[R] source_node alignment warning

[jonkeane]

Describe the enhancement requested

When debugging some sanitizer issues, I noticed:

Start test: streaming map_batches into an ExecPlan
  'test-dataset.R:666:3' [success]
  'test-dataset.R:677:3' [success]
  'test-dataset.R:689:3' [success]
  'test-dataset.R:698:3' [success]
/Users/runner/work/crossbow/crossbow/arrow/cpp/src/arrow/acero/source_node.cc:76: An input buffer was poorly aligned.  This could lead to crashes or poor performance on some hardware.  Please ensure that all Acero sources generate aligned buffers, or change the unaligned buffer handling configuration to silence this warning.
  'test-dataset.R:709:3' [success]
End test: streaming map_batches into an ExecPlan

arrow/r/tests/testthat/test-dataset.R

Lines 698 to 706 in c2ec1cd

	expect_equal(
	ds %>%
	filter(int > 5) %>%
	select(int, lgl) %>%
	map_batches(~ .$Take(0)) %>%
	arrange(int) %>%
	collect(),
	tibble(int = c(6, 101), lgl = c(TRUE, TRUE))
	)

is the test that the warning is popping up on.

Component(s)

R

[thisisnic]

Hey, I'm experimenting with using LLMs for debugging, and I'm going to summarise what I found here from inputting some code and docs files into chatGPT. I'm going to go slow on this as I don't want to waste folks' time looking into inaccurate solutions, so if this is nonsense, I'll stop with this approach in areas of the codebase I'm not familiar with.

The problem is that in TransferZeroCopy, the Buffers are created directly from memory owned by the Parquet page reader. When the RecordReader or file reader is destroyed, the memory backing those Buffers can disappear - but the Arrow Array still assumes the Buffers stay valid.

Arrow expects Buffers to own or strongly reference their memory. It doesn't track when memory can be deallocated; it trusts that if a Buffer exists, its memory is alive.

Fix:
When creating Buffers from RecordReader memory, the code should attach a shared_ptr back to the owner (e.g., the page reader or file reader) so that the memory stays alive for as long as the Buffer does.
Otherwise, downstream operations like Take, filter, or materializing lazy Arrays can cause use-after-free bugs.

[github-actions]

This issue has been marked as stale because it has had no activity in the past 365 days. Please remove the stale label or comment below, or this issue will be closed in 14 days. If this improvement is still desired but has no current owner, please add the 'Status: needs champion' label.

[thisisnic]

Issue resolved by pull request 50120
#50120