ARROW-8354: [R] Fix segfault in Table to Array conversion

The Converter::Make did not like receiving empty ArrayVector. The bug was exposed in ARROW-8216 which could return an empty selection vector due to a randomly generated fixture in test-dplyr.R

Closes #6871 from fsaintjacques/ARROW-8354

Lead-authored-by: François Saint-Jacques <fsaintjacques@gmail.com>
Co-authored-by: Neal Richardson <neal.p.richardson@gmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>

r/R/table.R

@@ -194,7 +194,7 @@ Table$create <- function(..., schema = NULL) {
 }
 
 #' @export
-as.data.frame.Table <- function(x, row.names = NULL, optional = FALSE, use_threads = TRUE, ...){
+as.data.frame.Table <- function(x, row.names = NULL, optional = FALSE, ...) {
   Table__to_dataframe(x, use_threads = option_use_threads())
 }
 

r/src/array_from_vector.cpp

@@ -295,7 +295,9 @@ std::shared_ptr<Array> MakeStructArray(SEXP df, const std::shared_ptr<DataType>&
   for (int i = 0; i < n; i++) {
     children[i] = Array__from_vector(VECTOR_ELT(df, i), type->child(i)->type(), true);
   }
-  return std::make_shared<StructArray>(type, children[0]->length(), children);
+
+  int64_t rows = n ? children[0]->length() : 0;
+  return std::make_shared<StructArray>(type, rows, children);
 }
 
 std::shared_ptr<Array> MakeListArray(SEXP x, const std::shared_ptr<DataType>& type) {

r/src/array_to_vector.cpp

@@ -656,8 +656,15 @@ class Converter_Null : public Converter {
     return Status::OK();
   }
 };
+
 std::shared_ptr<Converter> Converter::Make(const ArrayVector& arrays) {
-  switch (arrays[0]->type_id()) {
+  if (arrays.empty()) {
+    Rcpp::stop(tfm::format("Must have at least one array to create a converter"));
+  }
+
+  auto type = arrays[0]->type();
+
+  switch (type->id()) {
     // direct support
     case Type::INT32:
       return std::make_shared<arrow::r::Converter_SimpleArray<INTSXP>>(arrays);
@@ -742,7 +749,7 @@ std::shared_ptr<Converter> Converter::Make(const ArrayVector& arrays) {
       break;
   }
 
-  Rcpp::stop(tfm::format("cannot handle Array of type %s", arrays[0]->type()->name()));
+  Rcpp::stop(tfm::format("cannot handle Array of type %s", type->name()));
   return nullptr;
 }
 
@@ -813,7 +820,6 @@ SEXP Array__as_vector(const std::shared_ptr<arrow::Array>& array) {
 
 // [[arrow::export]]
 SEXP ChunkedArray__as_vector(const std::shared_ptr<arrow::ChunkedArray>& chunked_array) {
-  // NB: this segfaults if there are 0 chunks (presumably something tries chunks[0])
   return arrow::r::ArrayVector__as_vector(chunked_array->length(),
                                           chunked_array->chunks());
 }
@@ -849,7 +855,6 @@ Rcpp::List Table__to_dataframe(const std::shared_ptr<arrow::Table>& table,
   std::vector<std::shared_ptr<arrow::r::Converter>> converters(nc);
 
   for (int64_t i = 0; i < nc; i++) {
-    // This crashes if num_chunks == 0
     converters[i] = arrow::r::Converter::Make(table->column(i)->chunks());
     names[i] = table->field(i)->name();
   }

r/src/compute.cpp

@@ -234,7 +234,14 @@ std::shared_ptr<arrow::Table> Table__Filter(const std::shared_ptr<arrow::Table>&
     options.null_selection_behavior = arrow::compute::FilterOptions::EMIT_NULL;
   }
   STOP_IF_NOT_OK(arrow::compute::Filter(&context, table, filter, options, &out));
-  return out.table();
+  std::shared_ptr<arrow::Table> tab = out.table();
+  if (tab->num_rows() == 0) {
+    // Slight hack: if there are no rows in the result, instead do a 0-length
+    // slice so that we get chunked arrays with 1 chunk (itself length 0).
+    // We need that because the Arrow-to-R converter fails when there are 0 chunks.
+    return table->Slice(0, 0);
+  }
+  return tab;
 }
 
 // [[arrow::export]]
@@ -249,6 +256,13 @@ std::shared_ptr<arrow::Table> Table__FilterChunked(
     options.null_selection_behavior = arrow::compute::FilterOptions::EMIT_NULL;
   }
   STOP_IF_NOT_OK(arrow::compute::Filter(&context, table, filter, options, &out));
-  return out.table();
+  std::shared_ptr<arrow::Table> tab = out.table();
+  if (tab->num_rows() == 0) {
+    // Slight hack: if there are no rows in the result, instead do a 0-length
+    // slice so that we get chunked arrays with 1 chunk (itself length 0).
+    // We need that because the Arrow-to-R converter fails when there are 0 chunks.
+    return table->Slice(0, 0);
+  }
+  return tab;
 }
 #endif

r/tests/testthat/helper-arrow.R

@@ -23,6 +23,8 @@ if (tolower(Sys.info()[["sysname"]]) == "windows") {
   options(arrow.use_threads = FALSE)
 }
 
+set.seed(1)
+
 test_that <- function(what, code) {
   testthat::test_that(what, {
     skip_if(getOption("..skip.tests", TRUE), "arrow C++ library not available")

r/tests/testthat/test-Array.R

@@ -413,6 +413,11 @@ test_that("Array$create() can handle data frame with custom struct type (not inf
   expect_error(Array$create(df, type = type), regexp = "Expecting a character vector")
 })
 
+test_that("Array$create() supports tibble with no columns (ARROW-8354)", {
+  df <- tibble::tibble()
+  expect_equal(Array$create(df)$as_vector(), df)
+})
+
 test_that("Array$create() handles vector -> list arrays (ARROW-7662)", {
   # Should be able to create an empty list with a type hint.
   expect_is(Array$create(list(), list_of(bool())), "ListArray")

r/tests/testthat/test-dplyr.R

@@ -80,6 +80,7 @@ tbl <- tibble::tibble(
   int = 1:10,
   dbl = as.numeric(1:10),
   lgl = sample(c(TRUE, FALSE, NA), 10, replace = TRUE),
+  false = logical(10),
   chr = letters[1:10],
   fct = factor(letters[1:10])
 )
@@ -118,6 +119,16 @@ test_that("filter() with NAs in selection", {
   )
 })
 
+test_that("Filter returning an empty Table should not segfault (ARROW-8354)", {
+  expect_dplyr_equal(
+    input %>%
+      filter(false) %>%
+      select(chr, int, lgl) %>%
+      collect(),
+    tbl
+  )
+})
+
 test_that("filtering with expression", {
   char_sym <- "b"
   expect_dplyr_equal(