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| template<typename Derived, typename Base> | ||
| class CloneableError : public Base | ||
| { | ||
| public: | ||
| using Base::Base; | ||
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| /** | ||
| * Rethrow a copy of this exception. Useful when the exception can get | ||
| * modified when appending traces. | ||
| */ | ||
| [[noreturn]] void throwClone() const override | ||
| { | ||
| throw Derived(static_cast<const Derived &>(*this)); | ||
| } |
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A necessary but mild annoyance that looks well executed to me.
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| … forcing b | ||
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This is from the initial memoised stack trace. Pretty unavoidable and adds more context.
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This is needed to make it possible to store exceptions in failed values with each new rethrow getting a fresh copy of the exception object.
In the multithreaded evaluator, it's possible for multiple threads to wait on the same thunk. If evaluation of the thunk results in an exception, the waiting threads shouldn't try to re-force the thunk. Instead, they should rethrow the same exception, without duplicating any work. Therefore, there is now a new value type `tFailed` that stores an std::exception_ptr. If evaluation of a thunk/app results in an exception, `forceValue()` overwrites the value with a `tFailed`. If `forceValue()` encounters a `tFailed`, it rethrows the exception. So you normally never need to check for failed values (since forcing them causes a rethrow). Co-authored-by: Robert Hensing <robert@roberthensing.nl>
This provides an explicit API for call-fail-retry-succeed evaluation flows, such as currently used in NixOps4. An alternative design would simply reset the `Value` to the original thunk instead of `tFailed` under the condition of catching a `RecoverableEvalError`. That is somewhat simpler, but I believe the presence of `tFailed` is beneficial for possible use in the repl; being able to show the error sooner, without re-evaluation. The hasPos method is required in order to avoid an include loop.
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Motivation
Rebased version of #13930 that addresses concerns that @roberth voiced in #13930 (review) and in other review comments. My hope is to start working on getting async/concurrent evaluation upstream and this seems like a very natural way to propagate exceptions for suspendable work.
To address the trace mutation issue each error type now has an implementation of
ErrorBase::throwCloneaided by a CRTP mixinCloneableError. It's a bit of a churn, but I think this is the best way forward.Context
Closes #13930.
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