Support streaming to an intermediate layer

The current way to obtain the list of existing block jobs is to iterate over all root nodes and check which ones own a job.

Since we want to be able to support block jobs in other nodes as well, this patch keeps a list of jobs that is updated every time one is created or destroyed.

This also updates qmp_query_block_jobs() and bdrv_drain_all() to use this new list.

Signed-off-by Alberto Garcia <[email protected]>

Cc Max Reitz <[email protected]>

Cc Eric Blake <[email protected]>

---
 block/io.c               | 21 ++++++++-------------
 blockdev.c               | 19 ++++++++-----------
 blockjob.c               | 13 +++++++++++++
 include/block/blockjob.h | 14 ++++++++++++++
 4 files changed, 43 insertions(+), 24 deletions(-)

block/io.c +8 -13

index 1ce62c4..c38d776 100644
@@ -310,21 +310,19 @@ void bdrv_drain_all(void)
 {
     /* Always run first iteration so any pending completion BHs run */
     bool busy = true;
-    BlockDriverState *bs = NULL;
+    BlockJob *job = NULL;
 
-    while ((bs = bdrv_next(bs))) {
-        AioContext *aio_context = bdrv_get_aio_context(bs);
+    while ((job = block_job_next(job))) {
+        AioContext *aio_context = bdrv_get_aio_context(job->bs);
 
         aio_context_acquire(aio_context);
-        if (bs->job) {
-            block_job_pause(bs->job);
-        }
+        block_job_pause(job);
         aio_context_release(aio_context);
     }
 
     while (busy) {
+        BlockDriverState *bs = NULL;
         busy = false;
-        bs = NULL;
 
         while ((bs = bdrv_next(bs))) {
             AioContext *aio_context = bdrv_get_aio_context(bs);

@@ -335,14 +333,11 @@ void bdrv_drain_all(void)
         }
     }
 
-    bs = NULL;
-    while ((bs = bdrv_next(bs))) {
-        AioContext *aio_context = bdrv_get_aio_context(bs);
+    while ((job = block_job_next(job))) {
+        AioContext *aio_context = bdrv_get_aio_context(job->bs);
 
         aio_context_acquire(aio_context);
-        if (bs->job) {
-            block_job_resume(bs->job);
-        }
+        block_job_resume(job);
         aio_context_release(aio_context);
     }
 }

blockdev.c +8 -11

index 5eaf77e..bf36a0e 100644
@@ -3073,21 +3073,18 @@ fail:
 BlockJobInfoList *qmp_query_block_jobs(Error **errp)
 {
     BlockJobInfoList *head = NULL, **p_next = &head;
-    BlockDriverState *bs;
+    BlockJob *job;
 
-    for (bs = bdrv_next(NULL); bs; bs = bdrv_next(bs)) {
-        AioContext *aio_context = bdrv_get_aio_context(bs);
+    for (job = block_job_next(NULL); job; job = block_job_next(job)) {
+        BlockJobInfoList *elem = g_new0(BlockJobInfoList, 1);
+        AioContext *aio_context = bdrv_get_aio_context(job->bs);
 
         aio_context_acquire(aio_context);
-
-        if (bs->job) {
-            BlockJobInfoList *elem = g_new0(BlockJobInfoList, 1);
-            elem->value = block_job_query(bs->job);
-            *p_next = elem;
-            p_next = &elem->next;
-        }
-
+        elem->value = block_job_query(job);
         aio_context_release(aio_context);
+
+        *p_next = elem;
+        p_next = &elem->next;
     }
 
     return head;

blockjob.c +13 -0

index 2755465..c46984d 100644
@@ -35,6 +35,16 @@
 #include "qemu/timer.h"
 #include "qapi-event.h"
 
+static QLIST_HEAD(, BlockJob) block_jobs = QLIST_HEAD_INITIALIZER(block_jobs);
+
+BlockJob *block_job_next(BlockJob *job)
+{
+    if (!job) {
+        return QLIST_FIRST(&block_jobs);
+    }
+    return QLIST_NEXT(job, job_list);
+}
+
 void *block_job_create(const BlockJobDriver *driver, BlockDriverState *bs,
                        int64_t speed, BlockCompletionFunc *cb,
                        void *opaque, Error **errp)

@@ -73,6 +83,8 @@ void *block_job_create(const BlockJobDriver *driver, BlockDriverState *bs,
             return NULL;
         }
     }
+
+    QLIST_INSERT_HEAD(&block_jobs, job, job_list);
     return job;
 }
 

@@ -85,6 +97,7 @@ void block_job_completed(BlockJob *job, int ret)
     bs->job = NULL;
     bdrv_op_unblock_all(bs, job->blocker);
     error_free(job->blocker);
+    QLIST_REMOVE(job, job_list);
     g_free(job);
 }
 

include/block/blockjob.h +14 -0

index 57d8ef1..5431dd2 100644
@@ -102,6 +102,9 @@ struct BlockJob {
      */
     bool ready;
 
+    /** Element of the list of block jobs */
+    QLIST_ENTRY(BlockJob) job_list;
+
     /** Status that is published by the query-block-jobs QMP API */
     BlockDeviceIoStatus iostatus;
 

@@ -125,6 +128,17 @@ struct BlockJob {
 };
 
 /**
+ * block_job_next:
+ * @job: A block job, or %NULL.
+ *
+ * Get the next element from the list of block jobs after @job, or the
+ * first one if @job is %NULL.
+ *
+ * Returns the requested job, or %NULL if there are no more jobs left.
+ */
+BlockJob *block_job_next(BlockJob *job);
+
+/**
  * block_job_create:
  * @job_type: The class object for the newly-created job.
  * @bs: The block

Signature · 1 line

2.1.4

The current way to obtain the list of existing block jobs is to iterate over all root nodes and check which ones own a job.

Since we want to be able to support block jobs in other nodes as well, this patch keeps a list of jobs that is updated every time one is created or destroyed.

This also updates qmp_query_block_jobs() and bdrv_drain_all() to use this new list.

Signed-off-by Alberto Garcia <[email protected]>

Cc Max Reitz <[email protected]>

Cc Eric Blake <[email protected]>

---
  block/io.c               | 21 ++++++++-------------
  blockdev.c               | 19 ++++++++-----------
  blockjob.c               | 13 +++++++++++++
  include/block/blockjob.h | 14 ++++++++++++++
  4 files changed, 43 insertions(+), 24 deletions(-)

Currently, block jobs can only be owned by root nodes. This patch allows block jobs to be in any arbitrary node, by making the following changes:

- Block jobs can now be identified by the node name of their BlockDriverState in addition to the device name. Since both device and node names live in the same namespace there's no ambiguity.

