block: Add @exact parameter to bdrv_co_truncate()

We have two drivers (iscsi and file-posix) that (in some cases) return
success from their .bdrv_co_truncate() implementation if the block
device is larger than the requested offset, but cannot be shrunk.  Some
callers do not want that behavior, so this patch adds a new parameter
that they can use to turn off that behavior.

This patch just adds the parameter and lets the block/io.c and
block/block-backend.c functions pass it around.  All other callers
always pass false and none of the implementations evaluate it, so that
this patch does not change existing behavior.  Future patches take care
of that.

Suggested-by: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20190918095144.955-5-mreitz@redhat.com
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>

1 files changed, 13 insertions, 7 deletions

diff --git a/block/io.c b/block/io.c
index 8ff3b47..6e2b598 100644
--- a/block/io.c
+++ b/block/io.c
@@ -3291,8 +3291,12 @@ static void bdrv_parent_cb_resize(BlockDriverState *bs)
 
 /**
  * Truncate file to 'offset' bytes (needed only for file protocols)
+ *
+ * If 'exact' is true, the file must be resized to exactly the given
+ * 'offset'.  Otherwise, it is sufficient for the node to be at least
+ * 'offset' bytes in length.
  */
-int coroutine_fn bdrv_co_truncate(BdrvChild *child, int64_t offset,
+int coroutine_fn bdrv_co_truncate(BdrvChild *child, int64_t offset, bool exact,
                                   PreallocMode prealloc, Error **errp)
 {
     BlockDriverState *bs = child->bs;
@@ -3348,9 +3352,9 @@ int coroutine_fn bdrv_co_truncate(BdrvChild *child, int64_t offset,
     }
 
     if (drv->bdrv_co_truncate) {
-        ret = drv->bdrv_co_truncate(bs, offset, prealloc, errp);
+        ret = drv->bdrv_co_truncate(bs, offset, exact, prealloc, errp);
     } else if (bs->file && drv->is_filter) {
-        ret = bdrv_co_truncate(bs->file, offset, prealloc, errp);
+        ret = bdrv_co_truncate(bs->file, offset, exact, prealloc, errp);
     } else {
         error_setg(errp, "Image format driver does not support resize");
         ret = -ENOTSUP;
@@ -3381,6 +3385,7 @@ out:
 typedef struct TruncateCo {
     BdrvChild *child;
     int64_t offset;
+    bool exact;
     PreallocMode prealloc;
     Error **errp;
     int ret;
@@ -3389,18 +3394,19 @@ typedef struct TruncateCo {
 static void coroutine_fn bdrv_truncate_co_entry(void *opaque)
 {
     TruncateCo *tco = opaque;
-    tco->ret = bdrv_co_truncate(tco->child, tco->offset, tco->prealloc,
-                                tco->errp);
+    tco->ret = bdrv_co_truncate(tco->child, tco->offset, tco->exact,
+                                tco->prealloc, tco->errp);
     aio_wait_kick();
 }
 
-int bdrv_truncate(BdrvChild *child, int64_t offset, PreallocMode prealloc,
-                  Error **errp)
+int bdrv_truncate(BdrvChild *child, int64_t offset, bool exact,
+                  PreallocMode prealloc, Error **errp)
 {
     Coroutine *co;
     TruncateCo tco = {
         .child      = child,
         .offset     = offset,
+        .exact      = exact,
         .prealloc   = prealloc,
         .errp       = errp,
         .ret        = NOT_DONE,