block: add bdrv_add_before_write_notifier()

The bdrv_add_before_write_notifier() function installs a callback that
is invoked before a write request is processed.  This will be used to
implement copy-on-write point-in-time snapshots where we need to copy
out old data before overwriting it.

Note that BdrvTrackedRequest is moved to block_int.h since it is passed
to .notify() functions.

Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 22 insertions, 1 deletions

diff --git a/include/block/block_int.h b/include/block/block_int.h
index ba52247c1a..2d009556b0 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -59,7 +59,16 @@
 #define BLOCK_OPT_LAZY_REFCOUNTS    "lazy_refcounts"
 #define BLOCK_OPT_ADAPTER_TYPE      "adapter_type"
 
-typedef struct BdrvTrackedRequest BdrvTrackedRequest;
+typedef struct BdrvTrackedRequest {
+    BlockDriverState *bs;
+    int64_t sector_num;
+    int nb_sectors;
+    bool is_write;
+    QLIST_ENTRY(BdrvTrackedRequest) list;
+    Coroutine *co; /* owner, used for deadlock detection */
+    CoQueue wait_queue; /* coroutines blocked on this request */
+} BdrvTrackedRequest;
+
 
 typedef struct BlockIOLimit {
     int64_t bps[3];
@@ -248,6 +257,9 @@ struct BlockDriverState {
 
     NotifierList close_notifiers;
 
+    /* Callback before write request is processed */
+    NotifierWithReturnList before_write_notifiers;
+
     /* number of in-flight copy-on-read requests */
     unsigned int copy_on_read_in_flight;
 
@@ -299,6 +311,15 @@ void bdrv_set_io_limits(BlockDriverState *bs,
                         BlockIOLimit *io_limits);
 
 /**
+ * bdrv_add_before_write_notifier:
+ *
+ * Register a callback that is invoked before write requests are processed but
+ * after any throttling or waiting for overlapping requests.
+ */
+void bdrv_add_before_write_notifier(BlockDriverState *bs,
+                                    NotifierWithReturn *notifier);
+
+/**
  * bdrv_get_aio_context:
  *
  * Returns: the currently bound #AioContext