diff options
Diffstat (limited to 'support')
| -rw-r--r-- | support/Makefile | 1 | ||||
| -rw-r--r-- | support/support.h | 6 | ||||
| -rw-r--r-- | support/support_descriptor_supports_holes.c | 87 |
3 files changed, 94 insertions, 0 deletions
diff --git a/support/Makefile b/support/Makefile index 652d2cdf69..9063046c23 100644 --- a/support/Makefile +++ b/support/Makefile @@ -43,6 +43,7 @@ libsupport-routines = \ support_capture_subprocess \ support_capture_subprocess_check \ support_chroot \ + support_descriptor_supports_holes \ support_enter_mount_namespace \ support_enter_network_namespace \ support_format_address_family \ diff --git a/support/support.h b/support/support.h index b61fe0735c..34ae6e10cd 100644 --- a/support/support.h +++ b/support/support.h @@ -65,6 +65,12 @@ void support_write_file_string (const char *path, const char *contents); the result). */ char *support_quote_blob (const void *blob, size_t length); +/* Returns non-zero if the file descriptor is a regular file on a file + system which supports holes (that is, seeking and writing does not + allocate storage for the range of zeros). FD must refer to a + regular file open for writing, and initially empty. */ +int support_descriptor_supports_holes (int fd); + /* Error-checking wrapper functions which terminate the process on error. */ diff --git a/support/support_descriptor_supports_holes.c b/support/support_descriptor_supports_holes.c new file mode 100644 index 0000000000..c7099ca67c --- /dev/null +++ b/support/support_descriptor_supports_holes.c @@ -0,0 +1,87 @@ +/* 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 (see + block_limit below). */ + 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); + + /* This limit is arbitrary. The file system needs to store + somewhere that data exists at the write offset, and this may + moderately increase the number of blocks used by the file, in + proportion to the initial block count, but not in proportion to + the write offset. */ + 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; +} |