[v4,2/5] The discard flag for block stream operation

diff --git a/block/stream.c b/block/stream.c
index 9e85954..e844e94 100644
--- a/block/stream.c
+++ b/block/stream.c
@@ -584,10 +584,9 @@  static const BlockJobDriver stream_job_driver = {
 
 void stream_start(const char *job_id, BlockDriverState *bs,
                   BlockDriverState *base, const char *backing_file_str,
-                  int creation_flags, int64_t speed,
+                  int creation_flags, int64_t speed, bool discard,
                   BlockdevOnError on_error, Error **errp)
 {
-    const bool discard = false;
     StreamBlockJob *s = NULL;
     BlockDriverState *iter;
     int orig_bs_flags;
diff --git a/blockdev.c b/blockdev.c
index 81f95d9..333592e 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -3141,6 +3141,7 @@  void qmp_block_stream(bool has_job_id, const char *job_id, const char *device,
                       bool has_base_node, const char *base_node,
                       bool has_backing_file, const char *backing_file,
                       bool has_speed, int64_t speed,
+                      bool has_discard, bool discard,
                       bool has_on_error, BlockdevOnError on_error,
                       bool has_auto_finalize, bool auto_finalize,
                       bool has_auto_dismiss, bool auto_dismiss,
@@ -3157,6 +3158,10 @@  void qmp_block_stream(bool has_job_id, const char *job_id, const char *device,
         on_error = BLOCKDEV_ON_ERROR_REPORT;
     }
 
+    if (!has_discard) {
+        discard = false;
+    }
+
     bs = bdrv_lookup_bs(device, device, errp);
     if (!bs) {
         return;
@@ -3221,7 +3226,8 @@  void qmp_block_stream(bool has_job_id, const char *job_id, const char *device,
     }
 
     stream_start(has_job_id ? job_id : NULL, bs, base_bs, base_name,
-                 job_flags, has_speed ? speed : 0, on_error, &local_err);
+                 job_flags, has_speed ? speed : 0,
+                 discard, on_error, &local_err);
     if (local_err) {
         error_propagate(errp, local_err);
         goto out;
diff --git a/hmp-commands.hx b/hmp-commands.hx
index db0c681..a7e2a10 100644
--- a/hmp-commands.hx
+++ b/hmp-commands.hx
@@ -95,8 +95,8 @@  ETEXI
 
     {
         .name       = "block_stream",
-        .args_type  = "device:B,speed:o?,base:s?",
-        .params     = "device [speed [base]]",
+        .args_type  = "device:B,speed:o?,base:s?,discard:-d",
+        .params     = "device [speed [base]] [-d]",
         .help       = "copy data from a backing file into a block device",
         .cmd        = hmp_block_stream,
     },
diff --git a/hmp.c b/hmp.c
index 7828f93..0d263e4 100644
--- a/hmp.c
+++ b/hmp.c
@@ -1920,9 +1920,11 @@  void hmp_block_stream(Monitor *mon, const QDict *qdict)
     const char *device = qdict_get_str(qdict, "device");
     const char *base = qdict_get_try_str(qdict, "base");
     int64_t speed = qdict_get_try_int(qdict, "speed", 0);
+    bool discard = qdict_get_try_bool(qdict, "discard", false);
 
     qmp_block_stream(true, device, device, base != NULL, base, false, NULL,
-                     false, NULL, qdict_haskey(qdict, "speed"), speed, true,
+                     false, NULL, qdict_haskey(qdict, "speed"), speed,
+                     true, discard, true,
                      BLOCKDEV_ON_ERROR_REPORT, false, false, false, false,
                      &error);
 
diff --git a/include/block/block_int.h b/include/block/block_int.h
index f605622..2660336 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -970,7 +970,7 @@  int is_windows_drive(const char *filename);
  */
 void stream_start(const char *job_id, BlockDriverState *bs,
                   BlockDriverState *base, const char *backing_file_str,
-                  int creation_flags, int64_t speed,
+                  int creation_flags, int64_t speed, bool discard,
                   BlockdevOnError on_error, Error **errp);
 
 /**
diff --git a/qapi/block-core.json b/qapi/block-core.json
index d4fe710..f4538fa 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -2334,6 +2334,9 @@ 
 #
 # @speed:  the maximum speed, in bytes per second
 #
+# @discard: true to delete blocks duplicated in old backing files.
+#           (default: false). Since 4.0.
+#
 # @on-error: the action to take on an error (default report).
 #            'stop' and 'enospc' can only be used if the block device
 #            supports io-status (see BlockInfo).  Since 1.3.
@@ -2366,7 +2369,7 @@ 
 { 'command': 'block-stream',
   'data': { '*job-id': 'str', 'device': 'str', '*base': 'str',
             '*base-node': 'str', '*backing-file': 'str', '*speed': 'int',
-            '*on-error': 'BlockdevOnError',
+            '*discard': 'bool', '*on-error': 'BlockdevOnError',
             '*auto-finalize': 'bool', '*auto-dismiss': 'bool' } }
 
 ##