qapi: Introduce blockdev-group-snapshot-sync command

This is a QAPI/QMP only command to take a snapshot of a group of
devices. This is similar to the blockdev-snapshot-sync command, except
blockdev-group-snapshot-sync accepts a list devices, filenames, and
formats.

It is attempted to keep the snapshot of the group atomic; if the
creation or open of any of the new snapshots fails, then all of
the new snapshots are abandoned, and the name of the snapshot image
that failed is returned.  The failure case should not interrupt
any operations.

Rather than use bdrv_close() along with a subsequent bdrv_open() to
perform the pivot, the original image is never closed and the new
image is placed 'in front' of the original image via manipulation
of the BlockDriverState fields.  Thus, once the new snapshot image
has been successfully created, there are no more failure points
before pivoting to the new snapshot.

This allows the group of disks to remain consistent with each other,
even across snapshot failures.

Signed-off-by: Jeff Cody <jcody@redhat.com>
Acked-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block.c

@@ -882,6 +882,87 @@ void bdrv_make_anon(BlockDriverState *bs)
     bs->device_name[0] = '\0';
 }
 
+/*
+ * Add new bs contents at the top of an image chain while the chain is
+ * live, while keeping required fields on the top layer.
+ *
+ * This will modify the BlockDriverState fields, and swap contents
+ * between bs_new and bs_top. Both bs_new and bs_top are modified.
+ *
+ * This function does not create any image files.
+ */
+void bdrv_append(BlockDriverState *bs_new, BlockDriverState *bs_top)
+{
+    BlockDriverState tmp;
+
+    /* the new bs must not be in bdrv_states */
+    bdrv_make_anon(bs_new);
+
+    tmp = *bs_new;
+
+    /* there are some fields that need to stay on the top layer: */
+
+    /* dev info */
+    tmp.dev_ops           = bs_top->dev_ops;
+    tmp.dev_opaque        = bs_top->dev_opaque;
+    tmp.dev               = bs_top->dev;
+    tmp.buffer_alignment  = bs_top->buffer_alignment;
+    tmp.copy_on_read      = bs_top->copy_on_read;
+
+    /* i/o timing parameters */
+    tmp.slice_time        = bs_top->slice_time;
+    tmp.slice_start       = bs_top->slice_start;
+    tmp.slice_end         = bs_top->slice_end;
+    tmp.io_limits         = bs_top->io_limits;
+    tmp.io_base           = bs_top->io_base;
+    tmp.throttled_reqs    = bs_top->throttled_reqs;
+    tmp.block_timer       = bs_top->block_timer;
+    tmp.io_limits_enabled = bs_top->io_limits_enabled;
+
+    /* geometry */
+    tmp.cyls              = bs_top->cyls;
+    tmp.heads             = bs_top->heads;
+    tmp.secs              = bs_top->secs;
+    tmp.translation       = bs_top->translation;
+
+    /* r/w error */
+    tmp.on_read_error     = bs_top->on_read_error;
+    tmp.on_write_error    = bs_top->on_write_error;
+
+    /* i/o status */
+    tmp.iostatus_enabled  = bs_top->iostatus_enabled;
+    tmp.iostatus          = bs_top->iostatus;
+
+    /* keep the same entry in bdrv_states */
+    pstrcpy(tmp.device_name, sizeof(tmp.device_name), bs_top->device_name);
+    tmp.list = bs_top->list;
+
+    /* The contents of 'tmp' will become bs_top, as we are
+     * swapping bs_new and bs_top contents. */
+    tmp.backing_hd = bs_new;
+    pstrcpy(tmp.backing_file, sizeof(tmp.backing_file), bs_top->filename);
+
+    /* swap contents of the fixed new bs and the current top */
+    *bs_new = *bs_top;
+    *bs_top = tmp;
+
+    /* clear the copied fields in the new backing file */
+    bdrv_detach_dev(bs_new, bs_new->dev);
+
+    qemu_co_queue_init(&bs_new->throttled_reqs);
+    memset(&bs_new->io_base,   0, sizeof(bs_new->io_base));
+    memset(&bs_new->io_limits, 0, sizeof(bs_new->io_limits));
+    bdrv_iostatus_disable(bs_new);
+
+    /* we don't use bdrv_io_limits_disable() for this, because we don't want
+     * to affect or delete the block_timer, as it has been moved to bs_top */
+    bs_new->io_limits_enabled = false;
+    bs_new->block_timer       = NULL;
+    bs_new->slice_time        = 0;
+    bs_new->slice_start       = 0;
+    bs_new->slice_end         = 0;
+}
+
 void bdrv_delete(BlockDriverState *bs)
 {
     assert(!bs->dev);