- The "device" parameter used by all commands that operate on block jobs can also be a node name now.

- The node name is used as a fallback to fill in the BlockJobInfo structure and all BLOCK_JOB_* events if there is no device name for that job.

Signed-off-by Alberto Garcia <[email protected]>

Reviewed-by Max Reitz <[email protected]>

Reviewed-by Eric Blake <[email protected]>

---
 block/mirror.c            |  5 +++--
 blockdev.c                | 16 ++++++++--------
 blockjob.c                | 18 ++++++++++--------
 docs/qmp/qmp-events.txt   |  8 ++++----
 include/qapi/qmp/qerror.h |  3 ---
 qapi/block-core.json      | 20 ++++++++++----------
 6 files changed, 35 insertions(+), 35 deletions(-)

block/mirror.c +3 -2

index 58f391a..4b36e0b 100644
@@ -598,8 +598,9 @@ static void mirror_complete(BlockJob *job, Error **errp)
         return;
     }
     if (!s->synced) {
-        error_set(errp, QERR_BLOCK_JOB_NOT_READY,
-                  bdrv_get_device_name(job->bs));
+        error_setg(errp,
+                   "The active block job for device '%s' cannot be completed",
+                   bdrv_get_device_name(job->bs));
         return;
     }
 

blockdev.c +8 -8

index bf36a0e..1fcf466 100644
@@ -2813,32 +2813,32 @@ out:
     aio_context_release(aio_context);
 }
 
-/* Get the block job for a given device name and acquire its AioContext */
-static BlockJob *find_block_job(const char *device, AioContext **aio_context,
+/* Get the block job for a given device or node name
+ * and acquire its AioContext */
+static BlockJob *find_block_job(const char *device_or_node,
+                                AioContext **aio_context,
                                 Error **errp)
 {
-    BlockBackend *blk;
     BlockDriverState *bs;
 
-    blk = blk_by_name(device);
-    if (!blk) {
+    bs = bdrv_lookup_bs(device_or_node, device_or_node, errp);
+    if (!bs) {
         goto notfound;
     }
-    bs = blk_bs(blk);
 
     *aio_context = bdrv_get_aio_context(bs);
     aio_context_acquire(*aio_context);
 
     if (!bs->job) {
         aio_context_release(*aio_context);
+        error_set(errp, ERROR_CLASS_DEVICE_NOT_ACTIVE,
+              "No active block job on node '%s'", device_or_node);
         goto notfound;
     }
 
     return bs->job;
 
 notfound:
-    error_set(errp, ERROR_CLASS_DEVICE_NOT_ACTIVE,
-              "No active block job on device '%s'", device);
     *aio_context = NULL;
     return NULL;
 }

blockjob.c +10 -8

index c46984d..96471c6 100644
@@ -52,7 +52,8 @@ void *block_job_create(const BlockJobDriver *driver, BlockDriverState *bs,
     BlockJob *job;
 
     if (bs->job) {
-        error_set(errp, QERR_DEVICE_IN_USE, bdrv_get_device_name(bs));
+        error_setg(errp, "Node '%s' is in use",
+                   bdrv_get_device_or_node_name(bs));
         return NULL;
     }
     bdrv_ref(bs);

@@ -121,8 +122,9 @@ void block_job_set_speed(BlockJob *job, int64_t speed, Error **errp)
 void block_job_complete(BlockJob *job, Error **errp)
 {
     if (job->pause_count || job->cancelled || !job->driver->complete) {
-        error_set(errp, QERR_BLOCK_JOB_NOT_READY,
-                  bdrv_get_device_name(job->bs));
+        error_setg(errp,
+                   "The active block job for node '%s' cannot be completed",
+                   bdrv_get_device_or_node_name(job->bs));
         return;
     }
 

@@ -277,7 +279,7 @@ BlockJobInfo *block_job_query(BlockJob *job)
 {
     BlockJobInfo *info = g_new0(BlockJobInfo, 1);
     info->type      = g_strdup(BlockJobType_lookup[job->driver->job_type]);
-    info->device    = g_strdup(bdrv_get_device_name(job->bs));
+    info->device    = g_strdup(bdrv_get_device_or_node_name(job->bs));
     info->len       = job->len;
     info->busy      = job->busy;
     info->paused    = job->pause_count > 0;

@@ -299,7 +301,7 @@ static void block_job_iostatus_set_err(BlockJob *job, int error)
 void block_job_event_cancelled(BlockJob *job)
 {
     qapi_event_send_block_job_cancelled(job->driver->job_type,
-                                        bdrv_get_device_name(job->bs),
+                                        bdrv_get_device_or_node_name(job->bs),
                                         job->len,
                                         job->offset,
                                         job->speed,

@@ -309,7 +311,7 @@ void block_job_event_cancelled(BlockJob *job)
 void block_job_event_completed(BlockJob *job, const char *msg)
 {
     qapi_event_send_block_job_completed(job->driver->job_type,
-                                        bdrv_get_device_name(job->bs),
+                                        bdrv_get_device_or_node_name(job->bs),
                                         job->len,
                                         job->offset,
                                         job->speed,

@@ -323,7 +325,7 @@ void block_job_event_ready(BlockJob *job)
     job->ready = true;
 
     qapi_event_send_block_job_ready(job->driver->job_type,
-                                    bdrv_get_device_name(job->bs),
+                                    bdrv_get_device_or_node_name(job->bs),
                                     job->len,
                                     job->offset,
                                     job->speed, &error_abort);

@@ -352,7 +354,7 @@ BlockErrorAction block_job_error_action(BlockJob *job, BlockDriverState *bs,
     default:
         abort();
     }
-    qapi_event_send_block_job_error(bdrv_get_device_name(job->bs),
+    qapi_event_send_block_job_error(bdrv_get_device_or_node_name(job->bs),
                                     is_read ? IO_OPERATION_TYPE_READ :
                                     IO_OPERATION_TYPE_WRITE,
                                     action, &error_abort);

docs/qmp/qmp-events.txt +4 -4

index 6dc2cca..791f9dd 100644
@@ -90,7 +90,7 @@ Data:
 
 - "type":     Job type (json-string; "stream" for image streaming
                                      "commit" for block commit)
-- "device":   Device name (json-string)
+- "device":   Device name, or node name if not present (json-string)
 - "len":      Maximum progress value (json-int)
 - "offset":   Current progress value (json-int)
               On success this is equal to len.

