xz: Add --synchronous #159
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xz's default behavior is to delete the input file after successful
compression or decompression (unless writing to standard output).
If the system crashes soon after the deletion, it is possible that
the newly written file has not yet hit the disk while the previous
delete operation might have. In that case neither the original file
nor the written file is available.
The --synchronous option makes xz call fsync() on the file and possibly
the directory where the file was created. A similar option was added to
GNU gzip 1.7 in 2016. There some differences in behavior:
- When writing to standard output and processing multiple input files,
xz calls fsync() after every file while gzip does so only after all
files have been processed.
- This has no effect on "xz --list". xz doesn't sync standard output
in --list mode but gzip does.
Portability notes:
- <libgen.h> and dirname() should be available on all POSIX systems,
and aren't needed on non-POSIX systems.
- fsync() is available on all POSIX systems. The directory syncing
could be changed to fdatasync() although at least on ext4 it
doesn't seem to make a performance difference in xz's usage.
fdatasync() would need a build system check to support (old)
special cases, for example, MINIX 3.3.0 doesn't have fdatasync()
and Solaris 10 needs -lrt.
- On native Windows, _commit() is used to replace fsync(). Directory
syncing isn't done and shouldn't be needed. (In Cygwin, fsync() on
directories is a no-op.) It is known that syncing will fail if
writing to stdout and stdout isn't a regular file.
- DJGPP has fsync() for files. ;-)
Co-authored-by: Sebastian Andrzej Siewior <[email protected]>
Link: https://bugs.debian.org/814089
Link: https://www.mail-archive.com/[email protected]/msg00282.html
Closes: #151
Closes: #159
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xz's default behavior is to delete the input file after successful compression or decompression (unless writing to standard output). If the system crashes soon after the deletion, it is possible that the newly written file has not yet hit the disk while the previous delete operation might have. In that case neither the original file nor the written file is available.
The
--synchronousoption makes xz callfsync()on the file and possibly the directory where the file was created. A similar option was added to GNU gzip 1.7 in 2016.