BUG: write MonteCarlo input and output rows atomically (#1110)

rocketpy/simulation/monte_carlo.py

@@ -267,6 +267,39 @@ def __setup_files(self, append):
         except OSError as error:
             raise OSError(f"Error creating files: {error}") from error
 
+    def _append_simulation_record(self, inputs_json, outputs_json):
+        """Append one simulation's inputs and outputs as a paired record.
+
+        Writes the inputs row first, then the outputs row. If the outputs write
+        fails, the inputs file is truncated back to its size before this call so
+        the two files do not drift out of alignment.
+
+        Parameters
+        ----------
+        inputs_json : str
+            Serialized inputs row, including its trailing newline.
+        outputs_json : str
+            Serialized outputs row, including its trailing newline.
+        """
+        input_path = self.input_file
+        output_path = self.output_file
+
+        try:
+            previous_input_size = os.path.getsize(input_path)
+        except OSError:
+            previous_input_size = 0
+
+        with open(input_path, "a", encoding="utf-8") as f:
+            f.write(inputs_json)
+
+        try:
+            with open(output_path, "a", encoding="utf-8") as f:
+                f.write(outputs_json)
+        except Exception:
+            with open(input_path, "rb+") as f:
+                f.truncate(previous_input_size)
+            raise
+
     def __run_in_serial(self):
         """
         Runs the monte carlo simulation in serial mode.
@@ -289,10 +322,7 @@ def __run_in_serial(self):
                 inputs_json = self.__evaluate_flight_inputs(sim_monitor.count)
                 outputs_json = self.__evaluate_flight_outputs(flight, sim_monitor.count)
 
-                with open(self.input_file, "a", encoding="utf-8") as f:
-                    f.write(inputs_json)
-                with open(self.output_file, "a", encoding="utf-8") as f:
-                    f.write(outputs_json)
+                self._append_simulation_record(inputs_json, outputs_json)
 
                 sim_monitor.print_update_status()
 
@@ -431,10 +461,7 @@ def __sim_producer(self, seed, sim_monitor, mutex, error_event):  # pylint: disa
 
                         break
 
-                    with open(self.input_file, "a", encoding="utf-8") as f:
-                        f.write(inputs_json)
-                    with open(self.output_file, "a", encoding="utf-8") as f:
-                        f.write(outputs_json)
+                    self._append_simulation_record(inputs_json, outputs_json)
 
                     sim_monitor.print_update_status()
                 finally:

tests/unit/simulation/test_monte_carlo.py

@@ -1,7 +1,10 @@
+import builtins
 import csv
 import json
+import os
 import pathlib
 from collections import namedtuple
+from unittest.mock import patch
 
 import matplotlib as plt
 import numpy as np
@@ -86,6 +89,37 @@ def __init__(self):
         }
 
 
+def test_append_simulation_record_rolls_back_inputs_on_output_failure(tmp_path):
+    """If the outputs append fails, the inputs row must not remain on disk."""
+    mc = MockMonteCarlo()
+    input_file = tmp_path / "inputs.json"
+    output_file = tmp_path / "outputs.json"
+    input_file.write_text('{"index": 0}\n', encoding="utf-8")
+    output_file.write_text('{"index": 0}\n', encoding="utf-8")
+    mc._input_file = str(input_file)
+    mc._output_file = str(output_file)
+
+    mc._append_simulation_record('{"index": 1}\n', '{"index": 1}\n')
+
+    original_open = builtins.open
+    output_path = os.fspath(output_file)
+
+    def failing_output_open(*args, **kwargs):
+        # Match builtins.open call shapes without keyword-before-vararg (W1113).
+        file = args[0] if args else kwargs["file"]
+        mode = args[1] if len(args) > 1 else kwargs.get("mode", "r")
+        if os.fspath(file) == output_path and "a" in mode:
+            raise OSError("no space left on device")
+        return original_open(*args, **kwargs)
+
+    with pytest.raises(OSError, match="no space left on device"):
+        with patch("builtins.open", side_effect=failing_output_open):
+            mc._append_simulation_record('{"index": 2}\n', '{"index": 2}\n')
+
+    assert input_file.read_text(encoding="utf-8") == '{"index": 0}\n{"index": 1}\n'
+    assert output_file.read_text(encoding="utf-8") == '{"index": 0}\n{"index": 1}\n'
+
+
 def test_estimate_confidence_interval_contains_known_mean():
     """Checks that the confidence interval contains the known mean."""
     mc = MockMonteCarlo()