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libexpr: Support structured error classes

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While preparing PRs like #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`.
This commit is contained in:
Rebecca Turner 2024-01-22 17:08:29 -08:00
parent c62c21e29a
commit c6a89c1a16
No known key found for this signature in database
40 changed files with 653 additions and 545 deletions
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91
src/libexpr/eval.hh
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@ -2,6 +2,7 @@
///@file
#include "attr-set.hh"
#include "eval-error.hh"
#include "types.hh"
#include "value.hh"
#include "nixexpr.hh"
@ -151,45 +152,6 @@ struct DebugTrace {
bool isError;
};
void debugError(Error * e, Env & env, Expr & expr);
class ErrorBuilder
{
private:
EvalState & state;
ErrorInfo info;
ErrorBuilder(EvalState & s, ErrorInfo && i): state(s), info(i) { }
public:
template<typename... Args>
[[nodiscard, gnu::noinline]]
static ErrorBuilder * create(EvalState & s, const Args & ... args)
{
return new ErrorBuilder(s, ErrorInfo { .msg = hintfmt(args...) });
}
[[nodiscard, gnu::noinline]]
ErrorBuilder & atPos(PosIdx pos);
[[nodiscard, gnu::noinline]]
ErrorBuilder & withTrace(PosIdx pos, const std::string_view text);
[[nodiscard, gnu::noinline]]
ErrorBuilder & withFrameTrace(PosIdx pos, const std::string_view text);
[[nodiscard, gnu::noinline]]
ErrorBuilder & withSuggestions(Suggestions & s);
[[nodiscard, gnu::noinline]]
ErrorBuilder & withFrame(const Env & e, const Expr & ex);
template<class ErrorType>
[[gnu::noinline, gnu::noreturn]]
void debugThrow();
};
class EvalState : public std::enable_shared_from_this<EvalState>
{
public:
@ -274,39 +236,10 @@ public:
void runDebugRepl(const Error * error, const Env & env, const Expr & expr);
template<class E>
[[gnu::noinline, gnu::noreturn]]
void debugThrowLastTrace(E && error)
{
debugThrow(error, nullptr, nullptr);
}
template<class E>
[[gnu::noinline, gnu::noreturn]]
void debugThrow(E && error, const Env * env, const Expr * expr)
{
if (debugRepl && ((env && expr) || !debugTraces.empty())) {
if (!env || !expr) {
const DebugTrace & last = debugTraces.front();
env = &last.env;
expr = &last.expr;
}
runDebugRepl(&error, *env, *expr);
}
throw std::move(error);
}
// This is dangerous, but gets in line with the idea that error creation and
// throwing should not allocate on the stack of hot functions.
// as long as errors are immediately thrown, it works.
ErrorBuilder * errorBuilder;
template<typename... Args>
template<class T, typename... Args>
[[nodiscard, gnu::noinline]]
ErrorBuilder & error(const Args & ... args) {
errorBuilder = ErrorBuilder::create(*this, args...);
return *errorBuilder;
EvalErrorBuilder<T> & error(const Args & ... args) {
return *new EvalErrorBuilder<T>(*this, args...);
}
private:
@ -845,22 +778,6 @@ SourcePath resolveExprPath(SourcePath path);
*/
bool isAllowedURI(std::string_view uri, const Strings & allowedPaths);
struct InvalidPathError : EvalError
{
Path path;
InvalidPathError(const Path & path);
#ifdef EXCEPTION_NEEDS_THROW_SPEC
~InvalidPathError() throw () { };
#endif
};
template<class ErrorType>
void ErrorBuilder::debugThrow()
{
// NOTE: We always use the -LastTrace version as we push the new trace in withFrame()
state.debugThrowLastTrace(ErrorType(info));
}
}
#include "eval-inline.hh"