Fix a severe performance regression on the posts/index page introduced
by 6ca42947.
Short answer: scan_query dynamically allocated a regex inside an
inner loop that was called thousands of times per pageload.
Long answer:
* The post index page checks each post to see if they're tagged loli/shota,
* This triggers a call to Post#tag_array for every post.
* Post#tag_array called scan_query to split the tag string.
* scan_query loops over the tag string, checking if each tag matches the
regex /#{METATAGS.join("|")}:/.
* This regex uses string interpolation, which makes Ruby treat as a
dynamic value rather than a static value. Ruby doesn't know the
interpolation is static here. This causes the regex to be reallocated
on every iteration of the loop, or in other words, for every tag in
the tag string.
* This caused us to do thousands of regex allocations per pageload. On
average, a posts/index pageload contains 20 posts with ~35 tags per
post, or 7000+ total tags. Doing this many allocations killed performance.
The fix:
* Don't use scan_query for Post#tag_array. We don't have to fully parse
the tag_string here, we can use a simple split.
* Use the /o regex flag to tell Ruby to treat the regex as static and
only evaluate the interpolation once.
626c2723d7