Avoid dynamic-plugins writing conflicts

docker/install-dynamic-plugins.py

@@ -25,6 +25,10 @@
 import subprocess
 import base64
 import binascii
+import atexit
+import time
+import signal
+
 # This script is used to install dynamic plugins in the Backstage application,
 # and is available in the container image to be called at container initialization,
 # for example in an init container when using Kubernetes.
@@ -181,8 +185,39 @@ def verify_package_integrity(plugin: dict, archive: str, working_directory: str)
     if hash_digest != output.decode('utf-8').strip():
       raise InstallException(f'{package}: The hash of the downloaded package {output.decode("utf-8").strip()} does not match the provided integrity hash {hash_digest} provided in the configuration file')
 
+# Create the lock file, so that other instances of the script will wait for this one to finish
+def create_lock(lock_file_path):
+    while True:
+      try:
+        with open(lock_file_path, 'x'):
+          print(f"======= Created lock file: {lock_file_path}")
+          return
+      except FileExistsError:
+        wait_for_lock_release(lock_file_path)
+
+# Remove the lock file
+def remove_lock(lock_file_path):
+   os.remove(lock_file_path)
+   print(f"======= Removed lock file: {lock_file_path}")
+
+# Wait for the lock file to be released
+def wait_for_lock_release(lock_file_path):
+   print("======= Waiting for lock release...")
+   while True:
+     if not os.path.exists(lock_file_path):
+       break
+     time.sleep(1)
+   print("======= Lock released.")
+
 def main():
+
     dynamicPluginsRoot = sys.argv[1]
+
+    lock_file_path = os.path.join(dynamicPluginsRoot, 'install-dynamic-plugins.lock')
+    atexit.register(remove_lock, lock_file_path)
+    signal.signal(signal.SIGTERM, lambda signum, frame: sys.exit(0))
+    create_lock(lock_file_path)
+
     maxEntrySize = int(os.environ.get('MAX_ENTRY_SIZE', 20000000))
     skipIntegrityCheck = os.environ.get("SKIP_INTEGRITY_CHECK", "").lower() == "true"
 

docs/dynamic-plugins/installing-plugins.md

@@ -119,3 +119,19 @@ When using RHDH Helm Chart you can just name the Secret using following pattern
 When using the Operator ....
 
 //TODO
+
+### Storage of Dynamic Plugins
+
+The directory where dynamic plugins are located is mounted as a volume to the _install-dynamic-plugins_ init container and the _backstage-backend_ container. The _install-dynamic-plugins_ init container is responsible for downloading and extracting the plugins into this directory. Depending on the deployment method, the directory is mounted as an ephemeral or persistent volume. In the latter case, the volume can be shared between several Pods, and the plugins installation script is also responsible for downloading and extracting the plugins only once, avoiding conflicts.
+
+**Important Note:** If _install-dynamic-plugins_ init container was killed with SIGKILL signal (for example in a case of OOM) the script is not able to remove the lock file and the next time the Pod starts, it will be waiting for the lock release. You can see the following message in the logs for all the Pods:
+
+```console
+oc logs -n <namespace-name> -f backstage-<backstage-name>-<pod-suffix> -c install-dynamic-plugins
+======= Waiting for lock release...
+```
+In such a case, you can delete the lock file manually from any of the Pods:
+
+```console
+oc exec -n <namespace-name> deploy/backstage-<backstage-name> -c install-dynamic-plugins -- rm -f /dynamic-plugins-root/dynamic-plugins.lock
+```