GH-40742: [R] fix max_rows_per_group must be a positive number (#49709)

### Rationale for this change

Error message due to faulty checking

### What changes are included in this PR?

Change checking circumstances

### Are these changes tested?

Yes

### Are there any user-facing changes?

No

### AI usage

Used Claude to generate this - will double check before marking ready for review
* GitHub Issue: #40742

Authored-by: Nic Crane <thisisnic@gmail.com>
Signed-off-by: Nic Crane <thisisnic@gmail.com>

Author: thisisnic

r/R/dataset-write.R

@@ -218,7 +218,12 @@ write_dataset <- function(
   existing_data_behavior_opts <- c("delete_matching", "overwrite", "error")
   existing_data_behavior <- match(match.arg(existing_data_behavior), existing_data_behavior_opts) - 1L
 
-  if (!missing(max_rows_per_file) && missing(max_rows_per_group) && max_rows_per_group > max_rows_per_file) {
+  if (
+    !missing(max_rows_per_file) &&
+      missing(max_rows_per_group) &&
+      max_rows_per_file > 0 &&
+      max_rows_per_group > max_rows_per_file
+  ) {
     max_rows_per_group <- max_rows_per_file
   }
 
@@ -290,7 +295,12 @@ write_delim_dataset <- function(
   quote = c("needed", "all", "none"),
   preserve_order = FALSE
 ) {
-  if (!missing(max_rows_per_file) && missing(max_rows_per_group) && max_rows_per_group > max_rows_per_file) {
+  if (
+    !missing(max_rows_per_file) &&
+      missing(max_rows_per_group) &&
+      max_rows_per_file > 0 &&
+      max_rows_per_group > max_rows_per_file
+  ) {
     max_rows_per_group <- max_rows_per_file
   }
 
@@ -343,7 +353,12 @@ write_csv_dataset <- function(
   quote = c("needed", "all", "none"),
   preserve_order = FALSE
 ) {
-  if (!missing(max_rows_per_file) && missing(max_rows_per_group) && max_rows_per_group > max_rows_per_file) {
+  if (
+    !missing(max_rows_per_file) &&
+      missing(max_rows_per_group) &&
+      max_rows_per_file > 0 &&
+      max_rows_per_group > max_rows_per_file
+  ) {
     max_rows_per_group <- max_rows_per_file
   }
 
@@ -395,7 +410,12 @@ write_tsv_dataset <- function(
   quote = c("needed", "all", "none"),
   preserve_order = FALSE
 ) {
-  if (!missing(max_rows_per_file) && missing(max_rows_per_group) && max_rows_per_group > max_rows_per_file) {
+  if (
+    !missing(max_rows_per_file) &&
+      missing(max_rows_per_group) &&
+      max_rows_per_file > 0 &&
+      max_rows_per_group > max_rows_per_file
+  ) {
     max_rows_per_group <- max_rows_per_file
   }
 

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

@@ -576,6 +576,16 @@ test_that("max_rows_per_group is adjusted if at odds with max_rows_per_file", {
   )
 })
 
+test_that("max_rows_per_file = 0 does not trigger max_rows_per_group adjustment (ARROW-40742)", {
+  skip_if_not_available("parquet")
+
+  # max_rows_per_file = 0 means "no limit" and should not error
+  dst_dir <- make_temp_dir()
+  expect_no_error(
+    write_dataset(df1, dst_dir, max_rows_per_file = 0L)
+  )
+})
+
 
 test_that("write_dataset checks for format-specific arguments", {
   df <- tibble::tibble(