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libexpr: Support structured error classes
While preparing PRs like NixOS#9753, I've had to change error messages in dozens of code paths. It would be nice if instead of EvalError("expected 'boolean' but found '%1%'", showType(v)) we could write TypeError(v, "boolean") or similar. Then, changing the error message could be a mechanical refactor with the compiler pointing out places the constructor needs to be changed, rather than the error-prone process of grepping through the codebase. Structured errors would also help prevent the "same" error from having multiple slightly different messages, and could be a first step towards error codes / an error index. This PR reworks the exception infrastructure in `libexpr` to support exception types with different constructor signatures than `BaseError`. Actually refactoring the exceptions to use structured data will come in a future PR (this one is big enough already, as it has to touch every exception in `libexpr`). The core design is in `eval-error.hh`. Generally, errors like this: state.error("'%s' is not a string", getAttrPathStr()) .debugThrow<TypeError>() are transformed like this: state.error<TypeError>("'%s' is not a string", getAttrPathStr()) .debugThrow() The type annotation has moved from `ErrorBuilder::debugThrow` to `EvalState::error`.
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,99 @@ | ||
| #include "eval-error.hh" | ||
| #include "eval.hh" | ||
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| namespace nix { | ||
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| template<class T> | ||
| EvalErrorBuilder<T> & EvalErrorBuilder<T>::withExitStatus(unsigned int exitStatus) | ||
| { | ||
| error.withExitStatus(exitStatus); | ||
| return *this; | ||
| } | ||
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| template<class T> | ||
| EvalErrorBuilder<T> & EvalErrorBuilder<T>::atPos(PosIdx pos) | ||
| { | ||
| error.err.pos = error.state.positions[pos]; | ||
| return *this; | ||
| } | ||
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| template<class T> | ||
| EvalErrorBuilder<T> & EvalErrorBuilder<T>::withTrace(PosIdx pos, const std::string_view text) | ||
| { | ||
| error.err.traces.push_front( | ||
| Trace{.pos = error.state.positions[pos], .hint = hintformat(std::string(text)), .frame = false}); | ||
| return *this; | ||
| } | ||
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| template<class T> | ||
| EvalErrorBuilder<T> & EvalErrorBuilder<T>::withFrameTrace(PosIdx pos, const std::string_view text) | ||
| { | ||
| error.err.traces.push_front( | ||
| Trace{.pos = error.state.positions[pos], .hint = hintformat(std::string(text)), .frame = true}); | ||
| return *this; | ||
| } | ||
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| template<class T> | ||
| EvalErrorBuilder<T> & EvalErrorBuilder<T>::withSuggestions(Suggestions & s) | ||
| { | ||
| error.err.suggestions = s; | ||
| return *this; | ||
| } | ||
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| template<class T> | ||
| EvalErrorBuilder<T> & EvalErrorBuilder<T>::withFrame(const Env & env, const Expr & expr) | ||
| { | ||
| // NOTE: This is abusing side-effects. | ||
| // TODO: check compatibility with nested debugger calls. | ||
| // TODO: What side-effects?? | ||
| error.state.debugTraces.push_front(DebugTrace{ | ||
| .pos = error.state.positions[expr.getPos()], | ||
| .expr = expr, | ||
| .env = env, | ||
| .hint = hintformat("Fake frame for debugging purposes"), | ||
| .isError = true}); | ||
| return *this; | ||
| } | ||
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| template<class T> | ||
| EvalErrorBuilder<T> & EvalErrorBuilder<T>::addTrace(PosIdx pos, hintformat hint, bool frame) | ||
| { | ||
| error.addTrace(error.state.positions[pos], hint, frame); | ||
| return *this; | ||
| } | ||
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| template<class T> | ||
| template<typename... Args> | ||
| EvalErrorBuilder<T> & | ||
| EvalErrorBuilder<T>::addTrace(PosIdx pos, std::string_view formatString, const Args &... formatArgs) | ||
| { | ||
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| addTrace(error.state.positions[pos], hintfmt(std::string(formatString), formatArgs...)); | ||
| return *this; | ||
| } | ||
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| template<class T> | ||
| void EvalErrorBuilder<T>::debugThrow() | ||
| { | ||
| if (error.state.debugRepl && !error.state.debugTraces.empty()) { | ||
| const DebugTrace & last = error.state.debugTraces.front(); | ||
| const Env * env = &last.env; | ||
| const Expr * expr = &last.expr; | ||
| error.state.runDebugRepl(&error, *env, *expr); | ||
| } | ||
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| throw error; | ||
| } | ||
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| template class EvalErrorBuilder<EvalError>; | ||
| template class EvalErrorBuilder<AssertionError>; | ||
| template class EvalErrorBuilder<ThrownError>; | ||
| template class EvalErrorBuilder<Abort>; | ||
| template class EvalErrorBuilder<TypeError>; | ||
| template class EvalErrorBuilder<UndefinedVarError>; | ||
| template class EvalErrorBuilder<MissingArgumentError>; | ||
