[PATCH 5/6] block/backup: don't insert filter if no writers

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

If source is immutable and there no writers on it, we don't need to
insert a filter, so let's detect it and use simple blk's for
block-copy.

Note, that it's possible, that user will try to add writers on source
during backup. It will fail, as our source blk doesn't share write.

In future we can add a tri-state source-mode parameter for backup job
with the following values:

 immutable: got without filter. blockdev-backup command fails if there
            are writers on source. Adding writers during backup will
            fail.
 filtered: insert filter unconditionally. Writers are supported on
           start. User may add new writers above copy-before-write
           filter during backup [current behavior]
 auto: go "immutable" if there no writers on start, go "filtered"
       otherwise

And "auto" would be a default behavior. For now, let's just change a
default behavior to not create extra filter when it's not necessary.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 block/backup.c | 55 +++++++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 50 insertions(+), 5 deletions(-)

diff --git a/block/backup.c b/block/backup.c
index 14652ac98a..e856e4ad73 100644
--- a/block/backup.c
+++ b/block/backup.c
@@ -34,6 +34,8 @@ typedef struct BackupBlockJob {
     BlockDriverState *cbw;
     BlockDriverState *source_bs;
     BlockDriverState *target_bs;
+    BlockBackend *source_blk;
+    BlockBackend *target_blk;
 
     BdrvDirtyBitmap *sync_bitmap;
 
@@ -102,7 +104,17 @@ static void backup_clean(Job *job)
 {
     BackupBlockJob *s = container_of(job, BackupBlockJob, common.job);
     block_job_remove_all_bdrv(&s->common);
-    bdrv_cbw_drop(s->cbw);
+    if (s->cbw) {
+        assert(!s->source_blk && !s->target_blk);
+        bdrv_cbw_drop(s->cbw);
+    } else {
+        block_copy_state_free(s->bcs);
+        s->bcs = NULL;
+        blk_unref(s->source_blk);
+        s->source_blk = NULL;
+        blk_unref(s->target_blk);
+        s->target_blk = NULL;
+    }
 }
 
 void backup_do_checkpoint(BlockJob *job, Error **errp)
@@ -368,6 +380,7 @@ BlockJob *backup_job_create(const char *job_id, BlockDriverState *bs,
     int64_t cluster_size;
     BlockDriverState *cbw = NULL;
     BlockCopyState *bcs = NULL;
+    BlockBackend *source_blk = NULL, *target_blk = NULL;
 
     assert(bs);
     assert(target);
@@ -450,9 +463,37 @@ BlockJob *backup_job_create(const char *job_id, BlockDriverState *bs,
         goto error;
     }
 
-    cbw = bdrv_cbw_append(bs, target, filter_node_name, compress, &bcs, errp);
-    if (!cbw) {
-        goto error;
+    source_blk = blk_new_with_bs(bs, BLK_PERM_CONSISTENT_READ,
+                                    BLK_PERM_WRITE_UNCHANGED |
+                                    BLK_PERM_CONSISTENT_READ, NULL);
+    if (source_blk) {
+        BdrvDirtyBitmap *copy_bitmap;
+
+        target_blk  = blk_new_with_bs(target, BLK_PERM_WRITE,
+                                      BLK_PERM_CONSISTENT_READ, errp);
+        if (!target_blk) {
+            goto error;
+        }
+
+        bcs = block_copy_state_new(blk_root(source_blk), blk_root(target_blk),
+                                   false, compress, errp);
+        if (!bcs) {
+            goto error;
+        }
+
+        /*
+         * initalize bitmap in a way copy-before-write filter do it, to have
+         * same code path later.
+         */
+        copy_bitmap = block_copy_dirty_bitmap(bcs);
+        bdrv_set_dirty_bitmap(copy_bitmap, 0,
+                              bdrv_dirty_bitmap_size(copy_bitmap));
+    } else {
+        cbw = bdrv_cbw_append(bs, target, filter_node_name, compress, &bcs,
+                              errp);
+        if (!cbw) {
+            goto error;
+        }
     }
 
     cluster_size = block_copy_cluster_size(bcs);
@@ -464,7 +505,7 @@ BlockJob *backup_job_create(const char *job_id, BlockDriverState *bs,
     }
 
     /* job->len is fixed, so we can't allow resize */
-    job = block_job_create(job_id, &backup_job_driver, txn, cbw,
+    job = block_job_create(job_id, &backup_job_driver, txn, cbw ?: bs,
                            0, BLK_PERM_ALL,
                            speed, creation_flags, cb, opaque, errp);
     if (!job) {
@@ -474,6 +515,8 @@ BlockJob *backup_job_create(const char *job_id, BlockDriverState *bs,
     job->cbw = cbw;
     job->source_bs = bs;
     job->target_bs = target;
+    job->source_blk = source_blk;
+    job->target_blk = target_blk;
     job->on_source_error = on_source_error;
     job->on_target_error = on_target_error;
     job->sync_mode = sync_mode;
@@ -500,6 +543,8 @@ BlockJob *backup_job_create(const char *job_id, BlockDriverState *bs,
     if (cbw) {
         bdrv_cbw_drop(cbw);
     }
+    blk_unref(source_blk);
+    blk_unref(target_blk);
 
     return NULL;
 }
-- 
2.29.2