From 3176671662bda79d4b4059b8cc22b4b31a6547e0 Mon Sep 17 00:00:00 2001
From: Maurizio Lombardi <mlombard@redhat.com>
Date: Fri, 13 Nov 2020 11:44:40 +0100
Subject: [PATCH] fileio backstore: fix sparse file creation

fallocate() can't be used to create sparse files because it
actually preallocates all the disk space that will be used by the file
backstore, sparse files do not have preallocated disk space
by definition.
We must therefore use ftruncate().

We can, on the other hand, use fallocate() to create non-sparse
files and fall back to the slower "while() fwrite()" if we
are running on Python version < 3.3 where fallocate() is not available

Fixes 3bd4d8ef7c9b154c53e8b8dd863a570bce7f5c2c

Signed-off-by: Maurizio Lombardi <mlombard@redhat.com>
---
 targetcli/ui_backstore.py | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/targetcli/ui_backstore.py b/targetcli/ui_backstore.py
index 8692f22..9bc0c58 100644
--- a/targetcli/ui_backstore.py
+++ b/targetcli/ui_backstore.py
@@ -423,17 +423,17 @@ class UIFileIOBackstore(UIBackstore):
             raise ExecutionError("Could not open %s" % filename)
         try:
             if sparse:
+                os.ftruncate(f.fileno(), size)
+            else:
+                self.shell.log.info("Writing %d bytes" % size)
                 try:
+                    # Prior to version 3.3, Python does not provide fallocate
                     os.posix_fallocate(f.fileno(), 0, size)
                 except AttributeError:
-                    # Prior to version 3.3, Python does not provide fallocate
-                    os.ftruncate(f.fileno(), size)
-            else:
-                self.shell.log.info("Writing %d bytes" % size)
-                while size > 0:
-                    write_size = min(size, 1024)
-                    f.write("\0" * write_size)
-                    size -= write_size
+                    while size > 0:
+                        write_size = min(size, 1024)
+                        f.write("\0" * write_size)
+                        size -= write_size
         except (OSError, IOError):
             os.remove(filename)
             raise ExecutionError("Could not expand file to %d bytes" % size)
-- 
2.28.0