block.h

@@ -114,6 +114,7 @@ int bdrv_create(BlockDriver *drv, const char* filename,
 int bdrv_create_file(const char* filename, QEMUOptionParameter *options);
 BlockDriverState *bdrv_new(const char *device_name);
 void bdrv_make_anon(BlockDriverState *bs);
+void bdrv_append(BlockDriverState *bs_new, BlockDriverState *bs_top);
 void bdrv_delete(BlockDriverState *bs);
 int bdrv_parse_cache_flags(const char *mode, int *flags);
 int bdrv_file_open(BlockDriverState **pbs, const char *filename, int flags);

block_int.h

@@ -221,6 +221,12 @@ struct BlockDriver {
     QLIST_ENTRY(BlockDriver) list;
 };
 
+/*
+ * Note: the function bdrv_append() copies and swaps contents of
+ * BlockDriverStates, so if you add new fields to this struct, please
+ * inspect bdrv_append() to determine if the new fields need to be
+ * copied as well.
+ */
 struct BlockDriverState {
     int64_t total_sectors; /* if we are reading a disk image, give its
                               size in sectors */

blockdev.c

@@ -714,6 +714,137 @@ void qmp_blockdev_snapshot_sync(const char *device, const char *snapshot_file,
     }
 }
 
+
+/* New and old BlockDriverState structs for group snapshots */
+typedef struct BlkGroupSnapshotStates {
+    BlockDriverState *old_bs;
+    BlockDriverState *new_bs;
+    QSIMPLEQ_ENTRY(BlkGroupSnapshotStates) entry;
+} BlkGroupSnapshotStates;
+
+/*
+ * 'Atomic' group snapshots.  The snapshots are taken as a set, and if any fail
+ *  then we do not pivot any of the devices in the group, and abandon the
+ *  snapshots
+ */
+void qmp_blockdev_group_snapshot_sync(SnapshotDevList *dev_list,
+                                      Error **errp)
+{
+    int ret = 0;
+    SnapshotDevList *dev_entry = dev_list;
+    SnapshotDev *dev_info = NULL;
+    BlkGroupSnapshotStates *states;
+    BlockDriver *proto_drv;
+    BlockDriver *drv;
+    int flags;
+    const char *format;
+    const char *snapshot_file;
+
+    QSIMPLEQ_HEAD(snap_bdrv_states, BlkGroupSnapshotStates) snap_bdrv_states;
+    QSIMPLEQ_INIT(&snap_bdrv_states);
+
+    /* drain all i/o before any snapshots */
+    bdrv_drain_all();
+
+    /* We don't do anything in this loop that commits us to the snapshot */
+    while (NULL != dev_entry) {
+        dev_info = dev_entry->value;
+        dev_entry = dev_entry->next;
+
+        states = g_malloc0(sizeof(BlkGroupSnapshotStates));
+        QSIMPLEQ_INSERT_TAIL(&snap_bdrv_states, states, entry);
+
+        states->old_bs = bdrv_find(dev_info->device);
+
+        if (!states->old_bs) {
+            error_set(errp, QERR_DEVICE_NOT_FOUND, dev_info->device);
+            goto delete_and_fail;
+        }
+
+        if (bdrv_in_use(states->old_bs)) {
+            error_set(errp, QERR_DEVICE_IN_USE, dev_info->device);
+            goto delete_and_fail;
+        }
+
+        if (!bdrv_is_read_only(states->old_bs) &&
+             bdrv_is_inserted(states->old_bs)) {
+
+            if (bdrv_flush(states->old_bs)) {
+                error_set(errp, QERR_IO_ERROR);
+                goto delete_and_fail;
+            }
+        }
+
+        snapshot_file = dev_info->snapshot_file;
+
+        flags = states->old_bs->open_flags;
+
+        if (!dev_info->has_format) {
+            format = "qcow2";
+        } else {
+            format = dev_info->format;
+        }
+
+        drv = bdrv_find_format(format);
+        if (!drv) {
+            error_set(errp, QERR_INVALID_BLOCK_FORMAT, format);
+            goto delete_and_fail;
+        }
+
+        proto_drv = bdrv_find_protocol(snapshot_file);
+        if (!proto_drv) {
+            error_set(errp, QERR_INVALID_BLOCK_FORMAT, format);
+            goto delete_and_fail;
+        }
+
+        /* create new image w/backing file */
+        ret = bdrv_img_create(snapshot_file, format,
+                              states->old_bs->filename,
+                              drv->format_name, NULL, -1, flags);
+        if (ret) {
+            error_set(errp, QERR_OPEN_FILE_FAILED, snapshot_file);
+            goto delete_and_fail;
+        }
+
+        /* We will manually add the backing_hd field to the bs later */
+        states->new_bs = bdrv_new("");
+        ret = bdrv_open(states->new_bs, snapshot_file,
+                        flags | BDRV_O_NO_BACKING, drv);
+        if (ret != 0) {
+            error_set(errp, QERR_OPEN_FILE_FAILED, snapshot_file);
+            goto delete_and_fail;
+        }
+    }
+
+
+    /* Now we are going to do the actual pivot.  Everything up to this point
+     * is reversible, but we are committed at this point */
+    QSIMPLEQ_FOREACH(states, &snap_bdrv_states, entry) {
+        /* This removes our old bs from the bdrv_states, and adds the new bs */
+        bdrv_append(states->new_bs, states->old_bs);
+    }
+
+    /* success */
+    goto exit;
+
+delete_and_fail:
+    /*
+    * failure, and it is all-or-none; abandon each new bs, and keep using
+    * the original bs for all images
+    */
+    QSIMPLEQ_FOREACH(states, &snap_bdrv_states, entry) {
+        if (states->new_bs) {
+             bdrv_delete(states->new_bs);
+        }
+    }
+exit:
+    QSIMPLEQ_FOREACH(states, &snap_bdrv_states, entry) {
+        g_free(states);
+    }
+    return;
+}
+
+
 static void eject_device(BlockDriverState *bs, int force, Error **errp)
 {
     if (bdrv_in_use(bs)) {

qapi-schema.json

@@ -1117,6 +1117,44 @@
 ##
 { 'command': 'block_resize', 'data': { 'device': 'str', 'size': 'int' }}
 
+##
+# @SnapshotDev
+#
+# @device:  the name of the device to generate the snapshot from.
+#
+# @snapshot-file: the target of the new image. A new file will be created.
+#
+# @format: #optional the format of the snapshot image, default is 'qcow2'.
+##
+{ 'type': 'SnapshotDev',
+  'data': {'device': 'str', 'snapshot-file': 'str', '*format': 'str' } }
+
+##
+# @blockdev-group-snapshot-sync
+#
+# Generates a synchronous snapshot of a group of one or more block devices,
+# as atomically as possible.  If the snapshot of any device in the group
+# fails, then the entire group snapshot will be abandoned and the
+# appropriate error returned.
+#
+#  List of:
+#  @SnapshotDev: information needed for the device snapshot
+#
+# Returns: nothing on success
+#          If @device is not a valid block device, DeviceNotFound
+#          If @device is busy, DeviceInUse will be returned
+#          If @snapshot-file can't be created, OpenFileFailed
+#          If @snapshot-file can't be opened, OpenFileFailed
+#          If @format is invalid, InvalidBlockFormat
+#
+# Note: The group snapshot attempt returns failure on the first snapshot
+# device failure.  Therefore, there will be only one device or snapshot file
+# returned in an error condition, and subsequent devices will not have been
+# attempted.
+##
+{ 'command': 'blockdev-group-snapshot-sync',
+  'data': { 'devlist': [ 'SnapshotDev' ] } }
+
 ##
 # @blockdev-snapshot-sync
 #