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/* Test for file system hole support.
Copyright (C) 2018 Free Software Foundation, Inc.
This file is part of the GNU C Library.
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
The GNU C Library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library; if not, see
<http://www.gnu.org/licenses/>. */
#include <stdbool.h>
#include <support.h>
#include <support/check.h>
#include <sys/stat.h>
#include <xunistd.h>
int
support_descriptor_supports_holes (int fd)
{
enum
{
/* Write offset for the enlarged file. This value is arbitrary
and hopefully large enough to trigger the creation of holes.
We cannot use the file system block size as a reference here
because it is incorrect for network file systems. */
write_offset = 16 * 1024 * 1024,
/* Our write may add this number of additional blocks. */
block_headroom = 8,
};
struct stat64 st;
xfstat (fd, &st);
if (!S_ISREG (st.st_mode))
FAIL_EXIT1 ("descriptor %d does not refer to a regular file", fd);
if (st.st_size != 0)
FAIL_EXIT1 ("descriptor %d does not refer to an empty file", fd);
if (st.st_blocks > block_headroom)
FAIL_EXIT1 ("descriptor %d refers to a pre-allocated file (%lld blocks)",
fd, (long long int) st.st_blocks);
/* Write a single byte at the start of the file to compute the block
usage for a single byte. */
xlseek (fd, 0, SEEK_SET);
char b = '@';
xwrite (fd, &b, 1);
/* Attempt to bypass delayed allocation. */
TEST_COMPARE (fsync (fd), 0);
xfstat (fd, &st);
unsigned long long int block_limit = 2 * st.st_blocks + block_headroom;
/* Write a single byte at 16 megabytes. */
xlseek (fd, write_offset, SEEK_SET);
xwrite (fd, &b, 1);
/* Attempt to bypass delayed allocation. */
TEST_COMPARE (fsync (fd), 0);
xfstat (fd, &st);
bool supports_holes = st.st_blocks <= block_limit;
/* Also check that extending the file does not fill up holes. */
xftruncate (fd, 2 * write_offset);
/* Attempt to bypass delayed allocation. */
TEST_COMPARE (fsync (fd), 0);
xfstat (fd, &st);
supports_holes = supports_holes && st.st_blocks <= block_limit;
/* Return to a zero-length file. */
xftruncate (fd, 0);
xlseek (fd, 0, SEEK_SET);
return supports_holes;
}
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