[Qemu-block] [PATCH 2/7] block: Add blk_truncate_for_formatting()

[Max Reitz]

Signed-off-by: Max Reitz <address@hidden>
---
 include/sysemu/block-backend.h | 12 ++++++++
 block/block-backend.c          | 54 ++++++++++++++++++++++++++++++++++
 2 files changed, 66 insertions(+)

diff --git a/include/sysemu/block-backend.h b/include/sysemu/block-backend.h
index 733c4957eb..cd9ec8bf52 100644
--- a/include/sysemu/block-backend.h
+++ b/include/sysemu/block-backend.h
@@ -236,6 +236,18 @@ int blk_pwrite_compressed(BlockBackend *blk, int64_t 
offset, const void *buf,
                           int bytes);
 int blk_truncate(BlockBackend *blk, int64_t offset, PreallocMode prealloc,
                  Error **errp);
+
+/**
+ * Wrapper of blk_truncate() for format drivers that need to truncate
+ * their protocol node before formatting it.
+ * Invoke blk_truncate() to truncate the file to @offset; if that
+ * fails with -ENOTSUP (and the file is already big enough), try to
+ * overwrite the first sector with zeroes.  If that succeeds, return
+ * success.
+ */
+int blk_truncate_for_formatting(BlockBackend *blk, int64_t offset,
+                                Error **errp);
+
 int blk_pdiscard(BlockBackend *blk, int64_t offset, int bytes);
 int blk_save_vmstate(BlockBackend *blk, const uint8_t *buf,
                      int64_t pos, int size);
diff --git a/block/block-backend.c b/block/block-backend.c
index a8d160fd5d..c0e64b1ee1 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -2041,6 +2041,60 @@ int blk_truncate(BlockBackend *blk, int64_t offset, 
PreallocMode prealloc,
     return bdrv_truncate(blk->root, offset, prealloc, errp);
 }
 
+int blk_truncate_for_formatting(BlockBackend *blk, int64_t offset, Error 
**errp)
+{
+    Error *local_err = NULL;
+    int64_t current_size;
+    int bytes_to_clear;
+    int ret;
+
+    ret = blk_truncate(blk, offset, PREALLOC_MODE_OFF, &local_err);
+    if (ret < 0 && ret != -ENOTSUP) {
+        error_propagate(errp, local_err);
+        return ret;
+    } else if (ret >= 0) {
+        return ret;
+    }
+
+    current_size = blk_getlength(blk);
+    if (current_size < 0) {
+        error_free(local_err);
+        error_setg_errno(errp, -current_size,
+                         "Failed to inquire new image file's current length");
+        return current_size;
+    }
+
+    if (current_size < offset) {
+        /* Need to grow the image, but we failed to do that */
+        error_propagate(errp, local_err);
+        return -ENOTSUP;
+    }
+
+    error_free(local_err);
+    /*
+     * We can deal with images that are too big.  We just need to
+     * clear the first sector.
+     */
+
+    bytes_to_clear = MIN(current_size, BDRV_SECTOR_SIZE) - offset;
+    if (bytes_to_clear) {
+        if (!(blk->root->perm & BLK_PERM_WRITE)) {
+            error_setg(errp, "Cannot clear first sector of new image: "
+                       "Write permission missing");
+            return -EPERM;
+        }
+
+        ret = blk_pwrite_zeroes(blk, offset, bytes_to_clear, 0);
+        if (ret < 0) {
+            error_setg_errno(errp, -ret, "Failed to clear the first sector of "
+                             "the new image");
+            return ret;
+        }
+    }
+
+    return 0;
+}
+
 static void blk_pdiscard_entry(void *opaque)
 {
     BlkRwCo *rwco = opaque;
-- 
2.21.0