[Qemu-devel] [RFC 2/5] block: Generic truncation fallback

[Max Reitz <mreitz@redhat.com>]

If a protocol driver does not support truncation, we call fall back to
effectively not doing anything if the new size is less than the actual
file size.  This is what we have been doing for some host device drivers
already.

The only caveat is that we have to zero out everything in the first
sector that lies beyond the new "EOF" so we do not get any surprises
with format probing.

Signed-off-by: Max Reitz <mreitz@redhat.com>
---
 block/io.c | 69 ++++++++++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 65 insertions(+), 4 deletions(-)

diff --git a/block/io.c b/block/io.c
index 24a18759fd..382728fa9a 100644
--- a/block/io.c
+++ b/block/io.c
@@ -3064,6 +3064,57 @@ static void bdrv_parent_cb_resize(BlockDriverState *bs)
     }
 }
 
+static int coroutine_fn bdrv_co_truncate_fallback(BdrvChild *child,
+                                                  int64_t offset,
+                                                  PreallocMode prealloc,
+                                                  Error **errp)
+{
+    BlockDriverState *bs = child->bs;
+    int64_t cur_size = bdrv_getlength(bs);
+
+    if (cur_size < 0) {
+        error_setg_errno(errp, -cur_size,
+                         "Failed to inquire current file size");
+        return cur_size;
+    }
+
+    if (prealloc != PREALLOC_MODE_OFF) {
+        error_setg(errp, "Unsupported preallocation mode: %s",
+                   PreallocMode_str(prealloc));
+        return -ENOTSUP;
+    }
+
+    if (offset > cur_size) {
+        error_setg(errp, "Cannot grow this %s node", bs->drv->format_name);
+        return -ENOTSUP;
+    }
+
+    /*
+     * Overwrite first "post-EOF" parts of the first sector with
+     * zeroes so raw images will not be misprobed
+     */
+    if (offset < BDRV_SECTOR_SIZE && offset < cur_size) {
+        int64_t fill_len = MIN(BDRV_SECTOR_SIZE - offset, cur_size - offset);
+        int ret;
+
+        if (!(child->perm & BLK_PERM_WRITE)) {
+            error_setg(errp, "Cannot write to this node to clear the file past "
+                       "the truncated EOF");
+            return -EPERM;
+        }
+
+        ret = bdrv_co_pwrite_zeroes(child, offset, fill_len, 0);
+        if (ret < 0) {
+            error_setg_errno(errp, -ret,
+                             "Failed to clear file past the truncated EOF");
+            return ret;
+        }
+    }
+
+    return 0;
+}
+
+
 /**
  * Truncate file to 'offset' bytes (needed only for file protocols)
  */
@@ -3074,6 +3125,7 @@ int coroutine_fn bdrv_co_truncate(BdrvChild *child, int64_t offset,
     BlockDriver *drv = bs->drv;
     BdrvTrackedRequest req;
     int64_t old_size, new_bytes;
+    Error *local_err = NULL;
     int ret;
 
 
@@ -3127,15 +3179,24 @@ int coroutine_fn bdrv_co_truncate(BdrvChild *child, int64_t offset,
             ret = bdrv_co_truncate(bs->file, offset, prealloc, errp);
             goto out;
         }
-        error_setg(errp, "Image format driver does not support resize");
+        error_setg(&local_err, "Image format driver does not support resize");
         ret = -ENOTSUP;
-        goto out;
+    } else {
+        ret = drv->bdrv_co_truncate(bs, offset, prealloc, &local_err);
     }
 
-    ret = drv->bdrv_co_truncate(bs, offset, prealloc, errp);
-    if (ret < 0) {
+    if (ret == -ENOTSUP && drv->bdrv_file_open) {
+        error_free(local_err);
+
+        ret = bdrv_co_truncate_fallback(child, offset, prealloc, errp);
+        if (ret < 0) {
+            goto out;
+        }
+    } else if (ret < 0) {
+        error_propagate(errp, local_err);
         goto out;
     }
+
     ret = refresh_total_sectors(bs, offset >> BDRV_SECTOR_BITS);
     if (ret < 0) {
         error_setg_errno(errp, -ret, "Could not refresh total sector count");
-- 
2.21.0