@@ -114,7 +114,7 @@ Data:
 
 - "type":     Job type (json-string; "stream" for image streaming
                                      "commit" for block commit)
-- "device":   Device name (json-string)
+- "device":   Device name, or node name if not present (json-string)
 - "len":      Maximum progress value (json-int)
 - "offset":   Current progress value (json-int)
               On success this is equal to len.

@@ -141,7 +141,7 @@ Emitted when a block job encounters an error.
 
 Data:
 
-- "device": device name (json-string)
+- "device": device name, or node name if not present (json-string)
 - "operation": I/O operation (json-string, "read" or "write")
 - "action": action that has been taken, it's one of the following (json-string):
     "ignore": error has been ignored, the job may fail later

@@ -165,7 +165,7 @@ Data:
 
 - "type":     Job type (json-string; "stream" for image streaming
                                      "commit" for block commit)
-- "device":   Device name (json-string)
+- "device":   Device name, or node name if not present (json-string)
 - "len":      Maximum progress value (json-int)
 - "offset":   Current progress value (json-int)
               On success this is equal to len.

include/qapi/qmp/qerror.h +0 -3

index e567339..4ba442e 100644
@@ -37,9 +37,6 @@ void qerror_report_err(Error *err);
 #define QERR_BASE_NOT_FOUND \
     ERROR_CLASS_GENERIC_ERROR, "Base '%s' not found"
 
-#define QERR_BLOCK_JOB_NOT_READY \
-    ERROR_CLASS_GENERIC_ERROR, "The active block job for device '%s' cannot be completed"
-
 #define QERR_BUS_NO_HOTPLUG \
     ERROR_CLASS_GENERIC_ERROR, "Bus '%s' does not support hotplugging"
 

qapi/block-core.json +10 -10

index 863ffea..9b5cddd 100644
@@ -547,7 +547,7 @@
 #
 # @type: the job type ('stream' for image streaming)
 #
-# @device: the block device name
+# @device: the block device name, or node name if not present
 #
 # @len: the maximum progress value
 #

@@ -1146,7 +1146,7 @@
 #
 # Throttling can be disabled by setting the speed to 0.
 #
-# @device: the device name
+# @device: the device or node name of the owner of the block job.
 #
 # @speed:  the maximum speed, in bytes per second, or 0 for unlimited.
 #          Defaults to 0.

@@ -1177,7 +1177,7 @@
 # operation can be started at a later time to finish copying all data from the
 # backing file.
 #
-# @device: the device name
+# @device: the device or node name of the owner of the block job.
 #
 # @force: #optional whether to allow cancellation of a paused job (default
 #         false).  Since 1.3.

@@ -1203,7 +1203,7 @@
 # the operation is actually paused.  Cancelling a paused job automatically
 # resumes it.
 #
-# @device: the device name
+# @device: the device or node name of the owner of the block job.
 #
 # Returns: Nothing on success
 #          If no background operation is active on this device, DeviceNotActive

@@ -1223,7 +1223,7 @@
 #
 # This command also clears the error status of the job.
 #
-# @device: the device name
+# @device: the device or node name of the owner of the block job.
 #
 # Returns: Nothing on success
 #          If no background operation is active on this device, DeviceNotActive

@@ -1249,7 +1249,7 @@
 #
 # A cancelled or paused job cannot be completed.
 #
-# @device: the device name
+# @device: the device or node name of the owner of the block job.
 #
 # Returns: Nothing on success
 #          If no background operation is active on this device, DeviceNotActive

@@ -1903,7 +1903,7 @@
 #
 # @type: job type
 #
-# @device: device name
+# @device: device name, or node name if not present
 #
 # @len: maximum progress value
 #

@@ -1934,7 +1934,7 @@
 #
 # @type: job type
 #
-# @device: device name
+# @device: device name, or node name if not present
 #
 # @len: maximum progress value
 #

@@ -1957,7 +1957,7 @@
 #
 # Emitted when a block job encounters an error
 #
-# @device: device name
+# @device: device name, or node name if not present
 #
 # @operation: I/O operation
 #

@@ -1977,7 +1977,7 @@
 #
 # @type: job type
 #
-# @device: device name
+# @device: device name, or node name if not present
 #
 # @len: maximum progress value
 #

Signature · 1 line

2.1.4

We need to call stream_complete() in order to do all the necessary clean-ups, even if there's an early failure. At the moment it's only useful to make sure that s->backing_file_str is not leaked, but it will become more important as we introduce support for streaming to any intermediate node.

Signed-off-by Alberto Garcia <[email protected]>

Reviewed-by Max Reitz <[email protected]>

---
 block/stream.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

block/stream.c +6 -5

index a628901..37bfd8b 100644
@@ -114,21 +114,21 @@ static void coroutine_fn stream_run(void *opaque)
     StreamCompleteData *data;
     BlockDriverState *bs = s->common.bs;
     BlockDriverState *base = s->base;
-    int64_t sector_num, end;
+    int64_t sector_num = 0;
+    int64_t end = -1;
     int error = 0;
     int ret = 0;
     int n = 0;
     void *buf;
 
     if (!bs->backing_hd) {
-        block_job_completed(&s->common, 0);
-        return;
+        goto out;
     }
 
     s->common.len = bdrv_getlength(bs);
     if (s->common.len < 0) {
-        block_job_completed(&s->common, s->common.len);
-        return;
+        ret = s->common.len;
+        goto out;
     }
 
     end = s->common.len >> BDRV_SECTOR_BITS;

@@ -215,6 +215,7 @@ wait:
 
     qemu_vfree(buf);
 
+out:
     /* Modify backing chain and close BDSes in main loop */
     data = g_malloc(sizeof(*data));
     data->ret = ret;

Signature · 1 line

2.1.4

This makes sure that the image we are steaming into is open in read-write mode during the operation.

Operation blockers are also set in all intermediate nodes, since they will be removed from the chain afterwards.

Finally, this also unblocks the stream operation in backing files.

