Fix: A VM could be prepared in parallel

Problem: The method `prepare()` of a VM could be launched multiple times in parallel.

Solution: Use a lock and skip the preparation if it has already been done.

src/aleph/vm/models.py

@@ -112,6 +112,7 @@ def __init__(
  self.ready_event = asyncio.Event()
  self.concurrent_runs = 0
  self.runs_done_event = asyncio.Event()
+ self.preparation_pending_lock = asyncio.Lock()
  self.stop_pending_lock = asyncio.Lock()
  self.snapshot_manager = snapshot_manager
 
@@ -126,17 +127,22 @@ def to_json(self, indent: Optional[int] = None) -> str:
 
  async def prepare(self):
  """Download VM required files"""
- self.times.preparing_at = datetime.now(tz=timezone.utc)
- if self.is_program:
- resources = AlephProgramResources(self.message, namespace=self.vm_hash)
- elif self.is_instance:
- resources = AlephInstanceResources(self.message, namespace=self.vm_hash)
- else:
- msg = "Unknown executable message type"
- raise ValueError(msg)
- await resources.download_all()
- self.times.prepared_at = datetime.now(tz=timezone.utc)
- self.resources = resources
+ async with self.preparation_pending_lock:
+ if self.resources:
+ # Already prepared
+ return
+
+ self.times.preparing_at = datetime.now(tz=timezone.utc)
+ if self.is_program:
+ resources = AlephProgramResources(self.message, namespace=self.vm_hash)
+ elif self.is_instance:
+ resources = AlephInstanceResources(self.message, namespace=self.vm_hash)
+ else:
+ msg = "Unknown executable message type"
+ raise ValueError(msg)
+ await resources.download_all()
+ self.times.prepared_at = datetime.now(tz=timezone.utc)
+ self.resources = resources
 
  async def create(self, vm_id: int, tap_interface: Optional[TapInterface] = None) -> AlephFirecrackerExecutable:
  if not self.resources: