| 1 | #!/usr/bin/perl |
| 2 | # |
| 3 | # This script lets you update a hierarchy of files in an atomic way by |
| 4 | # first creating a new hierarchy using rsync's --link-dest option, and |
| 5 | # then swapping the hierarchy into place. **See the usage message for |
| 6 | # more details and some important caveats!** |
| 7 | |
| 8 | use strict; |
| 9 | use warnings; |
| 10 | use Cwd 'abs_path'; |
| 11 | |
| 12 | my $RSYNC_PROG = '/usr/bin/rsync'; |
| 13 | my $RM_PROG = '/bin/rm'; |
| 14 | |
| 15 | my $dest_dir = $ARGV[-1]; |
| 16 | &usage if !defined $dest_dir || $dest_dir =~ /(^-|^$)/ || grep(/^--help/, @ARGV); |
| 17 | $dest_dir =~ s{(?<=.)/+$} {}; |
| 18 | |
| 19 | if (!-d $dest_dir) { |
| 20 | die "$dest_dir is not a directory.\nUse --help for help.\n"; |
| 21 | } |
| 22 | |
| 23 | if (@_ = grep(/^--[a-z]+-dest\b/, @ARGV)) { |
| 24 | $_ = join(' or ', @_); |
| 25 | die "You cannot use the $_ option with atomic-rsync.\nUse --help for help.\n"; |
| 26 | } |
| 27 | |
| 28 | my $symlink_content = readlink $dest_dir; # undef when a real dir |
| 29 | |
| 30 | my $dest_arg = $dest_dir; |
| 31 | # This gives us the real destination dir, with all symlinks dereferenced. |
| 32 | $dest_dir = abs_path($dest_dir); |
| 33 | if ($dest_dir eq '/') { |
| 34 | die qq|You must not use "/" as the destination directory.\nUse --help for help.\n|; |
| 35 | } |
| 36 | |
| 37 | my($old_dir, $new_dir); |
| 38 | if (defined $symlink_content && $dest_dir =~ /-([12])$/) { |
| 39 | my $num = 3 - $1; |
| 40 | $old_dir = undef; |
| 41 | ($new_dir = $dest_dir) =~ s/-[12]$/-$num/; |
| 42 | $symlink_content =~ s/-[12]$/-$num/; |
| 43 | } else { |
| 44 | $old_dir = "$dest_dir~old~"; |
| 45 | $new_dir = "$dest_dir~new~"; |
| 46 | } |
| 47 | |
| 48 | $ARGV[-1] = "$new_dir/"; |
| 49 | |
| 50 | system($RM_PROG, '-rf', $old_dir) if defined $old_dir && -d $old_dir; |
| 51 | system($RM_PROG, '-rf', $new_dir) if -d $new_dir; |
| 52 | |
| 53 | if (system($RSYNC_PROG, "--link-dest=$dest_dir", @ARGV)) { |
| 54 | if ($? == -1) { |
| 55 | print "failed to execute $RSYNC_PROG: $!\n"; |
| 56 | } elsif ($? & 127) { |
| 57 | printf "child died with signal %d, %s coredump\n", |
| 58 | ($? & 127), ($? & 128) ? 'with' : 'without'; |
| 59 | } else { |
| 60 | printf "child exited with value %d\n", $? >> 8; |
| 61 | } |
| 62 | exit $?; |
| 63 | } |
| 64 | |
| 65 | if (!defined $old_dir) { |
| 66 | atomic_symlink($symlink_content, $dest_arg); |
| 67 | exit; |
| 68 | } |
| 69 | |
| 70 | rename($dest_dir, $old_dir) or die "Unable to rename $dest_dir to $old_dir: $!"; |
| 71 | rename($new_dir, $dest_dir) or die "Unable to rename $new_dir to $dest_dir: $!"; |
| 72 | |
| 73 | exit; |
| 74 | |
| 75 | sub atomic_symlink |
| 76 | { |
| 77 | my($target, $link) = @_; |
| 78 | my $newlink = "$link~new~"; |
| 79 | |
| 80 | unlink($newlink); # Just in case |
| 81 | symlink($target, $newlink) or die "Unable to symlink $newlink -> $target: $!\n"; |
| 82 | rename($newlink, $link) or die "Unable to rename $newlink to $link: $!\n"; |
| 83 | } |
| 84 | |
| 85 | |
| 86 | sub usage |
| 87 | { |
| 88 | die <<EOT; |
| 89 | Usage: atomic-rsync [RSYNC-OPTIONS] HOST:/SOURCE/DIR/ /DEST/DIR/ |
| 90 | atomic-rsync [RSYNC-OPTIONS] HOST::MOD/DIR/ /DEST/DIR/ |
| 91 | |
| 92 | This script lets you update a hierarchy of files in an atomic way by first |
| 93 | creating a new hierarchy (using hard-links to leverage the existing files), |
| 94 | and then swapping the new hierarchy into place. You must be pulling files |
| 95 | to a local directory, and that directory must already exist. For example: |
| 96 | |
| 97 | mkdir /local/files-1 |
| 98 | ln -s files-1 /local/files |
| 99 | atomic-rsync -av host:/remote/files/ /local/files/ |
| 100 | |
| 101 | If /local/files is a symlink to a directory that ends in -1 or -2, the |
| 102 | copy will go to the alternate suffix and the symlink will be changed to |
| 103 | point to the new dir. This is a fully atomic update. If the destination |
| 104 | is not a symlink (or not a symlink to a *-1 or a *-2 directory), this |
| 105 | will instead create a directory with "~new~" suffixed, move the current |
| 106 | directory to a name with "~old~" suffixed, and then move the ~new~ |
| 107 | directory to the original destination name (this double rename is not |
| 108 | fully atomic, but is rapid). In both cases, the prior destintaion |
| 109 | directory will be preserved until the next update, at which point it |
| 110 | will be deleted. |
| 111 | |
| 112 | In all likelihood, you do NOT want to specify this command: |
| 113 | |
| 114 | atomic-rsync -av host:/remote/files /local/ |
| 115 | |
| 116 | ... UNLESS you want the entire /local dir to be swapped out! |
| 117 | |
| 118 | See the "rsync" command for its list of options. You may not use the |
| 119 | --link-dest, --compare-dest, or --copy-dest options (since this script |
| 120 | uses --link-dest to make the transfer efficient). |
| 121 | EOT |
| 122 | } |