Signed-off-by Alberto Garcia <[email protected]>

Reviewed-by Max Reitz <[email protected]>

---
 block.c        |  4 +++-
 block/stream.c | 33 +++++++++++++++++++++++++++++++++
 2 files changed, 36 insertions(+), 1 deletion(-)

block.c +3 -1

index 7904098..8fab7e4 100644
@@ -1044,9 +1044,11 @@ void bdrv_set_backing_hd(BlockDriverState *bs, BlockDriverState *backing_hd)
             backing_hd->drv ? backing_hd->drv->format_name : "");
 
     bdrv_op_block_all(bs->backing_hd, bs->backing_blocker);
-    /* Otherwise we won't be able to commit due to check in bdrv_commit */
+    /* Otherwise we won't be able to commit or stream */
     bdrv_op_unblock(bs->backing_hd, BLOCK_OP_TYPE_COMMIT_TARGET,
                     bs->backing_blocker);
+    bdrv_op_unblock(bs->backing_hd, BLOCK_OP_TYPE_STREAM,
+                    bs->backing_blocker);
 out:
     bdrv_refresh_limits(bs, NULL);
 }

block/stream.c +16 -0

index 37bfd8b..8ba5a24 100644
@@ -33,6 +33,8 @@ typedef struct StreamBlockJob {
     BlockDriverState *base;
     BlockdevOnError on_error;
     char *backing_file_str;
+    int bs_flags;
+    Error *blocker;
 } StreamBlockJob;
 
 static int coroutine_fn stream_populate(BlockDriverState *bs,

@@ -89,6 +91,13 @@ static void stream_complete(BlockJob *job, void *opaque)
     StreamBlockJob *s = container_of(job, StreamBlockJob, common);
     StreamCompleteData *data = opaque;
     BlockDriverState *base = s->base;
+    BlockDriverState *bs;
+
+    /* Remove all blockers set in stream_start() */
+    for (bs = job->bs->backing_hd; bs && bs != s->base; bs = bs->backing_hd) {
+        bdrv_op_unblock_all(bs, s->blocker);
+    }
+    error_free(s->blocker);
 
     if (!block_job_is_cancelled(&s->common) && data->reached_end &&
         data->ret == 0) {

@@ -103,6 +112,11 @@ static void stream_complete(BlockJob *job, void *opaque)
         close_unused_images(job->bs, base, base_id);
     }
 
+    /* Reopen the image back in read-only mode if necessary */
+    if (s->bs_flags != bdrv_get_flags(job->bs)) {
+        bdrv_reopen(job->bs, s->bs_flags, NULL);
+    }
+
     g_free(s->backing_file_str);
     block_job_completed(&s->common, data->ret);
     g_free(data);

@@ -246,7 +260,9 @@ void stream_start(BlockDriverState *bs, BlockDriverState *base,
                   BlockCompletionFunc *cb,
                   void *opaque, Error **errp)
 {
+    BlockDriverState *iter;
     StreamBlockJob *s;
+    int orig_bs_flags;
 
     if ((on_error == BLOCKDEV_ON_ERROR_STOP ||
          on_error == BLOCKDEV_ON_ERROR_ENOSPC) &&

block/stream.c +13 -0

+            return;
+        }
+    }
+
     s = block_job_create(&stream_job_driver, bs, speed, cb, opaque, errp);
     if (!s) {
         return;
     }
 
+    /* Block all intermediate nodes between bs and base, because they
+     * will disappear from the chain after this operation */
+    error_setg(&s->blocker, "blocked by the block-stream operation in '%s'",
+               bdrv_get_device_or_node_name(bs));
+    for (iter = bs->backing_hd; iter && iter != base; iter = iter->backing_hd) {
+        bdrv_op_block_all(iter, s->blocker);
+    }
+
     s->base = base;
     s->backing_file_str = g_strdup(backing_file_str);
+    s->bs_flags = orig_bs_flags;
 
     s->on_error = on_error;
     s->common.co = qemu_coroutine_create(stream_run);

Signature · 1 line

2.1.4

block/stream.c void stream_start(BlockDriverState *bs, BlockDriverState *base, · line 277

Quoted 4 lines

block/stream.c +4 -0

+    /* Make sure that the image is opened in read-write mode */
+    orig_bs_flags = bdrv_get_flags(bs);
+    if (!(orig_bs_flags & BDRV_O_RDWR)) {
+        if (bdrv_reopen(bs, orig_bs_flags | BDRV_O_RDWR, errp) != 0) {

I don't think all bdrv_reopen implementation will guarantee to set errp on error :(. For example:

static int qcow2_reopen_prepare(BDRVReopenState *state,
BlockReopenQueue *queue, Error **errp)
{

Quoted 11 lines · Fam Zheng

block/stream.c void stream_start(BlockDriverState *bs, BlockDriverState *base, · line 277

Quoted 4 lines

block/stream.c +4 -0

+    /* Make sure that the image is opened in read-write mode */
+    orig_bs_flags = bdrv_get_flags(bs);
+    if (!(orig_bs_flags & BDRV_O_RDWR)) {
+        if (bdrv_reopen(bs, orig_bs_flags | BDRV_O_RDWR, errp) != 0) {

I don't think all bdrv_reopen implementation will guarantee to set errp on error :(. For example:

static int qcow2_reopen_prepare(BDRVReopenState *state,
BlockReopenQueue *queue, Error **errp)
{

bdrv_reopen_prepare() already sets a generic error if the implementation (qcow2_reopen_prepare() in this case) doesn't provide one:

    ret = drv->bdrv_reopen_prepare(reopen_state, queue, &local_err);
    if (ret) {
        if (local_err != NULL) {
            error_propagate(errp, local_err);
        } else {
            error_setg(errp, "failed while preparing to reopen image '%s'",
                       reopen_state->bs->filename);
        }
        goto error;
    }

This patch makes the 'device' parameter of the 'block-stream' command accept a node name as well as a device name.

In addition to that, operation blockers will be checked in all intermediate nodes between the top and the base node.

Since qmp_block_stream() now uses the error from bdrv_lookup_bs() and no longer returns DeviceNotFound, iotest 030 is updated to expect GenericError instead.

Signed-off-by Alberto Garcia <[email protected]>

Reviewed-by Max Reitz <[email protected]>

Reviewed-by Eric Blake <[email protected]>

Signed-off-by Alberto Garcia <[email protected]>

Reviewed-by Max Reitz <[email protected]>

Reviewed-by Eric Blake <[email protected]>

---
 docs/live-block-ops.txt | 31 ++++++++++++++++++++-----------
 1 file changed, 20 insertions(+), 11 deletions(-)

docs/live-block-ops.txt +20 -11

index a257087..a05d869 100644
@@ -10,9 +10,9 @@ Snapshot live merge
 Given a snapshot chain, described in this document in the following
 format:
 
-[A] -> [B] -> [C] -> [D]
+[A] <- [B] <- [C] <- [D] <- [E]
 
-Where the rightmost object ([D] in the example) described is the current
+Where the rightmost object ([E] in the example) described is the current
 image which the guest OS has write access to. To the left of it is its base
 image, and so on accordingly until the leftmost image, which has no
 base.

