Allow custom backing-file string in commit/stream

index a91809c..84ddb46 100644
@@ -3781,6 +3781,17 @@ BlockDriverState *bdrv_lookup_bs(const char *device,
     return NULL;
 }
 
+/* If 'base' is in the same chain as 'top', return true. Otherwise,
+ * return false.  If either argument is NULL, return false. */
+bool bdrv_chain_contains(BlockDriverState *top, BlockDriverState *base)
+{
+    while (top && top != base) {
+        top = top->backing_hd;
+    }
+
+    return top != NULL;
+}
+
 BlockDriverState *bdrv_next(BlockDriverState *bs)
 {
     if (!bs) {

index d0baf4f..1055990 100644
@@ -399,6 +399,7 @@ BlockDeviceInfoList *bdrv_named_nodes_list(void);
 BlockDriverState *bdrv_lookup_bs(const char *device,
                                  const char *node_name,
                                  Error **errp);
+bool bdrv_chain_contains(BlockDriverState *top, BlockDriverState *base);
 BlockDriverState *bdrv_next(BlockDriverState *bs);
 void bdrv_iterate(void (*it)(void *opaque, BlockDriverState *bs),
                   void *opaque);

index 84ddb46..def7e67 100644
@@ -2560,12 +2560,15 @@ typedef struct BlkIntermediateStates {
  *
  * base <- active
  *
+ * If backing_file_str is non-NULL, it will be used when modifying top's
+ * overlay image metadata.
+ *
  * Error conditions:
  *  if active == top, that is considered an error
  *
  */
 int bdrv_drop_intermediate(BlockDriverState *active, BlockDriverState *top,
-                           BlockDriverState *base)
+                           BlockDriverState *base, const char *backing_file_str)
 {
     BlockDriverState *intermediate;
     BlockDriverState *base_bs = NULL;

@@ -2617,7 +2620,8 @@ int bdrv_drop_intermediate(BlockDriverState *active, BlockDriverState *top,
     }
 
     /* success - we can delete the intermediate states, and link top->base */
-    ret = bdrv_change_backing_file(new_top_bs, base_bs->filename,
+    backing_file_str = backing_file_str ? backing_file_str : base_bs->filename;
+    ret = bdrv_change_backing_file(new_top_bs, backing_file_str,
                                    base_bs->drv ? base_bs->drv->format_name : "");
     if (ret) {
         goto exit;

index 5c09f44..91517d3 100644
@@ -37,6 +37,7 @@ typedef struct CommitBlockJob {
     BlockdevOnError on_error;
     int base_flags;
     int orig_overlay_flags;
+    char *backing_file_str;
 } CommitBlockJob;
 
 static int coroutine_fn commit_populate(BlockDriverState *bs,

@@ -141,7 +142,7 @@ wait:
 
     if (!block_job_is_cancelled(&s->common) && sector_num == end) {
         /* success */
-        ret = bdrv_drop_intermediate(active, top, base);
+        ret = bdrv_drop_intermediate(active, top, base, s->backing_file_str);
     }
 
 exit_free_buf:

@@ -158,7 +159,7 @@ exit_restore_reopen:
     if (overlay_bs && s->orig_overlay_flags != bdrv_get_flags(overlay_bs)) {
         bdrv_reopen(overlay_bs, s->orig_overlay_flags, NULL);
     }
-
+    g_free(s->backing_file_str);
     block_job_completed(&s->common, ret);
 }
 

@@ -182,7 +183,7 @@ static const BlockJobDriver commit_job_driver = {
 void commit_start(BlockDriverState *bs, BlockDriverState *base,
                   BlockDriverState *top, int64_t speed,
                   BlockdevOnError on_error, BlockDriverCompletionFunc *cb,
-                  void *opaque, Error **errp)
+                  void *opaque, const char *backing_file_str, Error **errp)
 {
     CommitBlockJob *s;
     BlockReopenQueue *reopen_queue = NULL;

