fix: do not use shutil.which to find npm binary

This works around a bug in python 3.12.0's `shutil.which` when running
on Windows.

See python/cpython#109590

build_frontend.py

@@ -26,6 +26,9 @@
 # path to compiled JS entry point
 APP_JS = "lektor/admin/static/app.js"
 
+# name of NPM program (patched by tests)
+NPM = "npm"
+
 
 class FrontendBuildHook(BuildHookInterface):
     """Hatching build hook to compile our frontend JS."""
@@ -65,16 +68,20 @@ def initialize(self, version, build_data):
             app.display_info(f"{APP_JS} exists, skipping frontend build")
             return
 
-        npm = shutil.which("npm")
-        if npm is None:
+        try:
+            proc = subprocess.run(
+                (NPM, "-v"), capture_output=True, text=True, check=True
+            )
+        except FileNotFoundError:
             app.abort("npm is not available. can not build frontend")
+        app.display_info(f"found npm version {proc.stdout.strip()}")
 
         frontend = Path(root, FRONTEND)
         if not frontend.is_dir():
             app.abort("frontend source is missing. can not build frontend")
 
         app.display_info("npm install")
-        subprocess.run((npm, "install"), cwd=frontend, check=True)
+        subprocess.run((NPM, "install"), cwd=frontend, check=True)
         app.display_info("npm run build")
-        subprocess.run((npm, "run", "build"), cwd=frontend, check=True)
+        subprocess.run((NPM, "run", "build"), cwd=frontend, check=True)
         app.display_success("built frontend static files")

tests/test_build_frontend.py

@@ -81,8 +81,10 @@ def test_initialize_skips_build_if_output_exists(
 
 
 @pytest.mark.usefixtures("frontend_src")
-def test_initialize_aborts_if_no_npm(frontend_build_hook, mocker):
-    mocker.patch("shutil.which").return_value = None
+def test_initialize_aborts_if_no_npm(
+    frontend_build_hook, frontend_build_module, monkeypatch
+):
+    monkeypatch.setattr(frontend_build_module, "NPM", "NPM_IS_MISSING")
     with pytest.raises(SystemExit) as exc_info:
         frontend_build_hook.initialize("standard", build_data={})
     assert exc_info.value.args[0] == 1