Fix python_server.py infinite loop on EOF (fixes #25620) (#25746)

## Summary

Fixes #25620 - Leftover process `python_server.py` with 100% CPU after
closing VS Code.

## The Problem

When VS Code closes, the STDIN stream to `python_server.py` is closed.
The `readline()` method returns empty bytes (`b''`) to signal EOF.
However, the previous code incorrectly treated this as an empty line
separator, causing:

1. `get_headers()` to return an empty headers dict
2. `content_length` to be 0
3. The main loop to continue immediately back to `get_headers()`
4. An infinite loop consuming 100% CPU

## The Fix

This PR properly detects EOF by checking for `b''` (empty bytes) vs
`b'\r\n'` or `b'\n'` (actual empty line with newline characters):

**In `get_headers()`:**
```python
raw = STDIN.buffer.readline()
# Detect EOF: readline() returns empty bytes when input stream is closed
if raw == b"":
    raise EOFError("EOF reached while reading headers")
```

**In all callers (main loop, `handle_response()`, `custom_input()`):**
- Catch `EOFError` and exit gracefully with `sys.exit(0)`

## Key Insight

```python
# EOF: readline() returns empty bytes
io.BytesIO(b"").readline()  # Returns b''

# Empty line: readline() returns newline bytes
io.BytesIO(b"\r\n").readline()  # Returns b'\r\n'
```

## Testing

Added comprehensive unit tests in
`python_files/tests/test_python_server.py`:
- `test_get_headers_normal` - verifies normal header parsing still works
- `test_get_headers_eof_raises_error` - verifies EOF detection
- `test_get_headers_eof_mid_headers_raises_error` - EOF during header
reading
- `test_get_headers_empty_line_terminates` - empty line still terminates
headers
- `test_custom_input_exits_on_eof` - graceful exit from `custom_input()`
- `test_handle_response_exits_on_eof` - graceful exit from
`handle_response()`
- `test_main_loop_exits_on_eof` - simulates the main loop behavior
- `test_readline_eof_vs_empty_line` - documents the EOF vs empty line
distinction

All 8 tests pass.

## How to Verify

1. Open a Python file in VS Code
2. Use Shift+Enter to start a Native REPL session and run some commands
3. Close VS Code
4. Check for leftover processes: `ps aux | grep python_server`
5. With this fix, no leftover processes should remain

Author: anthonykim1

python_files/python_server.py

@@ -64,6 +64,9 @@ def custom_input(prompt=""):
             message_text = STDIN.buffer.read(content_length).decode()
             message_json = json.loads(message_text)
             return message_json["result"]["userInput"]
+    except EOFError:
+        # Input stream closed, exit gracefully
+        sys.exit(0)
     except Exception:
         print_log(traceback.format_exc())
 
@@ -74,7 +77,7 @@ def custom_input(prompt=""):
 
 
 def handle_response(request_id):
-    while not STDIN.closed:
+    while True:
         try:
             headers = get_headers()
             # Content-Length is the data size in bytes.
@@ -88,8 +91,10 @@ def handle_response(request_id):
                     send_response(our_user_input, message_json["id"])
                 elif message_json["method"] == "exit":
                     sys.exit(0)
-
-        except Exception:  # noqa: PERF203
+        except EOFError:  # noqa: PERF203
+            # Input stream closed, exit gracefully
+            sys.exit(0)
+        except Exception:
             print_log(traceback.format_exc())
 
 
@@ -164,7 +169,11 @@ def get_value(self) -> str:
 def get_headers():
     headers = {}
     while True:
-        line = STDIN.buffer.readline().decode().strip()
+        raw = STDIN.buffer.readline()
+        # Detect EOF: readline() returns empty bytes when input stream is closed
+        if raw == b"":
+            raise EOFError("EOF reached while reading headers")
+        line = raw.decode().strip()
         if not line:
             break
         name, value = line.split(":", 1)
@@ -183,7 +192,7 @@ def get_headers():
     while "" in sys.path:
         sys.path.remove("")
     sys.path.insert(0, "")
-    while not STDIN.closed:
+    while True:
         try:
             headers = get_headers()
             # Content-Length is the data size in bytes.
@@ -198,6 +207,8 @@ def get_headers():
                     check_valid_command(request_json)
                 elif request_json["method"] == "exit":
                     sys.exit(0)
-
-        except Exception:  # noqa: PERF203
+        except EOFError:  # noqa: PERF203
+            # Input stream closed (VS Code terminated), exit gracefully
+            sys.exit(0)
+        except Exception:
             print_log(traceback.format_exc())

