Add/send a metadata signal.

[vsajip]

Pull Request Checklist

• I’ve read the guidelines for contributing.
• I updated AUTHORS.txt and CHANGES.txt (if the change is non-trivial) and documentation (if applicable).
• I tested my changes.

Description

Send a "metadata" signal to allow interested parties to make additions/changes to the metadata. One example use of this would be to set the "updated" time of a post to the actual modification time of the source file for the post.

[Kwpolska]

I don’t like the approach of mutating the meta dictionary in a signal. Perhaps a MetadataExtractor plugin would suit your needs better?

In any case, this should be documented in at least extending.rst

[vsajip]

I don’t like the approach of mutating the meta dictionary in a signal. Perhaps a MetadataExtractor plugin would suit your needs better?

I looked at that approach first. For my specific use case (setting the "updated" metadata for any post to the modification time of the source file) I couldn't see a way of doing it using MetadataExtractor, because extract_filename() is only called with filenames for paths which match FILE_METADATA_REGEXP. Since you can only have one such regexp, using it to match all posts would mean it couldn't be used with e.g. an actual regexp.

nikola/nikola/post.py

Lines 1196 to 1201 in 650b954

	if config.get('FILE_METADATA_REGEXP'):
	extractors = metadata_extractors_by['source'].get(metadata_extractors.MetaSource.filename, [])
	for extractor in extractors:
	if not metadata_extractors.check_conditions(post, post.source_path, extractor.conditions, config, None):
	continue
	meta.update(extractor.extract_filename(post.source_path, lang))

[vsajip]

In any case, this should be documented in at least extending.rst

If this functionality is going to be merged, I will happily update the documentation to cover it.