Handle teardown race, add comment about shm subdirs

piker/data/_sharedmem.py

@@ -98,7 +98,12 @@ class SharedInt:
         if _USE_POSIX:
             # We manually unlink to bypass all the "resource tracker"
             # nonsense meant for non-SC systems.
-            shm_unlink(self._shm.name)
+            name = self._shm.name
+            try:
+                shm_unlink(name)
+            except FileNotFoundError:
+                # might be a teardown race here?
+                log.warning(f'Shm for {name} already unlinked?')
 
 
 class _Token(BaseModel):
@@ -536,6 +541,10 @@ def attach_shm_array(
     if key in _known_tokens:
         assert _Token.from_msg(_known_tokens[key]) == token, "WTF"
 
+    # XXX: ugh, looks like due to the ``shm_open()`` C api we can't
+    # actually place files in a subdir, see discussion here:
+    # https://stackoverflow.com/a/11103289
+
     # attach to array buffer and view as per dtype
     shm = SharedMemory(name=key)
     shmarr = np.ndarray(