| template class EvalErrorBuilder<InfiniteRecursionError>; | ||
| template class EvalErrorBuilder<CachedEvalError>; | ||
| template class EvalErrorBuilder<InvalidPathError>; | ||
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| } |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,116 @@ | ||
| #pragma once | ||
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| #include <algorithm> | ||
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| #include "error.hh" | ||
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| namespace nix { | ||
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| class PosIdx; | ||
| struct Env; | ||
| struct Expr; | ||
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| class EvalState; | ||
| template<class T> | ||
| class EvalErrorBuilder; | ||
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| class EvalError : public Error | ||
| { | ||
| template<class T> | ||
| friend class EvalErrorBuilder; | ||
| public: | ||
| EvalState & state; | ||
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| EvalError(EvalState & state, ErrorInfo && errorInfo) | ||
| : Error(errorInfo) | ||
| , state(state) | ||
| { | ||
| } | ||
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| template<typename... Args> | ||
| explicit EvalError(EvalState & state, const std::string & formatString, const Args &... formatArgs) | ||
| : Error(formatString, formatArgs...) | ||
| , state(state) | ||
| { | ||
| } | ||
| }; | ||
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| MakeError(ParseError, Error); | ||
| MakeError(AssertionError, EvalError); | ||
| MakeError(ThrownError, AssertionError); | ||
| MakeError(Abort, EvalError); | ||
| MakeError(TypeError, EvalError); | ||
| MakeError(UndefinedVarError, EvalError); | ||
| MakeError(MissingArgumentError, EvalError); | ||
| MakeError(CachedEvalError, EvalError); | ||
| MakeError(InfiniteRecursionError, EvalError); | ||
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| struct InvalidPathError : public EvalError | ||
| { | ||
| public: | ||
| Path path; | ||
| InvalidPathError(EvalState & state, const Path & path) | ||
| : EvalError(state, "path '%s' is not valid", path) | ||
| { | ||
| } | ||
| #ifdef EXCEPTION_NEEDS_THROW_SPEC | ||
| ~InvalidPathError() throw(){}; | ||
| #endif | ||
| }; | ||
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| template<class T> | ||
| class EvalErrorBuilder | ||
| { | ||
| public: | ||
| T error; | ||
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| template<typename... Args> | ||
| explicit EvalErrorBuilder(EvalState & state, const Args &... args) | ||
| : error(T(state, args...)) | ||
| { | ||
| } | ||
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| [[nodiscard, gnu::noinline]] EvalErrorBuilder<T> & withExitStatus(unsigned int exitStatus); | ||
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| [[nodiscard, gnu::noinline]] EvalErrorBuilder<T> & atPos(PosIdx pos); | ||
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| [[nodiscard, gnu::noinline]] EvalErrorBuilder<T> & withTrace(PosIdx pos, const std::string_view text); | ||
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| [[nodiscard, gnu::noinline]] EvalErrorBuilder<T> & withFrameTrace(PosIdx pos, const std::string_view text); | ||
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| [[nodiscard, gnu::noinline]] EvalErrorBuilder<T> & withSuggestions(Suggestions & s); | ||
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| [[nodiscard, gnu::noinline]] EvalErrorBuilder<T> & withFrame(const Env & e, const Expr & ex); | ||
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| [[nodiscard, gnu::noinline]] EvalErrorBuilder<T> & addTrace(PosIdx pos, hintformat hint, bool frame = false); | ||
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| template<typename... Args> | ||
| [[nodiscard, gnu::noinline]] EvalErrorBuilder<T> & | ||
| addTrace(PosIdx pos, std::string_view formatString, const Args &... formatArgs); | ||
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| [[gnu::noinline, gnu::noreturn]] virtual void debugThrow(); | ||
| }; | ||
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| /** | ||
| * The size needed to allocate any `EvalErrorBuilder<T>`. | ||
| * | ||
| * The list of classes here needs to be kept in sync with the list of `template | ||
| * class` declarations in `eval-error.cc`. | ||
| * | ||
| * This is used by `EvalState` to preallocate a buffer of sufficient size for | ||
| * any `EvalErrorBuilder<T>` to avoid allocating while evaluating Nix code. | ||
| */ | ||
| constexpr size_t EVAL_ERROR_BUILDER_SIZE = std::max({ | ||
| sizeof(EvalErrorBuilder<EvalError>), | ||
| sizeof(EvalErrorBuilder<AssertionError>), | ||
| sizeof(EvalErrorBuilder<ThrownError>), | ||
| sizeof(EvalErrorBuilder<Abort>), | ||
| sizeof(EvalErrorBuilder<TypeError>), | ||
| sizeof(EvalErrorBuilder<UndefinedVarError>), | ||
| sizeof(EvalErrorBuilder<MissingArgumentError>), | ||
| sizeof(EvalErrorBuilder<InfiniteRecursionError>), | ||
| sizeof(EvalErrorBuilder<CachedEvalError>), | ||
| sizeof(EvalErrorBuilder<InvalidPathError>), | ||
| }); | ||
|
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| } |
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