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Out-of-date determination

The main task for build system is deciding if the target is out-of-date and needs rebuilding. The single most reliable way to do that is to compare file’s content with previously recorded one. But that is too expensive.

So direct content storage/comparison can be replaced with collision-resistant hash function of enough length. goredo uses BLAKE3 with 256-bit output for that purpose.

Also it stores file’s size. Obviously if size differs, then file’s content too and there is no need to read and hash it.

But still it could be relatively expensive. So there are additional possible checks that can skip need of hash checking, based on some trust to the underlying filesystem and operating system behaviour, controlled by $REDO_INODE_TRUST environment variable value:

$REDO_INODE_TRUST=none

Do not trust filesystem at all, except for file’s size knowledge. Most reliable mode.

$REDO_INODE_TRUST=ctime

Trust ctime value of file’s inode. It should change every time inode is updated. If nothing is touched and ctime is the same, then assume that file was not modified and we do not try to read its content. Unfortunately ctime also changes if link count is updated and ownership, that could give false positive decision and force file’s rereading.

$REDO_INODE_TRUST=mtime

Trust mtime value of file’s inode. It should change every time file’s content is updated. But unfortunately there are many reasons it won’t.

Pay attention that although mtime is considered harmful (link above), and is hardly acceptable in build system like Make, because it compares timestamps of two files, redo is satisfied only with the fact of its changing, so badly jumping clocks are not so devastating. Modern filesystem and operating systems with micro- and nano-seconds resolution timestamps should be pretty good choice for $REDO_INODE_TRUST=mtime. However GNU/Linux with ext4 filesystem can easily have pretty big granularity of 10ms.

goredo uses $REDO_INODE_TRUST=ctime by default.


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