@@ -21,11 +21,14 @@ The snapshot live merge operation transforms such a chain into a
 smaller one with fewer elements, such as this transformation relative
 to the first example:
 
-[A] -> [D]
+[A] <- [E]
 
-Currently only forward merge with target being the active image is
-supported, that is, data copy is performed in the right direction with
-destination being the rightmost image.
+Data is copied in the right direction with destination being the
+rightmost image, but any other intermediate image can be specified
+instead. In this example data is copied from [C] into [D], so [D] can
+be backed by [B]:
+
+[A] <- [B] <- [D] <- [E]
 
 The operation is implemented in QEMU through image streaming facilities.
 

@@ -35,14 +38,20 @@ streaming operation completes it raises a QMP event. 'block_stream'
 copies data from the backing file(s) into the active image. When finished,
 it adjusts the backing file pointer.
 
-The 'base' parameter specifies an image which data need not be streamed from.
-This image will be used as the backing file for the active image when the
-operation is finished.
+The 'base' parameter specifies an image which data need not be
+streamed from. This image will be used as the backing file for the
+destination image when the operation is finished.
+
+In the first example above, the command would be:
+
+(qemu) block_stream virtio0 file-A.img
 
-In the example above, the command would be:
+In order to specify a destination image different from the active
+(rightmost) one we can use its (previously set) node name instead.
 
-(qemu) block_stream virtio0 A
+In the second example above, the command would be:
 
+(qemu) block_stream node-D file-B.img
 
 Live block copy
 ===============

Signature · 1 line

2.1.4

This test is streaming to the top layer using the intermediate image as the base. This is a mistake since block-stream never copies data from the base image and its backing chain, so this is effectively a no-op.

In addition to fixing the base parameter, this patch also writes some data to the intermediate image before the test, so there's something to copy and the test is meaningful.

Signed-off-by Alberto Garcia <[email protected]>

Reviewed-by Max Reitz <[email protected]>

---
 tests/qemu-iotests/030 | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

tests/qemu-iotests/030 +2 -1

index 7ca9b25..6e6cb5a 100755
@@ -35,6 +35,7 @@ class TestSingleDrive(iotests.QMPTestCase):
         qemu_img('create', '-f', iotests.imgfmt, '-o', 'backing_file=%s' % backing_img, mid_img)
         qemu_img('create', '-f', iotests.imgfmt, '-o', 'backing_file=%s' % mid_img, test_img)
         qemu_io('-f', 'raw', '-c', 'write -P 0x1 0 512', backing_img)
+        qemu_io('-f', iotests.imgfmt, '-c', 'write -P 0x1 524288 512', mid_img)
         self.vm = iotests.VM().add_drive("blkdebug::" + test_img)
         self.vm.launch()
 

@@ -93,7 +94,7 @@ class TestSingleDrive(iotests.QMPTestCase):
     def test_stream_partial(self):
         self.assert_no_active_block_jobs()
 
-        result = self.vm.qmp('block-stream', device='drive0', base=mid_img)
+        result = self.vm.qmp('block-stream', device='drive0', base=backing_img)
         self.assert_qmp(result, 'return', {})
 
         self.wait_until_completed()

Signature · 1 line

2.1.4

This patch tests that in a partial block-stream operation, no data is ever copied from the base image.

Signed-off-by Alberto Garcia <[email protected]>

Reviewed-by Max Reitz <[email protected]>

---
 tests/qemu-iotests/030     | 18 ++++++++++++++++++
 tests/qemu-iotests/030.out |  4 ++--
 2 files changed, 20 insertions(+), 2 deletions(-)

tests/qemu-iotests/030 +18 -0

index 6e6cb5a..bc53885 100755
@@ -91,6 +91,24 @@ class TestSingleDrive(iotests.QMPTestCase):
                          qemu_io('-f', iotests.imgfmt, '-c', 'map', test_img),
                          'image file map does not match backing file after streaming')
 
+    def test_stream_no_op(self):
+        self.assert_no_active_block_jobs()
+
+        # The image map is empty before the operation
+        empty_map = qemu_io('-f', iotests.imgfmt, '-c', 'map', test_img)
+
+        # This is a no-op: no data should ever be copied from the base image
+        result = self.vm.qmp('block-stream', device='drive0', base=mid_img)
+        self.assert_qmp(result, 'return', {})
+
+        self.wait_until_completed()
+
+        self.assert_no_active_block_jobs()
+        self.vm.shutdown()
+
+        self.assertEqual(qemu_io('-f', iotests.imgfmt, '-c', 'map', test_img),
+                         empty_map, 'image file map changed after a no-op')
+
     def test_stream_partial(self):
         self.assert_no_active_block_jobs()
 

tests/qemu-iotests/030.out +2 -2

index fa16b5c..6323079 100644
@@ -1,5 +1,5 @@
-.............
+..............
 ----------------------------------------------------------------------
-Ran 13 tests
+Ran 14 tests
 
 OK

Signature · 1 line

2.1.4

This adds test_stream_intermediate(), similar to test_stream() but streams to the intermediate image instead.

Signed-off-by Alberto Garcia <[email protected]>

Reviewed-by Max Reitz <[email protected]>

---
 tests/qemu-iotests/030     | 18 +++++++++++++++++-
 tests/qemu-iotests/030.out |  4 ++--
 2 files changed, 19 insertions(+), 3 deletions(-)

tests/qemu-iotests/030 +17 -1

index bc53885..0927457 100755
@@ -36,7 +36,8 @@ class TestSingleDrive(iotests.QMPTestCase):
         qemu_img('create', '-f', iotests.imgfmt, '-o', 'backing_file=%s' % mid_img, test_img)
         qemu_io('-f', 'raw', '-c', 'write -P 0x1 0 512', backing_img)
         qemu_io('-f', iotests.imgfmt, '-c', 'write -P 0x1 524288 512', mid_img)
-        self.vm = iotests.VM().add_drive("blkdebug::" + test_img)
+        opts = "backing.node-name=mid"
+        self.vm = iotests.VM().add_drive("blkdebug::" + test_img, opts)
         self.vm.launch()
 
     def tearDown(self):

