Merge pull request #2801 from mquinnfd/prebuild-yarn-warning

Provide warning for local installs where yarn is not present

pre_build.py

@@ -1,13 +1,21 @@
 import os
 import subprocess
+from shutil import which
+from sys import exit
 
 
 def build() -> None:
     if os.environ.get("SKIP_PRE_BUILD", "") == "true":
         print("Skipping front end build...")
         return
+    if which("yarn") is None:
+        print(
+            "Locust requires the yarn binary to be available in this shell to build the web front-end.\nSee: https://docs.locust.io/en/stable/developing-locust.html#making-changes-to-locust-s-web-ui"
+        )
+        exit(1)
     print("Building front end...")
-    subprocess.run(["make", "frontend_build"])
+    subprocess.run(["yarn", "webui:install"])
+    subprocess.run(["yarn", "webui:build"])
 
 
 if __name__ == "__main__":