copy(): only overwrite the list when necessary, consider the attributes

[aitap]

Rewrite copy() in C so that instead of calling y[] <- lapply(y, reallocate) we can consider each individual list element and only call SET_VECTOR_ELT when necessary. This avoids the problem with xgboost's special altlist that must never be altered.

Also walk the attributes of the object being copied, look for data.tables and reallocate them. Maybe that's not necessary (copy() has been working fine without it) and can be taken out.

Test 2224.3 points out an important detail: when y is a LISTSXP pairlist, y[] <- lapply(y, reallocate) makes it into a VECSXP list instead. It turns out we've been relying on copy() automatically coercing pairlist columns into vector lists when constructing data.tables. Should this become more explicit and be moved somewhere into the data.table() constructor, leaving copy() to just duplicate + reallocate data.tables?

Unfortunately this is hard to test without loading xgboost or creating a different altclass.

Fixes: #7456

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.02%. Comparing base (94649d5) to head (cf6449a).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7820   +/-   ##
=======================================
  Coverage   99.01%   99.02%           
=======================================
  Files          88       88           
  Lines       17234    17250   +16     
=======================================
+ Hits        17065    17081   +16     
  Misses        169      169           

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[github-actions]

• HEAD=fix7456 much slower for transform improved in #5493
  

Generated via commit cf6449a

Download link for the artifact containing the test results: ↓ atime-results.zip

Task	Duration
R setup and installing dependencies	3 minutes and 27 seconds
Installing different package versions	24 seconds
Running and plotting the test cases	5 minutes and 29 seconds