fix: add custom 404 support to dev server

src/rockgarden/cli.py

@@ -4,7 +4,6 @@
 import shutil
 import socketserver
 import tomllib
-from functools import partial
 from pathlib import Path
 from typing import Annotated, NoReturn
 
@@ -201,6 +200,36 @@ def build(
                 typer.echo(f"  {page_slug}: [[{target}]]", err=True)
 
 
+def _make_handler(
+    output_dir: Path,
+) -> type[http.server.SimpleHTTPRequestHandler]:
+    """Create an HTTP handler that serves custom 404.html when present."""
+
+    class _Handler(http.server.SimpleHTTPRequestHandler):
+        def __init__(self, *args: object, **kwargs: object) -> None:
+            super().__init__(*args, directory=str(output_dir), **kwargs)  # type: ignore[arg-type]
+
+        def send_error(
+            self,
+            code: int,
+            message: str | None = None,
+            explain: str | None = None,
+        ) -> None:
+            if code == 404:
+                custom_404 = Path(self.directory) / "404.html"  # type: ignore[attr-defined]
+                if custom_404.is_file():
+                    body = custom_404.read_bytes()
+                    self.send_response(404)
+                    self.send_header("Content-Type", "text/html; charset=utf-8")
+                    self.send_header("Content-Length", str(len(body)))
+                    self.end_headers()
+                    self.wfile.write(body)
+                    return
+            super().send_error(code, message, explain)
+
+    return _Handler
+
+
 @app.command()
 def serve(
     output: Annotated[
@@ -226,7 +255,7 @@ def serve(
         typer.echo("Run 'rockgarden build' first.", err=True)
         raise typer.Exit(1)
 
-    handler = partial(http.server.SimpleHTTPRequestHandler, directory=str(output_dir))
+    handler = _make_handler(output_dir)
 
     class ReuseAddrServer(socketserver.TCPServer):
         allow_reuse_address = True

tests/test_cli.py

@@ -1,10 +1,13 @@
+import socketserver
 import tempfile
+import threading
+import urllib.request
 from pathlib import Path
 
 from typer.testing import CliRunner
 
 from rockgarden import __version__
-from rockgarden.cli import app
+from rockgarden.cli import _make_handler, app
 
 runner = CliRunner()
 
@@ -124,3 +127,59 @@ def test_build_unhandled_error_shows_type_and_message(tmp_path, monkeypatch):
     assert result.exit_code == 1
     assert "RuntimeError: something broke" in result.output
     assert "Traceback" not in result.output
+
+
+def _start_serve_server(output_dir):
+    """Start the real rockgarden handler on an OS-assigned port."""
+    handler = _make_handler(output_dir)
+
+    # Suppress request logging during tests
+    handler.log_message = lambda self, *a, **kw: None  # type: ignore[assignment]
+
+    class ReuseAddrServer(socketserver.TCPServer):
+        allow_reuse_address = True
+
+    httpd = ReuseAddrServer(("127.0.0.1", 0), handler)
+    port = httpd.server_address[1]
+    thread = threading.Thread(target=httpd.serve_forever, daemon=True)
+    thread.start()
+    return httpd, port
+
+
+def test_serve_custom_404(tmp_path):
+    output_dir = tmp_path / "_site"
+    output_dir.mkdir()
+    (output_dir / "index.html").write_text("<h1>Home</h1>")
+    (output_dir / "404.html").write_text("<h1>Custom Not Found</h1>")
+
+    httpd, port = _start_serve_server(output_dir)
+    try:
+        try:
+            urllib.request.urlopen(f"http://127.0.0.1:{port}/nonexistent")
+        except urllib.error.HTTPError as e:
+            assert e.code == 404
+            body = e.read().decode()
+            assert "Custom Not Found" in body
+        else:
+            raise AssertionError("Expected 404 HTTPError")
+    finally:
+        httpd.shutdown()
+
+
+def test_serve_default_404_without_custom_page(tmp_path):
+    output_dir = tmp_path / "_site"
+    output_dir.mkdir()
+    (output_dir / "index.html").write_text("<h1>Home</h1>")
+
+    httpd, port = _start_serve_server(output_dir)
+    try:
+        try:
+            urllib.request.urlopen(f"http://127.0.0.1:{port}/nonexistent")
+        except urllib.error.HTTPError as e:
+            assert e.code == 404
+            body = e.read().decode()
+            assert "Custom Not Found" not in body
+        else:
+            raise AssertionError("Expected 404 HTTPError")
+    finally:
+        httpd.shutdown()