fix(r): correct two int64 corruption bugs affecting list<int64> conversion

[meztez]

Fixes #932.

While investigating why converting a list<int64> array to a vctrs::list_of(ptype = bit64::integer64()) target silently produced wrong values (not just a lossy-precision warning), I found two separate, independent bugs, both in r/:

1. as_nanoarrow_array.list() corrupted integer64 list elements while building the child array

It built the flattened child vector with:

child <- unlist(x, recursive = FALSE, use.names = FALSE)

unlist() strips the integer64 S3 class from each element before concatenating, so the elements are concatenated as plain numeric, reinterpreting the raw 64-bit integer bit pattern as a double bit pattern. The resulting child array is corrupted before conversion even runs. Fixed by using do.call(c, x), which dispatches to c.integer64 (and any other registered c() S3 method) and preserves the class/underlying representation.

2. nanoarrow_materialize_int64() used the wrong buffer view for pointer arithmetic

case NANOARROW_TYPE_INT64:
  memcpy(result + dst->offset,
         src->array_view->buffer_views[1].data.as_int32 + raw_src_offset,
         dst->length * sizeof(int64_t));

raw_src_offset is added to data.as_int32, so the offset is scaled by 4 bytes instead of 8. Any int64 array/slice with a nonzero starting offset — such as the per-row child slices nanoarrow builds internally when materializing a list<int64> column — reads from the wrong memory location. Offset-0 conversions (the case covered by existing tests) are unaffected, which is presumably why this slipped through since it was introduced in #293.

Both bugs needed fixing to correctly round-trip a list<int64> through bit64::integer64; either one alone still corrupts values.

Testing

Added regression tests to test-as-array.R and test-convert-array.R. I confirmed both new tests fail against the pre-fix code and pass after the fix (see repro below), and that the full existing r/tests/testthat suite still passes (1408 passed, 0 failed).

Minimal repro of bug 1 (pre-fix):

library(nanoarrow)
schema <- na_list(na_int64())
arr <- as_nanoarrow_array(
  list(bit64::as.integer64(c("9223372036854775295", "2")), bit64::as.integer64("3")),
  schema = schema
)
to <- vctrs::new_list_of(list(), ptype = bit64::integer64())
convert_array(arr, to = to)
#> [[1]]
#> integer64
#> [1] <NA> 0
#>
#> [[2]]
#> integer64
#> [1] 0

After this PR, this returns list(c(9223372036854775295, 2), 3) as expected.

Originally reported against a downstream package: meztez/bigrquerystorage#86.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.30%. Comparing base (f55f85b) to head (6124762).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #933   +/-   ##
=======================================
  Coverage   79.30%   79.30%           
=======================================
  Files         106      106           
  Lines       16223    16223           
  Branches     1882     1882           
=======================================
  Hits        12865    12865           
  Misses       2177     2177           
  Partials     1181     1181           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[paleolimbot]

Thank you!