Provide R < 4.6 entry points for older versions of data.table in atime tests

[aitap]

Fixes: #7693

With these changes, atime::atime_pkg() passes for me on R-4.6.0 and R-devel.

[github-actions]

• HEAD=atime-backports slower P<0.001 for memrecycle regression fixed in #5463
  

Generated via commit a3d2921

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

Task	Duration
R setup and installing dependencies	6 minutes and 32 seconds
Installing different package versions	11 minutes and 49 seconds
Running and plotting the test cases	5 minutes and 23 seconds

[tdhock]

Wow Ivan!
This looks great!!
I’m impressed!!
Thanks very much.

[tdhock]

One thing before merging, can you please add a comment to explain how you landed on this result? (I see several commits in this branch, suggesting that some trials did not work. Maybe explain what did not work and what you learned from that? Or if it is too complex for a comment, provide a link to a blog etc? Or do you think the explanation not necessary?)

[aitap]

The process was mostly try to run tests, fix an error (following the approach of #6640), repeat. I had to backtrack once after I defined VECTOR_PTR(x) as SEXPPTR(x) and an earlier version of data.table failed to compile because it didn't have SEXPPTR yet, but I overwrote that commit before pushing.

Compilation or linking errors were easiest to fix. For example, SETLENGTH does not exist, data.table.so compiles with warnings and links but fails to load. Solution: copy enough definitions from Defn.h to implement the function. Another example: VECTOR_PTR does not exist. The code uses VECTOR_PTR(foo)[bar]. The implicit declaration provided by the C compiler is int VECTOR_PTR(SEXP), and dereferencing an int like an array/pointer doesn't make sense, which results in a compiler error. Solution: provide a VECTOR_PTR.

An interesting workaround is the -DSTRING_PTR_RO=STRING_PTR_RO flag in Makevars. There used to be a test for R ≥ 3.5 in src/data.table.h: if STRING_PTR_RO is not defined as a macro, use the preprocessor to remap it to STRING_PTR. It worked well as long as our use of USE_RINTERNALS caused R to define some internal entry points as macros. Modern versions of R only offer STRING_PTR_RO as a function, not a macro; thus the code tries to rename STRING_PTR_RO to STRING_PTR, which no longer exists. Solution: satisfy the test for the macro without breaking anything else.

Some problems were only visible as compilation warnings. For example, int OBJECT(SEXP) still remains exported from R despite not being declared, so when data.table tries to call OBJECT(foo), the implicit function declaration provided by the C compiler is enough to produce working code. It is important to notice and fix them as well, because C uses the int type for the return value of undeclared functions, and an int foo(...) in place of SEXP foo(...) may cause very confusing crashes after the code successfully compiles and links.

Another interesting workaround is #define REFCNT(x) NAMED(x). An old version of data.table defines USE_RINTERNALS, and Rinternals.h in R-4.6.0 still contains an #ifdef USE_RINTERNALS branch that defines MAYBE_SHARED() as a macro that calls REFCNT(). REFCNT() is still exported, but no longer declared, so the code compiles with a warning but links and works. Since we already have NAMED() because some other data.table version demanded it, it's easy enough to silence this compilation warning as well.

An old version of data.table did not include Rversion.h, so I had to include it a second time.

[MichaelChirico]

This is great! I wonder if other packages would benefit from something similar. Would adding this to {backports} or as a standalone package make sense?