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

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 finishes, assigning func()'s return value to d[k], then
2) execAsync's return value is assigned to d[k], overwriting it.

The same failure happens when replay mode temporarily uses
execSync in place of every execAsync call. 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.

Author: midichef

visidata/threads.py

@@ -30,7 +30,9 @@ 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)
+                t = vd.execAsync(_func, k, *args, **kwargs)
+                #atomic read/write to d[k]
+                d.setdefault(k, t)  #2826
             return d.get(k)
         return _execAsync
     return _decorator