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Improper dependency tracking for sass imports #1451

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Ecco opened this issue Aug 13, 2019 · 6 comments · Fixed by #1452

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@Ecco
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commented Aug 13, 2019

Steps to reproduce

  1. Run bundle exec nanoc compile
  2. Modify a single file
  3. Run bundle exec nanoc compile again

Expected behavior

  • Goes rather fast (hopefully less than a second)
  • Time taken is the sum of the time it took each "update/create/delete" steps

Actual behavior

Loading site… done
Compiling site…
      update  [0.00s]  output/jobs/index.html

Site compiled in 9.31s.
```

And yes, it really took 10 seconds…

### Details

- There are 324 files in the `content` directory of this nanoc website.
- The `Loading site... done` is super quick.
@Ecco

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commented Aug 13, 2019

Ok, first step towards solving my issue : running bundle exec nanoc compile --verbose shows a lot more info, and some "identical" files which do indeed take a lot of time to build.

Suggestion:

  • Maybe nanoc shouldn't hide "identical" log lines?
  • If the total time is much greater than the sum of lines displayed, maybe nanoc should display "identical lines"? Or suggest a re-run with --verbose?
@ddfreyne

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commented Aug 13, 2019

Hey @Ecco, do you have a copy of your site that I could get hold of? (I believe you'd be able to share a private GitHub repository with me.)

@Ecco

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commented Aug 14, 2019

Yeah, I'm trying to do my homework first and if really I can't find anything I'll ask for your help 😄

What I've found so far: the problem comes from a sass file that has a huge list of dependencies that don't make any sense. Here's a small extract of bundle exec nanoc show-data:

item /shared/site/stylesheets/style.sass depends on:
  [   item ] (r___) /en/legal/warranty/index.html
  [   item ] (r___) /favicon.ico
  [   item ] (r___) /shared/site/images/flags/fr.svg
  [   item ] (r___) /shared/site/images/flags/us.svg
  [   item ] (r___) /shared/site/images/flags/uk.svg

I'm trying to figure out where this dependency comes from, so far I haven't found anything intersting.

What happens is that if I modify "/en/legal/warranty/index.html", then nanoc think the sass file needs to be rebuilt. And it's kinda big, so it takes a while. Eventually nanoc figures out it built it for nothing because it's identical to the previous one, so it doesn't log an "identical" line, which causes a discrepancy between the "total" line and the actual sum of logged times.

@Ecco

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commented Aug 14, 2019

Ok, I think I found the bug. It's a big hairy one 😄 It happens when nanoc processes an @import sass directive.

  1. Nanoc processes an @import foobar directive using its sass filter
  2. Sass process this content, and turns back to nanoc, calling Nanoc::Filters::SassCommon::Importer#find_relative
  3. This calls raw_filename_to_item on the same class
  4. Which eventually calls Nanoc::Filters::SassCommon::Importer::raw_filename_to_item_map_for_config
  5. This class methods does this:
items.each do |item|
  if item.raw_filename

And, if I read Nanoc::Filter#depend_on correctly, that's how a filter registers a dependency. For the record, here's the code of this method:

def depend_on(items)
  items.flat_map(&:reps).flat_map(&:raw_path)
  items.each(&:raw_filename)
end

Long story short, the sass filter creates tons of spurrious, random dependencies. This doesn't yield a bad compilation, but it definitely slows things down an awful lot!

@Ecco Ecco changed the title Nanoc takes a really long time to rebuild Improper dependency tracking for sass imports Aug 14, 2019

@ddfreyne

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commented Aug 17, 2019

Thank you for the detective work and the quick fix! A release with this fix in it will be out soon.

@ddfreyne

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commented Aug 17, 2019

This is now released in Nanoc 4.11.9.

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