@@ -60,6 +61,21 @@ class TestSingleDrive(iotests.QMPTestCase):
                          qemu_io('-f', iotests.imgfmt, '-c', 'map', test_img),
                          'image file map does not match backing file after streaming')
 
+    def test_stream_intermediate(self):
+        self.assert_no_active_block_jobs()
+
+        result = self.vm.qmp('block-stream', device='mid')
+        self.assert_qmp(result, 'return', {})
+
+        self.wait_until_completed(drive='mid')
+
+        self.assert_no_active_block_jobs()
+        self.vm.shutdown()
+
+        self.assertEqual(qemu_io('-f', 'raw', '-c', 'map', backing_img),
+                         qemu_io('-f', iotests.imgfmt, '-c', 'map', mid_img),
+                         'image file map does not match backing file after streaming')
+
     def test_stream_pause(self):
         self.assert_no_active_block_jobs()
 

tests/qemu-iotests/030.out +2 -2

index 6323079..96961ed 100644
@@ -1,5 +1,5 @@
-..............
+...............
 ----------------------------------------------------------------------
-Ran 14 tests
+Ran 15 tests
 
 OK

Signature · 1 line

2.1.4

This test case checks that it's possible to launch several stream operations in parallel in the same snapshot chain, each one involving a different set of nodes.

Signed-off-by Alberto Garcia <[email protected]>

Reviewed-by Max Reitz <[email protected]>

---
 tests/qemu-iotests/030     | 80 ++++++++++++++++++++++++++++++++++++++++++++++
 tests/qemu-iotests/030.out |  4 +--
 2 files changed, 82 insertions(+), 2 deletions(-)

tests/qemu-iotests/030 +80 -0

index 0927457..c199cef 100755
@@ -145,6 +145,86 @@ class TestSingleDrive(iotests.QMPTestCase):
         self.assert_qmp(result, 'error/class', 'GenericError')
 
 
+class TestParallelOps(iotests.QMPTestCase):
+    image_len = 2 * 1024 * 1024 # MB
+    num_ops = 4 # Number of parallel block-stream operations
+    num_imgs = num_ops * 2 + 1
+    imgs = []
+
+    def setUp(self):
+        opts = []
+        self.imgs = []
+
+        # Initialize file names and command-line options
+        for i in range(self.num_imgs):
+            img_depth = self.num_imgs - i - 1
+            opts.append("backing." * img_depth + "node-name=node%d" % i)
+            self.imgs.append(os.path.join(iotests.test_dir, 'img-%d.img' % i))
+
+        # Create all images
+        iotests.create_image(self.imgs[0], self.image_len)
+        for i in range(1, self.num_imgs):
+            qemu_img('create', '-f', iotests.imgfmt,
+                     '-o', 'backing_file=%s' % self.imgs[i-1], self.imgs[i])
+
+        # Put data into the images we are copying data from
+        for i in range(1, self.num_imgs, 2):
+            qemu_io('-f', iotests.imgfmt,
+                    '-c', 'write -P %d %d 128K' % (i, i*128*1024), self.imgs[i])
+
+        # Attach the drive to the VM
+        self.vm = iotests.VM()
+        self.vm.add_drive("blkdebug::" + self.imgs[-1], ','.join(opts))
+        self.vm.launch()
+
+    def tearDown(self):
+        self.vm.shutdown()
+        for img in self.imgs:
+            os.remove(img)
+
+    # Test that it's possible to run several block-stream operations
+    # in parallel in the same snapshot chain
+    def test_stream_parallel(self):
+        self.assert_no_active_block_jobs()
+
+        # Check that the maps don't match before the streaming operations
+        for i in range(2, self.num_imgs, 2):
+            self.assertNotEqual(qemu_io('-f', iotests.imgfmt, '-c', 'map', self.imgs[i]),
+                                qemu_io('-f', iotests.imgfmt, '-c', 'map', self.imgs[i-1]),
+                                'image file map matches backing file before streaming')
+
+        # Create all streaming jobs
+        pending_jobs = []
+        for i in range(2, self.num_imgs, 2):
+            node_name = 'node%d' % i
+            pending_jobs.append(node_name)
+            result = self.vm.qmp('block-stream', device=node_name, base=self.imgs[i-2], speed=32768)
+            self.assert_qmp(result, 'return', {})
+
+        # The block job on the active image is always referenced by
+        # its device name. Therefore we have to replace the node name
+        # with the device name in the list of pending jobs
+        pending_jobs.pop()
+        pending_jobs.append("drive0")
+
+        # Wait for all jobs to be finished.
+        while len(pending_jobs) > 0:
+            for event in self.vm.get_qmp_events(wait=True):
+                if event['event'] == 'BLOCK_JOB_COMPLETED':
+                    node_name = self.dictpath(event, 'data/device')
+                    self.assertTrue(node_name in pending_jobs)
+                    self.assert_qmp_absent(event, 'data/error')
+                    pending_jobs.remove(node_name)
+
+        self.assert_no_active_block_jobs()
+        self.vm.shutdown()
+
+        # Check that all maps match now
+        for i in range(2, self.num_imgs, 2):
+            self.assertEqual(qemu_io('-f', iotests.imgfmt, '-c', 'map', self.imgs[i]),
+                             qemu_io('-f', iotests.imgfmt, '-c', 'map', self.imgs[i-1]),
+                             'image file map does not match backing file after streaming')
+
 class TestSmallerBackingFile(iotests.QMPTestCase):
     backing_len = 1 * 1024 * 1024 # MB
     image_len = 2 * backing_len

tests/qemu-iotests/030.out +2 -2

index 96961ed..b6f2576 100644
@@ -1,5 +1,5 @@
-...............
+................
 ----------------------------------------------------------------------
-Ran 15 tests
+Ran 16 tests
 
 OK

Signature · 1 line

2.1.4

This test case checks that it's not possible to perform two block-stream operations if there are nodes involved in both.

