Add parallel streams test

tests/notebooks/merge_streams_parallel.ipynb

@@ -0,0 +1,186 @@
+{
+  "cells": [
+   {
+    "cell_type": "code",
+    "execution_count": 1,
+    "metadata": {
+     "execution": {
+      "iopub.execute_input": "2024-09-19T21:44:29.809012Z",
+      "iopub.status.busy": "2024-09-19T21:44:29.808809Z",
+      "iopub.status.idle": "2024-09-19T21:44:29.978481Z",
+      "shell.execute_reply": "2024-09-19T21:44:29.977891Z"
+     }
+    },
+    "outputs": [
+     {
+      "name": "stdout",
+      "output_type": "stream",
+      "text": [
+       "0"
+      ]
+     },
+     {
+      "name": "stdout",
+      "output_type": "stream",
+      "text": [
+       "0"
+      ]
+     },
+     {
+      "name": "stdout",
+      "output_type": "stream",
+      "text": [
+       "0"
+      ]
+     },
+     {
+      "name": "stdout",
+      "output_type": "stream",
+      "text": [
+       "0"
+      ]
+     },
+     {
+      "name": "stdout",
+      "output_type": "stream",
+      "text": [
+       "0"
+      ]
+     },
+     {
+      "name": "stdout",
+      "output_type": "stream",
+      "text": [
+       "0"
+      ]
+     },
+     {
+      "name": "stdout",
+      "output_type": "stream",
+      "text": [
+       "0"
+      ]
+     },
+     {
+      "name": "stdout",
+      "output_type": "stream",
+      "text": [
+       "0"
+      ]
+     },
+     {
+      "name": "stdout",
+      "output_type": "stream",
+      "text": [
+       "0"
+      ]
+     },
+     {
+      "name": "stdout",
+      "output_type": "stream",
+      "text": [
+       "\n"
+      ]
+     },
+     {
+      "name": "stdout",
+      "output_type": "stream",
+      "text": [
+       "0"
+      ]
+     },
+     {
+      "name": "stdout",
+      "output_type": "stream",
+      "text": [
+       "\n"
+      ]
+     },
+     {
+      "name": "stdout",
+      "output_type": "stream",
+      "text": [
+       "\n"
+      ]
+     },
+     {
+      "name": "stdout",
+      "output_type": "stream",
+      "text": [
+       "\n"
+      ]
+     },
+     {
+      "name": "stdout",
+      "output_type": "stream",
+      "text": [
+       "\n"
+      ]
+     },
+     {
+      "name": "stdout",
+      "output_type": "stream",
+      "text": [
+       "\n"
+      ]
+     },
+     {
+      "name": "stdout",
+      "output_type": "stream",
+      "text": [
+       "\n"
+      ]
+     },
+     {
+      "name": "stdout",
+      "output_type": "stream",
+      "text": [
+       "\n"
+      ]
+     },
+     {
+      "name": "stdout",
+      "output_type": "stream",
+      "text": [
+       "\n"
+      ]
+     },
+     {
+      "name": "stdout",
+      "output_type": "stream",
+      "text": [
+       "\n"
+      ]
+     }
+    ],
+    "source": [
+     "from concurrent.futures import ProcessPoolExecutor\n",
+     "\n",
+     "with ProcessPoolExecutor() as executor:\n",
+     "    for i in executor.map(print, [0] * 10):\n",
+     "        pass"
+    ]
+   }
+  ],
+  "metadata": {
+   "kernelspec": {
+    "display_name": "Python 3",
+    "language": "python",
+    "name": "python3"
+   },
+   "language_info": {
+    "codemirror_mode": {
+     "name": "ipython",
+     "version": 3
+    },
+    "file_extension": ".py",
+    "mimetype": "text/x-python",
+    "name": "python",
+    "nbconvert_exporter": "python",
+    "pygments_lexer": "ipython3",
+    "version": "3.12.3"
+   }
+  },
+  "nbformat": 4,
+  "nbformat_minor": 4
+ }

tests/test_render_outputs.py

@@ -1,4 +1,5 @@
 """Tests for rendering code cell outputs."""
+
 import pytest
 
 from myst_nb.core.render import EntryPointError, load_renderer
@@ -103,6 +104,18 @@ def test_merge_streams(sphinx_run, file_regression):
     file_regression.check(doctree.pformat(), extension=".xml", encoding="utf-8")
 
 
+@pytest.mark.sphinx_params(
+    "merge_streams_parallel.ipynb",
+    conf={"nb_execution_mode": "off", "nb_merge_streams": True},
+)
+def test_merge_streams_parallel(sphinx_run, file_regression):
+    """Test configuring multiple concurrent stdout/stderr outputs to be merged."""
+    sphinx_run.build()
+    assert sphinx_run.warnings() == ""
+    doctree = sphinx_run.get_resolved_doctree("merge_streams_parallel")
+    file_regression.check(doctree.pformat(), extension=".xml", encoding="utf-8")
+
+
 @pytest.mark.sphinx_params(
     "metadata_image.ipynb",
     conf={"nb_execution_mode": "off", "nb_cell_metadata_key": "myst"},

tests/test_render_outputs/test_merge_streams_parallel.xml

@@ -0,0 +1,21 @@
+<document source="merge_streams_parallel">
+    <container cell_index="0" cell_metadata="{'execution': {'iopub.execute_input': '2024-09-19T21:44:29.809012Z', 'iopub.status.busy': '2024-09-19T21:44:29.808809Z', 'iopub.status.idle': '2024-09-19T21:44:29.978481Z', 'shell.execute_reply': '2024-09-19T21:44:29.977891Z'}}" classes="cell" exec_count="1" nb_element="cell_code">
+        <container classes="cell_input" nb_element="cell_code_source">
+            <literal_block language="ipython3" linenos="False" xml:space="preserve">
+                from concurrent.futures import ProcessPoolExecutor
+
+                with ProcessPoolExecutor() as executor:
+                    for i in executor.map(print, [0] * 10):
+                        pass
+        <container classes="cell_output" nb_element="cell_code_output">
+            <literal_block classes="output stream" language="myst-ansi" linenos="False" xml:space="preserve">
+                0
+                0
+                0
+                0
+                0
+                0
+                0
+                0
+                0
+                0