skipmap: Store value silently lost under concurrent LoadAndDelete

[jmasters-git]

Operating System

MacOS 26.3.1

Go Version

1.26.1

Package Version

v0.1.4

Affected Packages

• collections/skipmap

Expected Behavior

The effect of Store(key, value) operations must cause the value to be visible to subsequent operations.

Actual Behavior

A Store completes successfully but its value is never observed. A subsequent LoadAndDelete returns nil, and no operation accounts for the removal of the stored value. (see reproduction)

Reproduction Steps

See #263

A porcupine linearizability test reliably reproduces this with 16 goroutines performing a uniform mix of Load, LoadOrStore, Store, and LoadAndDelete on 10 keys (200 ops per client). The failure is intermittent but typically triggers within a few runs.

Observed timeline

LoadAndDelete(key_0) -> 700091   [deletes the old value]
Store(key_0, 200019)             [concurrent, should install new value]
LoadAndDelete(key_0) -> nil      [value from Store is gone]

There is no valid linearization of these operations: if Store(key_0, 200019) linearizes after the first LoadAndDelete, the key should hold 200019. The second LoadAndDelete returning nil means the Store's effect was lost without any corresponding delete.

Other Information

No response