Ensure npm install runs once at a time

On starting up the Lektor server, the server_spawn and before_build_all
events both fire, each one spawning `npm install`. This ensures that
only one runs at a time.

Author: zakj

lektor_webpack_support.py

@@ -1,4 +1,5 @@
 import os
+import threading
 
 from lektor.pluginsystem import Plugin
 from lektor.reporter import reporter
@@ -11,6 +12,7 @@ class WebpackSupportPlugin(Plugin):
 
     def __init__(self, *args, **kwargs):
         Plugin.__init__(self, *args, **kwargs)
+        self.npm_lock = threading.Lock()
         self.webpack_process = None
 
     def is_enabled(self, extra_flags):
@@ -26,7 +28,11 @@ def run_webpack(self, watch=False):
     def npm_install(self):
         reporter.report_generic('Running npm install')
         webpack_root = os.path.join(self.env.root_path, 'webpack')
-        portable_popen(['npm', 'install'], cwd=webpack_root).wait()
+        if self.npm_lock.acquire(False):
+            portable_popen(['npm', 'install'], cwd=webpack_root).wait()
+        else:
+            self.npm_lock.acquire()
+        self.npm_lock.release()
 
     def on_server_spawn(self, **extra):
         extra_flags = extra.get("extra_flags") or extra.get("build_flags") or {}

tests/test_plugin.py

@@ -1,4 +1,6 @@
 import py
+import threading
+import time
 
 
 def test_disabled_by_default(plugin):
@@ -66,3 +68,19 @@ def test_server_stop(plugin, mocker):
     plugin.webpack_process = mocker.Mock()
     plugin.on_server_stop()
     plugin.webpack_process.kill.assert_called_with()
+
+
+def test_single_npm_process(plugin, builder, mocker):
+    def pause(*args, **kwargs):
+        time.sleep(.1)
+        return mocker.DEFAULT
+
+    mock_popen = mocker.patch("lektor_webpack_support.portable_popen")
+    mock_popen.side_effect = pause
+    thread1 = threading.Thread(target=plugin.npm_install)
+    thread2 = threading.Thread(target=plugin.npm_install)
+    thread1.start()
+    thread2.start()
+    thread1.join()
+    thread2.join()
+    assert mock_popen.call_count == 1