@@ -244,6 +245,8 @@ void commit_start(BlockDriverState *bs, BlockDriverState *base,
     s->base_flags          = orig_base_flags;
     s->orig_overlay_flags  = orig_overlay_flags;
 
+    s->backing_file_str = g_strdup(backing_file_str);
+
     s->on_error = on_error;
     s->common.co = qemu_coroutine_create(commit_run);
 

index 858bd75..4a7885e 100644
@@ -1910,6 +1910,7 @@ void qmp_block_stream(const char *device, bool has_base,
 void qmp_block_commit(const char *device,
                       bool has_base, const char *base,
                       bool has_top, const char *top,
+                      bool has_backing_file, const char *backing_file,
                       bool has_speed, int64_t speed,
                       Error **errp)
 {

@@ -1975,11 +1976,16 @@ void qmp_block_commit(const char *device,
     }
 
     if (top_bs == bs) {
+        if (has_backing_file) {
+            error_setg(errp, "'backing-file' specified,"
+                             " but 'top' is the active layer");
+            return;
+        }
         commit_active_start(bs, base_bs, speed, on_error, block_job_cb,
                             bs, &local_err);
     } else {
         commit_start(bs, base_bs, top_bs, speed, on_error, block_job_cb, bs,
-                    &local_err);
+                     has_backing_file ? backing_file : NULL, &local_err);
     }
     if (local_err != NULL) {
         error_propagate(errp, local_err);

index 1055990..a6a5725 100644
@@ -284,7 +284,8 @@ int bdrv_change_backing_file(BlockDriverState *bs,
     const char *backing_file, const char *backing_fmt);
 void bdrv_register(BlockDriver *bdrv);
 int bdrv_drop_intermediate(BlockDriverState *active, BlockDriverState *top,
-                           BlockDriverState *base);
+                           BlockDriverState *base,
+                           const char *backing_file_str);
 BlockDriverState *bdrv_find_overlay(BlockDriverState *active,
                                     BlockDriverState *bs);
 BlockDriverState *bdrv_find_base(BlockDriverState *bs);

index 715c761..cba5dca 100644
@@ -459,13 +459,14 @@ void stream_start(BlockDriverState *bs, BlockDriverState *base,
  * @on_error: The action to take upon error.
  * @cb: Completion function for the job.
  * @opaque: Opaque pointer value passed to @cb.
+ * @backing_file_str: String to use as the backing file in @top's overlay
  * @errp: Error object.
  *
  */
 void commit_start(BlockDriverState *bs, BlockDriverState *base,
                  BlockDriverState *top, int64_t speed,
                  BlockdevOnError on_error, BlockDriverCompletionFunc *cb,
-                 void *opaque, Error **errp);
+                 void *opaque, const char *backing_file_str, Error **errp);
 /**
  * commit_active_start:
  * @bs: Active block device to be committed.

index 0433409..34de8ba 100644
@@ -32,7 +32,7 @@ typedef struct StreamBlockJob {
     RateLimit limit;
     BlockDriverState *base;
     BlockdevOnError on_error;
-    char backing_file_id[1024];
+    char *backing_file_str;
 } StreamBlockJob;
 
 static int coroutine_fn stream_populate(BlockDriverState *bs,

@@ -186,7 +186,7 @@ wait:
     if (!block_job_is_cancelled(&s->common) && sector_num == end && ret == 0) {
         const char *base_id = NULL, *base_fmt = NULL;
         if (base) {
-            base_id = s->backing_file_id;
+            base_id = s->backing_file_str;
             if (base->drv) {
                 base_fmt = base->drv->format_name;
             }

@@ -196,6 +196,7 @@ wait:
     }
 
     qemu_vfree(buf);
+    g_free(s->backing_file_str);
     block_job_completed(&s->common, ret);
 }
 

