Hugo 0.36.1: One Bugfix

This release fixes a multi-thread issue when reprocessing and reusing images across pages. When doing something like this with the same image from a partial used in, say, both the home page and the single page:

{{ with $img }}
{{ $big := .Fill "1024x512 top" }}
{{ $small := $big.Resize "512x" }}
{{ end }}

There would be timing issues making Hugo in some cases trying to process the same image twice at the same time.

You would experience errors of type:

png: invalid format: not enough pixel data

This commit fixes that by adding a mutex per image. This should also improve the performance, slightly, as it avoids duplicate work.

The current workaround before this fix is to always operate on the original:

{{ with $img }}
{{ $big := .Fill "1024x512 top" }}
{{ $small := .Fill "512x256 top" }}
{{ end }}

This error was rare (no reports on GitHub or the discussion forum), but very hard to debug for the end user.

• Fix multi-threaded image processing issue d8fdffb5 @bep #4404
• Improve error message in .Render 08521dac @bep
• Bump Travis/Snapcraft to Go 1.9.4 fc23a80f @bep
• Improve error processing error message 2851af02 @bep