✨ throw CircularTaskError when a task depends on itself or an ancestor#1162
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✨ throw CircularTaskError when a task depends on itself or an ancestor#1162
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A task may not wait on its own result or its own halt(), nor on any of its ancestors' — by structured concurrency the parent cannot complete until every child does, so any descendant waiting on an ancestor is a circular dependency that would deadlock. Detect synchronously inside the task[Symbol.iterator] and halt thenable[Symbol.iterator] wrappers via assertNonCircular: walk the calling scope's contexts prototype chain and throw if the targeted task's scope appears in it. Sibling and downward (parent→child) waits are unaffected. Expose createFuture's operation directly on FutureWithResolvers so the task iterator wrapper can yield to it without re-entering its own overridden Symbol.iterator (which would recurse).
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Motivation
A task that awaits its own result or the result of an ancestor is a circular dependency that should deadlock. The parent cannot complete until every child does, so any descendant waiting on itself or an ancestor should hang forever. This applies equally to the main task computation as well as its
halt()We currently try to fudge this situation when we encounter it and try to allow it to muddle through for historical reasons, but that ends up making things works by introducing complexity in order to paper over a pathological use-case. It's better to be up-front about it and when we detect a circular dependency between task, explicitly call that out
The options are:
Approach
Compare the scopes of the operation yielding to a task and the task itself. If the caller is, or is descended from the task being halted, then raise a
CircularTaskError.Potential Drawbacks