soft-pool: lock channel queue when trying to open new channels (#31)

sshfs/pools/soft.py

@@ -1,3 +1,4 @@
+import asyncio
 import heapq
 from collections import Counter
 from contextlib import asynccontextmanager
@@ -19,20 +20,23 @@ class SFTPSoftChannelPool(BaseSFTPChannelPool):
 
     def __init__(self, *args, **kwargs):
         self._channels = Counter()
+        self._channels_lock = asyncio.Lock()
         super().__init__(*args, **kwargs)
 
     @asynccontextmanager
     async def get(self):
-        [(least_used_channel, num_connections)] = (
-            heapq.nsmallest(1, self._channels.items(), lambda kv: kv[1])
-            or self._NO_CHANNELS
-        )
-
+        least_used_channel, num_connections = self._least_used()
         if least_used_channel is None or num_connections >= self._THRESHOLD:
-            channel = await self._maybe_new_channel()
-            if channel is not None:
-                least_used_channel = channel
-                num_connections = 0
+            async with self._channels_lock:
+                channel = await self._maybe_new_channel()
+                if channel is not None:
+                    least_used_channel = channel
+                    num_connections = 0
+                    self._channels[least_used_channel] = 0
+
+            if channel is None:
+                # another coroutine may have opened a channel while we waited
+                least_used_channel, num_connections = self._least_used()
 
         if least_used_channel is None:
             raise ValueError("Can't create any SFTP connections!")
@@ -46,6 +50,13 @@ async def get(self):
     async def _cleanup(self):
         self._channels.clear()
 
+    def _least_used(self):
+        [(least_used_channel, num_connections)] = (
+            heapq.nsmallest(1, self._channels.items(), lambda kv: kv[1])
+            or self._NO_CHANNELS
+        )
+        return least_used_channel, num_connections
+
     @property
     def active_channels(self):
         return len(self._channels)