util-lib: when copying files make sure to apply some chattrs early, some late

Some chattrs only work sensible if you set them right after opening a
file for create (think: FS_NOCOW_FL). Others only work when they are
applied when the file is fully written (think: FS_IMMUTABLE_FL). Let's
take that into account when copying files and applying a chattr to them.

src/basic/chattr-util.h

@@ -1,6 +1,20 @@
 /* SPDX-License-Identifier: LGPL-2.1+ */
 #pragma once
 
+#include <linux/fs.h>
+
+#include "missing_fs.h"
+
+/* The chattr() flags to apply when creating a new file *before* writing to it. In particular, flags such as
+ * FS_NOCOW_FL don't work if applied a-posteriori. All other flags are fine (or even necessary, think
+ * FS_IMMUTABLE_FL!) to apply after writing to the files. */
+#define CHATTR_EARLY_FL                         \
+        (FS_NOATIME_FL |                        \
+         FS_COMPR_FL   |                        \
+         FS_NOCOW_FL   |                        \
+         FS_NOCOMP_FL  |                        \
+         FS_PROJINHERIT_FL)
+
 int chattr_fd(int fd, unsigned value, unsigned mask, unsigned *previous);
 int chattr_path(const char *p, unsigned value, unsigned mask, unsigned *previous);