Signed-off-by Alberto Garcia <[email protected]>

Reviewed-by Max Reitz <[email protected]>

---
 tests/qemu-iotests/030     | 27 +++++++++++++++++++++++++++
 tests/qemu-iotests/030.out |  4 ++--
 2 files changed, 29 insertions(+), 2 deletions(-)

tests/qemu-iotests/030 +10 -0

index c199cef..f898c92 100755
@@ -225,6 +225,33 @@ class TestParallelOps(iotests.QMPTestCase):
                              qemu_io('-f', iotests.imgfmt, '-c', 'map', self.imgs[i-1]),
                              'image file map does not match backing file after streaming')
 
+    # Test that it's not possible to perform two block-stream
+    # operations if there are nodes involved in both.
+    def test_stream_overlapping(self):
+        self.assert_no_active_block_jobs()
+
+        # Set a speed limit to make sure that this job blocks the rest
+        result = self.vm.qmp('block-stream', device='node4', base=self.imgs[0], speed=32768)
+        self.assert_qmp(result, 'return', {})
+
+        result = self.vm.qmp('block-stream', device='node5', base=self.imgs[1])

tests/qemu-iotests/030 +7 -0

+            self.wait_until_completed(drive='drive0')
+        else:
+            self.wait_until_completed(drive='node4')
+        self.assert_no_active_block_jobs()
+
+        self.vm.shutdown()
+
 class TestSmallerBackingFile(iotests.QMPTestCase):
     backing_len = 1 * 1024 * 1024 # MB
     image_len = 2 * backing_len

tests/qemu-iotests/030.out +2 -2

index b6f2576..52d796e 100644
@@ -1,5 +1,5 @@
-................
+.................
 ----------------------------------------------------------------------
-Ran 16 tests
+Ran 17 tests
 
 OK

Signature · 1 line

2.1.4

tests/qemu-iotests/030 class TestParallelOps(iotests.QMPTestCase): · line 247

Quoted 2 lines

tests/qemu-iotests/030 +2 -0

+        # If node4 is the active node, the id of the block job is drive0
+        if self.num_imgs == 5:

Isn't self.num_imgs a constant (9) ?

Quoted 4 lines · Fam Zheng

tests/qemu-iotests/030 class TestParallelOps(iotests.QMPTestCase): · line 247

Quoted 2 lines

tests/qemu-iotests/030 +2 -0

+        # If node4 is the active node, the id of the block job is drive0
+        if self.num_imgs == 5:

Isn't self.num_imgs a constant (9) ?

Yes, but the number is arbitrary. The idea is to let users change it if they want to, so the test case must be prepared for that.

v7:
- Rebased against the current master
- Updated bdrv_drain_all() to use the new block_job_next() API.

v6: https://lists.gnu.org/archive/html/qemu-devel/2015-04/msg03046.html
- fix the no-op test following Max's suggestions

v5: https://lists.gnu.org/archive/html/qemu-devel/2015-04/msg03006.html
- Fix a few typos
- Minor documentation updates
- Update test_stream_partial() to test no-ops
- New test case: test_stream_parallel()
- New test case: test_stream_overlapping()

v4: https://lists.gnu.org/archive/html/qemu-devel/2015-04/msg01878.html
- Refactor find_block_job to use the error from bdrv_lookup_bs()
- Don't use QERR_DEVICE_IN_USE in block_job_create() since we can be dealing with nodes now.
- Fix @device comment in the BlockJobInfo documentation
- stream_start(): simplify the bdrv_reopen() call and use bdrv_get_device_or_node_name() for error messages.
- Use a different variable name for BlockDriverState *i
- Documentation fixes in docs/live-block-ops.txt
- Update iotest 30 since now test_device_not_found() returns GenericError
- Fix test case test_stream_partial()
- Add new test case test_stream_intermediate()
- Fix typos

v3: https://lists.gnu.org/archive/html/qemu-devel/2015-04/msg00806.html
- Keep a list of block jobs and make qmp_query_block_jobs() iterate over it.

v2: https://lists.gnu.org/archive/html/qemu-devel/2015-03/msg04798.html
- The 'block-stream' command does not have a 'node-name' parameter anymore and reuses 'device' for that purpose.
- Block jobs can now be owned by any intermediate node, and not just by the ones at the root. query-block-jobs is updated to reflect that change.
- The 'device' parameter of all 'block-job-*' commands can now take a node name.
- The BlockJobInfo type and all BLOCK_JOB_* events report the node name in the 'device' field if the node does not have a device name.
- All intermediate nodes are blocked (and checked for blockers) during the streaming operation.

v1: https://lists.gnu.org/archive/html/qemu-devel/2015-02/msg04116.html

Regards,

Berto

Alberto Garcia (11):
block: keep a list of block jobs
block: allow block jobs in any arbitrary node
block: never cancel a streaming job without running stream_complete()
block: Support streaming to an intermediate layer
block: Add QMP support for streaming to an intermediate layer
docs: Document how to stream to an intermediate layer
qemu-iotests: fix test_stream_partial()
qemu-iotests: add no-op streaming test
qemu-iotests: test streaming to an intermediate layer
qemu-iotests: test block-stream operations in parallel
qemu-iotests: test overlapping block-stream operations

 block.c                    |   4 +-
 block/io.c                 |  21 +++----
 block/mirror.c             |   5 +-
 block/stream.c             |  44 ++++++++++++--
 blockdev.c                 |  55 ++++++++---------
 blockjob.c                 |  31 +++++++---
 docs/live-block-ops.txt    |  31 ++++++----
 docs/qmp/qmp-events.txt    |   8 +--
 include/block/blockjob.h   |  14 +++++
 include/qapi/qmp/qerror.h  |   3 -
 qapi/block-core.json       |  30 +++++----
 tests/qemu-iotests/030     | 148 ++++++++++++++++++++++++++++++++++++++++++++-
 tests/qemu-iotests/030.out |   4 +-
 13 files changed, 304 insertions(+), 94 deletions(-)

Signature · 1 line

2.1.4

Ping...

https://lists.gnu.org/archive/html/qemu-devel/2015-05/msg02580.html

Quoted 84 lines

v7:
- Rebased against the current master
- Updated bdrv_drain_all() to use the new block_job_next() API.

v6: https://lists.gnu.org/archive/html/qemu-devel/2015-04/msg03046.html
- fix the no-op test following Max's suggestions

v5: https://lists.gnu.org/archive/html/qemu-devel/2015-04/msg03006.html
- Fix a few typos
- Minor documentation updates
- Update test_stream_partial() to test no-ops
- New test case: test_stream_parallel()
- New test case: test_stream_overlapping()

v4: https://lists.gnu.org/archive/html/qemu-devel/2015-04/msg01878.html
- Refactor find_block_job to use the error from bdrv_lookup_bs()
- Don't use QERR_DEVICE_IN_USE in block_job_create() since we can be dealing with nodes now.
- Fix @device comment in the BlockJobInfo documentation
- stream_start(): simplify the bdrv_reopen() call and use bdrv_get_device_or_node_name() for error messages.
- Use a different variable name for BlockDriverState *i
- Documentation fixes in docs/live-block-ops.txt
- Update iotest 30 since now test_device_not_found() returns GenericError
- Fix test case test_stream_partial()
- Add new test case test_stream_intermediate()
- Fix typos

v3: https://lists.gnu.org/archive/html/qemu-devel/2015-04/msg00806.html
- Keep a list of block jobs and make qmp_query_block_jobs() iterate over it.

