Fix UnicodeDecodeError crash on invalid UTF-8 input (#6456)

src/textual/drivers/linux_driver.py

@@ -412,7 +412,7 @@ def run_input_thread(self) -> None:
         feed = parser.feed
         tick = parser.tick
 
-        utf8_decoder = getincrementaldecoder("utf-8")().decode
+        utf8_decoder = getincrementaldecoder("utf-8")(errors="replace").decode
         decode = utf8_decoder
         read = os.read
 

src/textual/drivers/linux_inline_driver.py

@@ -130,7 +130,7 @@ def run_input_thread(self) -> None:
         feed = parser.feed
         tick = parser.tick
 
-        utf8_decoder = getincrementaldecoder("utf-8")().decode
+        utf8_decoder = getincrementaldecoder("utf-8")(errors="replace").decode
         decode = utf8_decoder
         read = os.read
 

src/textual/drivers/web_driver.py

@@ -185,7 +185,7 @@ def run_input_thread(self) -> None:
         """Wait for input and dispatch events."""
         input_reader = self._input_reader
         parser = XTermParser(debug=self._debug)
-        utf8_decoder = getincrementaldecoder("utf-8")().decode
+        utf8_decoder = getincrementaldecoder("utf-8")(errors="replace").decode
         decode = utf8_decoder
         # The server sends us a stream of bytes, which contains the equivalent of stdin, plus
         # in band data packets.

tests/test_utf8_decode_resilience.py

@@ -0,0 +1,63 @@
+"""Regression test for https://github.com/Textualize/textual/issues/6456
+
+Verify that the UTF-8 incremental decoders used in drivers are configured
+with ``errors="replace"`` so that invalid byte sequences produce U+FFFD
+instead of raising ``UnicodeDecodeError`` and crashing the input thread.
+
+This test inspects the driver source code to ensure the fix is in place.
+Without errors="replace", invalid UTF-8 input would crash the input thread.
+"""
+
+import re
+from pathlib import Path
+
+
+def _get_driver_files() -> dict[str, Path]:
+    """Get the paths to the three drivers that were modified."""
+    drivers_dir = Path(__file__).parent.parent / "src" / "textual" / "drivers"
+    return {
+        "linux_driver": drivers_dir / "linux_driver.py",
+        "linux_inline_driver": drivers_dir / "linux_inline_driver.py",
+        "web_driver": drivers_dir / "web_driver.py",
+    }
+
+
+def _check_driver_decoder_config(driver_path: Path) -> bool:
+    """Check if driver uses getincrementaldecoder with errors='replace'."""
+    if not driver_path.exists():
+        raise FileNotFoundError(f"Driver file not found: {driver_path}")
+
+    source = driver_path.read_text(encoding="utf-8")
+
+    # Look for the pattern: getincrementaldecoder("utf-8")(errors="replace")
+    # This regex matches the decoder instantiation with the replace error handler
+    pattern = r'getincrementaldecoder\s*\(\s*["\']utf-8["\']\s*\)\s*\(\s*errors\s*=\s*["\']replace["\']\s*\)'
+
+    return bool(re.search(pattern, source))
+
+
+def test_linux_driver_uses_replace_errors() -> None:
+    """Linux driver must use errors='replace' for UTF-8 decoder."""
+    drivers = _get_driver_files()
+    assert _check_driver_decoder_config(drivers["linux_driver"]), (
+        "linux_driver.py must use getincrementaldecoder('utf-8')(errors='replace'). "
+        "Without this, invalid UTF-8 bytes will crash the input thread."
+    )
+
+
+def test_linux_inline_driver_uses_replace_errors() -> None:
+    """Linux inline driver must use errors='replace' for UTF-8 decoder."""
+    drivers = _get_driver_files()
+    assert _check_driver_decoder_config(drivers["linux_inline_driver"]), (
+        "linux_inline_driver.py must use getincrementaldecoder('utf-8')(errors='replace'). "
+        "Without this, invalid UTF-8 bytes will crash the input thread."
+    )
+
+
+def test_web_driver_uses_replace_errors() -> None:
+    """Web driver must use errors='replace' for UTF-8 decoder."""
+    drivers = _get_driver_files()
+    assert _check_driver_decoder_config(drivers["web_driver"]), (
+        "web_driver.py must use getincrementaldecoder('utf-8')(errors='replace'). "
+        "Without this, invalid UTF-8 bytes will crash the input thread."
+    )