    }
+
+    /* final sanity check */
+    if (!bdrv_chain_contains(bs, image_bs)) {
+        error_setg(errp, "'%s' and image file are not in the same chain",
+                   device);
+        return;
+    }
+
+    /* if not r/w, reopen to make r/w */
+    open_flags = image_bs->open_flags;
+    ro = bdrv_is_read_only(image_bs);
+
+    if (ro) {
+        bdrv_reopen(image_bs, open_flags | BDRV_O_RDWR, &local_err);
+        if (local_err) {
+            error_propagate(errp, local_err);
+            return;
+        }
+    }
+
+    ret = bdrv_change_backing_file(image_bs, backing_file,
+                               image_bs->drv ? image_bs->drv->format_name : "");
+
+    if (ret < 0) {
+        error_setg_errno(errp, -ret, "Could not change backing file to '%s'",
+                         backing_file);
+        /* don't exit here, so we can try to restore open flags if
+         * appropriate */
+    }
+
+    if (ro) {
+        bdrv_reopen(image_bs, open_flags, &local_err);
+        if (local_err) {
+            error_propagate(errp, local_err); /* will preserve prior errp */
+        }
+    }
+}
+
 void qmp_blockdev_add(BlockdevOptions *options, Error **errp)
 {
     QmpOutputVisitor *ov = qmp_output_visitor_new();
diff --git a/qapi/block-core.json b/qapi/block-core.json
index c76d45d..0939143 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -680,6 +680,66 @@
   'data': 'BlockdevSnapshot' }
 
 ##
+# @change-backing-file
+#
+# Change the backing file in the image file metadata.  This does not cause QEMU
+# to reopen the image file to reparse the backing filename (it may, however,
+# perform a reopen to change permissions from r/o -> r/w -> r/o, if needed).
+# The new backing file string is written into the image file metadata, and the
+# QEMU internal strings are updated.
+#
+# The image file to perform the operation on can be specified by two different
+# methods:
+#
+#  Method 1: Supply the device name (e.g. 'virtio0'), and optionally the image
+#            filename.  This would use arguments @device and @image.
+#
+#  Method 2: Supply the device name, and the node-name of the image to modify,
+#            via @image-node-name.
+#
+# Arguments @image and @image-node-name are mutually exclusive.
+#
+# Method 1 interface
+#---------------------
+# @image:          #optional The file name of the image to modify.  If omitted,
+#                            and @image-node-name is not supplied, then the
+#                            default is the active layer of the chain described
+#                            by @device.
+#
+# Method 2 interface
+#---------------------
+# @image-node-name #optional The name of the block driver state node of the
+#                            image to modify.  The @device argument is used to
+#                            verify @image-node-name is in the chain described
+#                            by @device.
+#
+# Common arguments
+#---------------------
+# @device:          The name of the device.
+#
+# @backing-file:    The string to write as the backing file.  This string is
+#                   not validated, so care should be taken when specifying
+#                   the string or the image chain may not be able to be
+#                   reopened again.
+#
+#                   If a pathname string is such that it cannot be
+#                   resolved by QEMU, that means that subsequent QMP or
+#                   HMP commands must use node-names for the image in
+#                   question, as filename lookup methods will fail.
+#
+#
+# Returns: Nothing on success
+#          If @device does not exist or cannot be determined, DeviceNotFound
+#          If @image is specified, but not @device, GenericError
+#          If both @image and @image-node-name are specified, GenericError
+#
+# Since: 2.1
+##
+{ 'command': 'change-backing-file',
+  'data': { 'device': 'str', '*image': 'str', '*image-node-name': 'str',
+            'backing-file': 'str' } }
+
+##
 # @block-commit
 #
 # Live commit of data from overlay image nodes into backing nodes - i.e.,
diff --git a/qmp-commands.hx b/qmp-commands.hx
index 314cbba..c1cbfd2 100644
--- a/qmp-commands.hx
+++ b/qmp-commands.hx
@@ -1438,6 +1438,80 @@ Example:
 EQMP
 
     {
+        .name       = "change-backing-file",
+        .args_type  = "device:s,image:s?,image-node-name:s?,backing-file:s",
+        .mhandler.cmd_new = qmp_marshal_input_change_backing_file,
+    },
+
+SQMP
+change-backing-file
+-------------------
+Since: 2.1
+
+Change the backing file in the image file metadata.  This does not cause QEMU
+to reopen the image file to reparse the backing filename (it may, however,
+perform a reopen to change permissions from r/o -> r/w -> r/o, if needed).
+The new backing file string is written into the image file metadata, and the
+QEMU internal strings are updated.
+
+The image file to perform the operation on can be specified by two different
+methods:
+
+ Method 1: Supply the device name (e.g. 'virtio0'), and optionally the image
+           filename.  This would use arguments "device" and "image".
+
+ Method 2: Supply the device name, and the node-name of the image to modify,
+           via "image-node-name".
+
+Arguments:
+
+Arguments "image" or "image-node-name" are mutually exclusive.
+
+
+Method 1 interface
+--------------------
+- "image":              The file name of the image to modify.  If omitted,
+                        and "image-node-name" is not supplied, then the
+                        default is the active layer of the chain described
+                        by device.
+                        (json-string, optional)
+
+
+Method 2 interface
+--------------------
+- "image-node-name":    The name of the block driver state node of the
+                        image to modify.  The "device" is argument is used to
+                        verify "image-node-name" is in the chain described by
+                        "device".
+                        (json-string, optional)
+
+
+Common arguments
+--------------------
+- "device":             The name of the device.
+                        (json-string)
+
+- "backing-file":       The string to write as the backing file.  This string is
+                        not validated, so care should be taken when specifying
+                        the string or the image chain may not be able to be
+                        reopened again.
+                        (json-string)
+
+                        If a pathname string is such that it cannot be
+                        resolved by QEMU, that means that subsequent QMP or
+                        HMP commands must use node-names for the image in
+                        question, as filename lookup methods will fail.
+
+
+Returns: Nothing on success
+         If "device" does not exist or cannot be determined, DeviceNotFound
+         If "image" is specified, but not "device, GenericError
+         If both "image" and "image-node-name" are specified, GenericError
+
+
+EQMP
+
+    {
         .name       = "balloon",
         .args_type  = "value:M",
         .mhandler.cmd_new = qmp_marshal_input_balloon,
-- 
1.9.3