Lektor type to deduce page modification time from git

# Lektor-Git-Timestamp

This Lektor plugin implements a new datetime-like type, gittimestamp, which gets it's default value from git timestamps. This can be used to implement auto-updating pub_date and last_mod fields in Lektor datamodels.

## Description

The gittimestamp type behaves just like the built-in datetime type, except that if the field is left blank in contents.lr a default value will be deduced from git timestamps for the file (or possibly the file’s filesystem mtime.)

If an explicit value for the field is not found, the git log for the source file (typically contents.lr) is searched using git log --follow --remove-empty -- <source_filename>, and the author timestamp of all matching commits are considered. Additionally, if the source file is dirty with respect to git’s HEAD, or if the file is not checked into the git tree, the file’s mtime is prepended to that list of timestamps. That list of timestamps is filtered based on the ignore_commits and skip_first_commit options (see below); then, finally, a timestamp is selected from those that remain based on the setting of the strategy option.

### Options

The gittimestamp type supports the following options.

#### ignore_commits

This can be set to a string, which is interpreted as a regular expression. Any git commits whose commit message matches this pattern are ignored when computing a default timestamp value for the field. (The matching is performed using re.search.)

#### skip_first_commit

If this boolean option is set, the first commit in the git log for the source file will be ignored. This is useful for implementing a last_mod field which has a defined value only if the source file has actually been modified since the initial commit.

#### strategy

This option determines which timestamp is selected from the git log (and/or the file mtime). This can be set to one of four values:

• last: If the source file is dirty (with respect to the git HEAD tree), the mtime of the file is used. Otherwise, the timestamp of the last (nominally the most recent) non-ignored git commit is used. This is the default strategy.

• first: The timestamp of the first (nominally the earliest) commit is used.

• latest: The latest timestamp is used. Normally this produces the same result at last, however due to rebasing, cherry-picking, etc. the git timestamps may not be monotonically increasing, in which case this option causes the greatest (most recent) timestamp remaining after any filtering to be selected.

• earliest: The earliest timestamp is used. Normally this produces the same result at first, but if the timestamps in the git log are not monotonic, this will select the minimum of all the timestamps remaining after any filtering.

## Examples

Here is a simple example excerpt from a datamodel file:

<...>

[fields.last_mod]
type = gittimestamp


On a page using the above datamodel, so long as the last_mod field is left blank in the contents.lr file, the page modification time will be deduced from timestamp of the most recent git commit which affected that contents.lr. (Or if that file is dirty, the value of last_mod will be taken from the file’s filesystem mtime.)

Here is a more complicated example which demonstrates the use of all the options.

<...>

[fields.pub_date]
label = Time first published
type = gittimestamp
strategy = first

[fields.last_mod]
type = gittimestamp
ignore_commits = $nochange$
skip_first_commit = true


This will get the default value of the pub_date field from the timestamp of the first (earliest) git commit for the source file.

The default value for last_mod will, as in the previous example, be taken from the most recent commit for the file, except that:

• any commits whose commit message include the tag [nochange] will be ignored
• the first commit (the one whose timestamp is used for pub_date) is ignored

If there has only been one commit of the source file, last_mod will not have a default value. (It will evaluate to a jinja2 Undefined instance.)

## Warning: On sorting by gittimestamp in Lektor <= 3.2

A common use case for timestamps is for sorting records. E.g. in a blog one generally wants to display posts in reverse chronological order by post date. This generally won't work using gittimestamp timestamps under any currently released version of Lektor.

The gittimestamp type is implemented using a field descriptor. (This is required in order to defer computation of the field value until after the record for the page is available.) In lektor<=3.2, field descriptors are supported for most usages, the one glaring exception being when sorting records.

This was fixed in Lektor PR #789 which was merged to the master branch on February 6, 2021. Unfortunately, the last released version of Lektor (as of August 12, 2021) is version 3.2, released August 20, 2020. For now, you'll have to install Lektor from git if you want to be able to sort records by gittimestamp.

## Author

Jeff Dairiki dairiki@dairiki.org

## Changelog

### Release 0.1.0.post1 (2021-08-12)

No code changes.

Add warning to README about lektor > 3.2 (not yet released) being required in order to be able to sort records by gittimestamp fields.

### Release 0.1 (2021-02-05)

No code changes.

Update development status classifier to "stable".

### Release 0.1a2 (2021-02-03)

#### Bugs Fixed

Fixed attrocious typo which prevented the use of anything other than the default strategy=last for picking timestamps.

Initial release.

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