v2: https://lists.gnu.org/archive/html/qemu-devel/2015-03/msg04798.html
- The 'block-stream' command does not have a 'node-name' parameter anymore and reuses 'device' for that purpose.
- Block jobs can now be owned by any intermediate node, and not just by the ones at the root. query-block-jobs is updated to reflect that change.
- The 'device' parameter of all 'block-job-*' commands can now take a node name.
- The BlockJobInfo type and all BLOCK_JOB_* events report the node name in the 'device' field if the node does not have a device name.
- All intermediate nodes are blocked (and checked for blockers) during the streaming operation.

v1: https://lists.gnu.org/archive/html/qemu-devel/2015-02/msg04116.html

Regards,

Berto

Alberto Garcia (11):
block: keep a list of block jobs
block: allow block jobs in any arbitrary node
block: never cancel a streaming job without running stream_complete()
block: Support streaming to an intermediate layer
block: Add QMP support for streaming to an intermediate layer
docs: Document how to stream to an intermediate layer
qemu-iotests: fix test_stream_partial()
qemu-iotests: add no-op streaming test
qemu-iotests: test streaming to an intermediate layer
qemu-iotests: test block-stream operations in parallel
qemu-iotests: test overlapping block-stream operations

 block.c                    |   4 +-
 block/io.c                 |  21 +++----
 block/mirror.c             |   5 +-
 block/stream.c             |  44 ++++++++++++--
 blockdev.c                 |  55 ++++++++---------
 blockjob.c                 |  31 +++++++---
 docs/live-block-ops.txt    |  31 ++++++----
 docs/qmp/qmp-events.txt    |   8 +--
 include/block/blockjob.h   |  14 +++++
 include/qapi/qmp/qerror.h  |   3 -
 qapi/block-core.json       |  30 +++++----
 tests/qemu-iotests/030     | 148 ++++++++++++++++++++++++++++++++++++++++++++-
 tests/qemu-iotests/030.out |   4 +-
 13 files changed, 304 insertions(+), 94 deletions(-)

Signature · 1 line

2.1.4

I believe our conclusion from an earlier version of the series was that we need QAPI introspection so that libvirt can detect the presence of the feature.

Quoted 4 lines

I believe our conclusion from an earlier version of the series was that we need QAPI introspection so that libvirt can detect the presence of the feature.

The initial version of this series had an extra 'top' parameter to decide what image to stream data into. That's what prompted the discussion:

https://lists.gnu.org/archive/html/qemu-devel/2015-02/msg04182.html

That was later removed when we agreed that we could just reuse the 'device' parameter to refer to either a device or a node name.

I don't think that introspection support would make a difference in this case, or am I missing anything?

Quoted 9 lines

Quoted 3 lines

I believe our conclusion from an earlier version of the series was that we need QAPI introspection so that libvirt can detect the presence of the feature.

The initial version of this series had an extra 'top' parameter to decide what image to stream data into. [...] That was later removed when we agreed that we could just reuse the 'device' parameter to refer to either a device or a node name.

Quoted 2 lines

I don't think that introspection support would make a difference in this case, or am I missing anything?

Hm, yes, probably not. But how would libvirt detect the feature then?

Quoted 3 lines · Kevin Wolf

Quoted 3 lines

Quoted 3 lines

I believe our conclusion from an earlier version of the series was that we need QAPI introspection so that libvirt can detect the presence of the feature.

Quoted 10 lines

Quoted 8 lines

The initial version of this series had an extra 'top' parameter to decide what image to stream data into. [...] That was later removed when we agreed that we could just reuse the 'device' parameter to refer to either a device or a node name.

I don't think that introspection support would make a difference in this case, or am I missing anything?

Hm, yes, probably not. But how would libvirt detect the feature then?

One possibility is to try to stream to an intermediate node and see if it fails.

Example: in a chain like [A] <- [B] <- [C], streaming to [B] using [A] as the 'base' parameter is a no-op (there's even a test for that in iotest 030).

If QEMU does support streaming to [B], the operation will succeed but do nothing. Otherwise the operation will fail with a DeviceNotFound error.

That said, I would prefer a way to detect the feature that does not involve testing commands for their error codes, but is there any? What does libvirt generally do in order to detect new features that don't depend on API changes?

Quoted 5 lines · Alberto Garcia

Quoted 3 lines · Kevin Wolf

Quoted 3 lines

Quoted 3 lines

I believe our conclusion from an earlier version of the series was that we need QAPI introspection so that libvirt can detect the presence of the feature.

Detecting the presence of a feature allows libvirt the luxury of giving its own error message, rather than relying on the qemu message. But that's not to say libvirt HAS to use its own error message, and therefore being unable to detect the feature may not be the end of the world.

Quoted 4 lines

That said, I would prefer a way to detect the feature that does not involve testing commands for their error codes, but is there any? What does libvirt generally do in order to detect new features that don't depend on API changes?

But libvirt has not yet set up node name management (I'm about to revive Jeff's patch for auto-node-naming simultaneously with a libvirt patch series that proves that it helps libvirt), and libvirt will need a new API to allow users a way to request streams to an intermediate image. So anything libvirt does to interact with the new stream-to-intermediate will have to be new code, and I can worry about whether the qemu error message is good enough, or whether I have to contrive some probing test to see if it even works; but my initial thought is that merely probing to see if auto-node-naming is in place is a good approximation filter (if libvirt isn't managing its own node names, then the only way to use stream-to-intermediate is via a node name automatically supplied by qemu, especially nice if both features land in 2.4).

Quoted 10 lines

One possibility is to try to stream to an intermediate node and see if it fails.

Example: in a chain like [A] <- [B] <- [C], streaming to [B] using
[A] as the 'base' parameter is a no-op (there's even a test for that in iotest 030).

If QEMU does support streaming to [B], the operation will succeed but do nothing. Otherwise the operation will fail with a DeviceNotFound error.

In general, if a feature addition doesn't change API, but merely converts what was previously an error into something that works, then libvirt is probably okay with just trying the feature, and reporting the error message if it fails (assuming the qemu error message is sane).

v7:
- Rebased against the current master
- Updated bdrv_drain_all() to use the new block_job_next() API.