Adds diagnostics/remedy system

tests/unit/test_process.py

@@ -2,6 +2,7 @@ from __future__ import annotations
 
 from pathlib import Path
 import sys
+import time
 import unittest
 from unittest.mock import patch
 
@@ -51,6 +52,33 @@ class ProcessTests(unittest.TestCase):
         self.assertIn("Command timed out", err)
         self.assertIn(sys.executable, err)
 
+    def test_execute_process_can_stop_early_while_streaming_stderr(self):
+        start = time.monotonic()
+        observed_lines = []
+
+        out, err, rc = executeProcess(
+            [
+                sys.executable,
+                "-c",
+                (
+                    "import sys, time; "
+                    "sys.stderr.write('fatal warning\\n'); sys.stderr.flush(); "
+                    "time.sleep(2); "
+                    "sys.stderr.write('late line\\n'); sys.stderr.flush()"
+                ),
+            ],
+            stderrLineHandler=lambda line: observed_lines.append(line) or ("fatal warning" in line),
+        )
+
+        elapsed = time.monotonic() - start
+
+        self.assertLess(elapsed, 1.5)
+        self.assertNotEqual(0, rc)
+        self.assertEqual("", out)
+        self.assertIn("fatal warning", err)
+        self.assertNotIn("late line", err)
+        self.assertEqual(["fatal warning\n"], observed_lines)
+
     def test_get_wrapped_command_sequence_leaves_command_unwrapped_when_limits_disabled(self):
         wrapped = getWrappedCommandSequence(
             ["ffmpeg", "-i", "input.mkv"],