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https://issues.apache.org/jira/browse/ARROW-18101?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17620669#comment-17620669
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Dewey Dunnington commented on ARROW-18101:
------------------------------------------
After some sleuthing, it seems that's because an {{ExecPlanReader}} is
collected to a table explicitly using {{RunWithCapturedR}}:
https://github.com/apache/arrow/blob/5984d8a81a6fe8d53e89457d113c931aae59dcd8/r/src/compute-exec.cpp#L191-L198
...but a regular {{RecordBatchReader}} (like the one created by
{{head.RecordBatchReader()}}) is not:
https://github.com/apache/arrow/blob/5984d8a81a6fe8d53e89457d113c931aae59dcd8/r/src/recordbatchreader.cpp#L113-L117
I believe this was on purpose to limit number of places where
{{RunWithCapturedR()}} was used since it might be causing problems in
valgrind/autobrew nightlies (ARROW-17879). Ideally, {{RunWithCapturedR()}}
causes no problems and it's safe to use it to collect any RecordBatchReader
into a table.
> [R] RecordBatchReaderHead from ExecPlan with UDF cannot be read
> ---------------------------------------------------------------
>
> Key: ARROW-18101
> URL: https://issues.apache.org/jira/browse/ARROW-18101
> Project: Apache Arrow
> Issue Type: Bug
> Components: R
> Reporter: Neal Richardson
> Assignee: Dewey Dunnington
> Priority: Major
>
> {code}
> register_scalar_function(
> "times_32",
> function(context, x) x * 32.0,
> int32(),
> float64(),
> auto_convert = TRUE
> )
> record_batch(a = 1:1000) %>%
> dplyr::mutate(b = times_32(a)) %>%
> as_record_batch_reader() %>%
> head(11) %>%
> as_arrow_table()
> # Error: NotImplemented: Call to R (resolve scalar user-defined function
> output data type) from a non-R thread from an unsupported context
> # /arrow/cpp/src/arrow/compute/exec.cc:649
> kernel_->signature->out_type().Resolve(kernel_ctx_, args.inputs)
> # /arrow/cpp/src/arrow/compute/exec/expression.cc:602
> executor->Init(&kernel_context, {kernel, types, options})
> # /arrow/cpp/src/arrow/compute/exec/project_node.cc:91
> ExecuteScalarExpression(simplified_expr, target, plan()->exec_context())
> # /arrow/cpp/src/arrow/record_batch.cc:336 ReadNext(&batch)
> # /arrow/cpp/src/arrow/record_batch.cc:350 ToRecordBatches()
> {code}
> It works fine if you don't call {{as_record_batch_reader()}} in the middle.
> Oddly, it also works fine if you add {{as_adq()}} (aka {{collapse()}}) after
> head() and before evaluating to table--that is, if you run it through an
> ExecPlan again, it doesn't error.
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