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

When VS Code closes, the STDIN stream closes and readline() returns
empty bytes (b''). Previously this was incorrectly treated as an
empty line separator, causing an infinite loop with 100% CPU usage.

This fix:
- Detects EOF in get_headers() by checking for b'' and raising EOFError
- Handles EOFError in all three places that call get_headers():
  - The main loop
  - handle_response()
  - custom_input()
- Exits gracefully with sys.exit(0) when EOF is detected

The key insight is distinguishing between:
- EOF: readline() returns b'' (empty bytes)
- Empty line: readline() returns b'\r\n' or b'\n' (newline bytes)

Also added comprehensive unit tests to verify the fix.

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"