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| author | Laurent Bercot <ska-skaware@skarnet.org> | 2017-04-21 09:31:01 +0000 |
|---|---|---|
| committer | Laurent Bercot <ska-skaware@skarnet.org> | 2017-04-21 09:31:01 +0000 |
| commit | 4a22cc63fd2032308a062d2b8ddf7f283432f34f (patch) | |
| tree | b7ddf1087e83736aad74cb49685b3157818be65a | |
| parent | 5dda89c3050733c52320ea5ae3ee42435047ca7f (diff) | |
| download | s6-rc-4a22cc63fd2032308a062d2b8ddf7f283432f34f.tar.xz | |
Fix the advice on how to atomically replace directories with s6-ln or POSIX
| -rw-r--r-- | doc/faq.html | 21 |
1 files changed, 11 insertions, 10 deletions
diff --git a/doc/faq.html b/doc/faq.html index 8df36cb..09c3a99 100644 --- a/doc/faq.html +++ b/doc/faq.html @@ -215,20 +215,21 @@ To ensure that the new database will be used on the next boot, atomically update the link: <pre> olddb=`s6-linkname -f /etc/s6-rc/compiled` - s6-ln -sf compiled-$stamp /etc/s6-rc/compiled + s6-ln -nsf compiled-$stamp /etc/s6-rc/compiled </pre> The use of the <a href="http://skarnet.org/software/s6-portable-utils/s6-ln.html">s6-ln</a> -utility is recommended, because not all implementations of the +utility is recommended, because the <a href="http://pubs.opengroup.org/onlinepubs/9699919799/utilities/ln.html">ln</a> -standard actually perform an atomic replacement. (For instance, <tt>ln -sf</tt> -from GNU coreutils first removes the old link, then creates the new one: -this is not atomic and may break your system if you're unlucky and get a -power failure at the wrong time.) s6-ln is guaranteed to do the right thing. -Alternatively, you could run something like <tt>ln -sf compiled-$stamp -/etc/s6-rc/compiled.new ; mv -f /etc/s6-rc/compiled.new -/etc/s6-rc/compiled</tt>, but that's more complex and may also fail with -bad <tt>mv</tt> implementations. </li> +standard actually <em>forbids</em> an atomic replacement, so utilities that +follow it to the letter, for instance, <tt>ln</tt> from GNU coreutils, cannot +be atomic: they first remove the old link, then create the new one. If you +do not have +<a href="http://skarnet.org/software/s6-portable-utils/s6-ln.html">s6-ln</a>, +you need to perform an unintuitive workaround to get POSIX-compliant tools to +do the right thing: + <tt>ln -sf compiled-$stamp /etc/s6-rc/compiled/compiled && +mv -f /etc/s6-rc/compiled/compiled /etc/s6-rc</tt> </li> <li> Your current compiled database, stored in <tt>/etc/s6-rc/compiled-$stamp</tt>, is now accessible via the <tt>/etc/s6-rc/compiled</tt> symlink and will be used at boot time. You do |