@@ -217,7 +218,7 @@ static const BlockJobDriver stream_job_driver = {
 };
 
 void stream_start(BlockDriverState *bs, BlockDriverState *base,
-                  const char *base_id, int64_t speed,
+                  const char *backing_file_str, int64_t speed,
                   BlockdevOnError on_error,
                   BlockDriverCompletionFunc *cb,
                   void *opaque, Error **errp)

@@ -237,9 +238,7 @@ void stream_start(BlockDriverState *bs, BlockDriverState *base,
     }
 
     s->base = base;
-    if (base_id) {
-        pstrcpy(s->backing_file_id, sizeof(s->backing_file_id), base_id);
-    }
+    s->backing_file_str = g_strdup(backing_file_str);
 
     s->on_error = on_error;
     s->common.co = qemu_coroutine_create(stream_run);

index 4a7885e..61b84d0 100644
@@ -1866,14 +1866,17 @@ static void block_job_cb(void *opaque, int ret)
     bdrv_put_ref_bh_schedule(bs);
 }
 
-void qmp_block_stream(const char *device, bool has_base,
-                      const char *base, bool has_speed, int64_t speed,
+void qmp_block_stream(const char *device,
+                      bool has_base, const char *base,
+                      bool has_backing_file, const char *backing_file,
+                      bool has_speed, int64_t speed,
                       bool has_on_error, BlockdevOnError on_error,
                       Error **errp)
 {
     BlockDriverState *bs;
     BlockDriverState *base_bs = NULL;
     Error *local_err = NULL;
+    const char *base_name = NULL;
 
     if (!has_on_error) {
         on_error = BLOCKDEV_ON_ERROR_REPORT;

@@ -1889,15 +1892,27 @@ void qmp_block_stream(const char *device, bool has_base,
         return;
     }
 
-    if (base) {
+    if (has_base) {
         base_bs = bdrv_find_backing_image(bs, base);
         if (base_bs == NULL) {
             error_set(errp, QERR_BASE_NOT_FOUND, base);
             return;
         }
+        base_name = base;
     }
 
-    stream_start(bs, base_bs, base, has_speed ? speed : 0,
+    /* if we are streaming the entire chain, the result will have no backing
+     * file, and specifying one is therefore an error */
+    if (base_bs == NULL && has_backing_file) {
+        error_setg(errp, "backing file specified, but streaming the "
+                         "entire chain");
+        return;
+    }
+
+    /* backing_file string overrides base bs filename */
+    base_name = has_backing_file ? backing_file : base_name;
+
+    stream_start(bs, base_bs, base_name, has_speed ? speed : 0,
                  on_error, block_job_cb, bs, &local_err);
     if (local_err) {
         error_propagate(errp, local_err);

index dc3d279..0dc2499 100644
@@ -1175,7 +1175,7 @@ void hmp_block_stream(Monitor *mon, const QDict *qdict)
     const char *base = qdict_get_try_str(qdict, "base");
     int64_t speed = qdict_get_try_int(qdict, "speed", 0);
 
-    qmp_block_stream(device, base != NULL, base,
+    qmp_block_stream(device, base != NULL, base, false, NULL,
                      qdict_haskey(qdict, "speed"), speed,
                      true, BLOCKDEV_ON_ERROR_REPORT, &error);
 

index 61b84d0..c2fafd1 100644
@@ -2347,6 +2347,108 @@ 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();

index dae92bb..48bd9a3 100644
@@ -2388,6 +2388,85 @@ void qmp_block_job_complete(const char *device, Error **errp)
     block_job_complete(job, errp);
 }
 
+void qmp_change_backing_file(const char *device,
+                             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;
+
+    /* find the top layer BDS of the chain */
+    bs = bdrv_find(device);
+    if (!bs) {
+        error_set(errp, QERR_DEVICE_NOT_FOUND, device);
+        return;
+    }
+
+    image_bs = bdrv_lookup_bs(NULL, image_node_name, &local_err);
+    if (local_err) {
+        error_propagate(errp, local_err);
+        return;
+    }
+
+    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();