python_files/tests/test_python_server.py

@@ -0,0 +1,162 @@
+# Copyright (c) Microsoft Corporation. All rights reserved.
+# Licensed under the MIT License.
+
+"""Tests for python_server.py, specifically EOF handling to prevent infinite loops."""
+
+import io
+from unittest import mock
+
+import pytest
+
+
+class TestGetHeaders:
+    """Tests for the get_headers function."""
+
+    def test_get_headers_normal(self):
+        """Test get_headers with valid headers."""
+        # Arrange: Import the module
+        import python_server
+
+        # Create a mock stdin with valid headers
+        mock_input = b"Content-Length: 100\r\nContent-Type: application/json\r\n\r\n"
+        mock_stdin = io.BytesIO(mock_input)
+
+        # Act
+        with mock.patch.object(python_server, "STDIN", mock.Mock(buffer=mock_stdin)):
+            headers = python_server.get_headers()
+
+        # Assert
+        assert headers == {"Content-Length": "100", "Content-Type": "application/json"}
+
+    def test_get_headers_eof_raises_error(self):
+        """Test that get_headers raises EOFError when stdin is closed (EOF)."""
+        # Arrange: Import the module
+        import python_server
+
+        # Create a mock stdin that returns empty bytes (EOF)
+        mock_stdin = io.BytesIO(b"")
+
+        # Act & Assert
+        with mock.patch.object(python_server, "STDIN", mock.Mock(buffer=mock_stdin)), pytest.raises(
+            EOFError, match="EOF reached while reading headers"
+        ):
+            python_server.get_headers()
+
+    def test_get_headers_eof_mid_headers_raises_error(self):
+        """Test that get_headers raises EOFError when EOF occurs mid-headers."""
+        # Arrange: Import the module
+        import python_server
+
+        # Create a mock stdin with partial headers then EOF
+        mock_input = b"Content-Length: 100\r\n"  # No terminating empty line
+        mock_stdin = io.BytesIO(mock_input)
+
+        # Act & Assert
+        with mock.patch.object(python_server, "STDIN", mock.Mock(buffer=mock_stdin)), pytest.raises(
+            EOFError, match="EOF reached while reading headers"
+        ):
+            python_server.get_headers()
+
+    def test_get_headers_empty_line_terminates(self):
+        """Test that an empty line (not EOF) properly terminates header reading."""
+        # Arrange: Import the module
+        import python_server
+
+        # Create a mock stdin with headers followed by empty line
+        mock_input = b"Content-Length: 50\r\n\r\nsome body content"
+        mock_stdin = io.BytesIO(mock_input)
+
+        # Act
+        with mock.patch.object(python_server, "STDIN", mock.Mock(buffer=mock_stdin)):
+            headers = python_server.get_headers()
+
+        # Assert
+        assert headers == {"Content-Length": "50"}
+
+
+class TestEOFHandling:
+    """Tests for EOF handling in various functions that use get_headers."""
+
+    def test_custom_input_exits_on_eof(self):
+        """Test that custom_input exits gracefully on EOF."""
+        # Arrange: Import the module
+        import python_server
+
+        # Create a mock stdin that returns empty bytes (EOF)
+        mock_stdin = io.BytesIO(b"")
+        mock_stdout = io.BytesIO()
+
+        # Act & Assert
+        with mock.patch.object(
+            python_server, "STDIN", mock.Mock(buffer=mock_stdin)
+        ), mock.patch.object(python_server, "STDOUT", mock.Mock(buffer=mock_stdout)), pytest.raises(
+            SystemExit
+        ) as exc_info:
+            python_server.custom_input("prompt> ")
+
+        # Should exit with code 0 (graceful exit)
+        assert exc_info.value.code == 0
+
+    def test_handle_response_exits_on_eof(self):
+        """Test that handle_response exits gracefully on EOF."""
+        # Arrange: Import the module
+        import python_server
+
+        # Create a mock stdin that returns empty bytes (EOF)
+        mock_stdin = io.BytesIO(b"")
+
+        # Act & Assert
+        with mock.patch.object(python_server, "STDIN", mock.Mock(buffer=mock_stdin)), pytest.raises(
+            SystemExit
+        ) as exc_info:
+            python_server.handle_response("test-request-id")
+
+        # Should exit with code 0 (graceful exit)
+        assert exc_info.value.code == 0
+
+
+class TestMainLoopEOFHandling:
+    """Tests that simulate the main loop EOF scenario."""
+
+    def test_main_loop_exits_on_eof(self):
+        """Test that the main loop pattern exits gracefully on EOF.
+
+        This test verifies the fix for GitHub issue #25620 where the server
+        would spin at 100% CPU instead of exiting when VS Code closes.
+        """
+        # Arrange: Import the module
+        import python_server
+
+        # Create a mock stdin that returns empty bytes (EOF)
+        mock_stdin = io.BytesIO(b"")
+
+        # Simulate what happens in the main loop
+        with mock.patch.object(python_server, "STDIN", mock.Mock(buffer=mock_stdin)):
+            try:
+                python_server.get_headers()
+                # If we get here without raising EOFError, the fix isn't working
+                pytest.fail("Expected EOFError to be raised on EOF")
+            except EOFError:
+                # This is the expected behavior - the fix is working
+                pass
+
+    def test_readline_eof_vs_empty_line(self):
+        """Test that we correctly distinguish between EOF and empty line.
+
+        EOF: readline() returns b'' (empty bytes)
+        Empty line: readline() returns b'\\r\\n' or b'\\n' (newline bytes)
+        """
+        # Test EOF case
+        eof_stream = io.BytesIO(b"")
+        result = eof_stream.readline()
+        assert result == b"", "EOF should return empty bytes"
+
+        # Test empty line case
+        empty_line_stream = io.BytesIO(b"\r\n")
+        result = empty_line_stream.readline()
+        assert result == b"\r\n", "Empty line should return newline bytes"
+
+        # Test empty line with just newline
+        empty_line_stream2 = io.BytesIO(b"\n")
+        result = empty_line_stream2.readline()
+        assert result == b"\n", "Empty line should return newline bytes"