[threads-] asynccache/addcol-shell: fix replay/race condition

Author: midichef

visidata/threads.py

@@ -30,7 +30,17 @@ def _func(k, *args, **kwargs):
         def _execAsync(*args, **kwargs):
             k = key(*args, **kwargs)
             if k not in d:
-                d[k] = vd.execAsync(_func, k, *args, **kwargs)
+                # The straightforward way to assign to d[k] is not thread-safe:
+                #     d[k] = vd.execAsync(_func, k, *args, **kwargs)
+                # because events can happen in this order:
+                # 1) _func can finish, assigning the return value of func() to d[k], then
+                # 2) execAsync's return value is assigned to d[k], overwriting the return value of func()
+                # The same failure happens when replay mode temporarily uses execSync in place of every execAsync call.
+                t = vd.execAsync(_func, k, *args, **kwargs)
+                d.setdefault(k, t)  #2826
+                # Using setdefault() is safe, assigning to d[k] only if _func hasn't yet done so.
+                # The setdefault() technique is commonly said to be atomic on CPython, for a standard
+                # dictionary with no custom methods.
             return d.get(k)
         return _execAsync
     return _decorator