[llvm-dev] [RFC] Error handling in LLVM libraries.

James Y Knight via llvm-dev llvm-dev at lists.llvm.org
Tue Feb 2 18:05:20 PST 2016

Regarding one point in particular:

> 2. Difficult to drop - The 'checked' flag in TypedError ensures that it
> can't be dropped, it must be explicitly "handled", even if that only
> involves catching the error and doing nothing.

It seems to me that "[[clang::warn_unused_result]] class TypedError" is
probably sufficient for ensuring people check a status return value; I'm
not sure runtime checking really brings much additional value there.

On Tue, Feb 2, 2016 at 8:29 PM, Lang Hames via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi All,
> I've been thinking lately about how to improve LLVM's error model and
> error reporting. A lack of good error reporting in Orc and MCJIT has forced
> me to spend a lot of time investigating hard-to-debug errors that could
> easily have been identified if we provided richer error information to the
> client, rather than just aborting. Kevin Enderby has made similar
> observations about the state of libObject and the difficulty of producing
> good error messages for damaged object files. I expect to encounter more
> issues like this as I continue work on the MachO side of LLD. I see
> tackling the error modeling problem as a first step towards improving error
> handling in general: if we make it easy to model errors, it may pave the
> way for better error handling in many parts of our libraries.
> At present in LLVM we model errors with std::error_code (and its helper,
> ErrorOr) and use diagnostic streams for error reporting. Neither of these
> seem entirely up to the job of providing a solid error-handling mechanism
> for library code. Diagnostic streams are great if all you want to do is
> report failure to the user and then terminate, but they can't be used to
> distinguish between different kinds of errors, and so are unsuited to many
> use-cases (especially error recovery). On the other hand, std::error_code
> allows error kinds to be distinguished, but suffers a number of drawbacks:
> 1. It carries no context: It tells you what went wrong, but not where or
> why, making it difficult to produce good diagnostics.
> 2. It's extremely easy to ignore or forget: instances can be silently
> dropped.
> 3. It's not especially debugger friendly: Most people call the error_code
> constructors directly for both success and failure values. Breakpoints have
> to be set carefully to avoid stopping when success values are constructed.
> In fairness to std::error_code, it has some nice properties too:
> 1. It's extremely lightweight.
> 2. It's explicit in the API (unlike exceptions).
> 3. It doesn't require C++ RTTI (a requirement for use in LLVM).
> To address these shortcomings I have prototyped a new error-handling
> scheme partially inspired by C++ exceptions. The aim was to preserve the
> performance and API visibility of std::error_code, while allowing users to
> define custom error classes and inheritance relationships between them. My
> hope is that library code could use this scheme to model errors in a
> meaningful way, allowing clients to inspect the error information and
> recover where possible, or provide a rich diagnostic when aborting.
> The scheme has three major "moving parts":
> 1. A new 'TypedError' class that can be used as a replacement for
> std::error_code. E.g.
> std::error_code foo();
> becomes
> TypedError foo();
> The TypedError class serves as a lightweight wrapper for the real error
> information (see (2)). It also contains a 'Checked' flag, initially set to
> false, that tracks whether the error has been handled or not. If a
> TypedError is ever destructed without being checked (or passed on to
> someone else) it will call std::terminate(). TypedError cannot be silently
> dropped.
> 2. A utility class, TypedErrorInfo, for building error class hierarchies
> rooted at 'TypedErrorInfoBase' with custom RTTI. E.g.
> // Define a new error type implicitly inheriting from TypedErrorInfoBase.
> class MyCustomError : public TypedErrorInfo<MyCustomError> {
> public:
>   // Custom error info.
> };
> // Define a subclass of MyCustomError.
> class MyCustomSubError : public TypedErrorInfo<MyCustomSubError,
> MyCustomError> {
> public:
>   // Extends MyCustomError, adds new members.
> };
> 3.  A set of utility functions that use the custom RTTI system to inspect
> and handle typed errors. For example 'catchAllTypedErrors' and
> 'handleTypedError' cooperate to handle error instances in a type-safe way:
> TypedError foo() {
>   if (SomeFailureCondition)
>     return make_typed_error<MyCustomError>();
> }
> TypedError Err = foo();
> catchAllTypedErrors(std::move(Err),
>   handleTypedError<MyCustomError>(
>     [](std::unique_ptr<MyCustomError> E) {
>       // Handle the error.
>       return TypedError(); // <- Indicate success from handler.
>     }
>   )
> );
> If your initial reaction is "Too much boilerplate!" I understand, but take
> comfort: (1) In the overwhelmingly common case of simply returning errors,
> the usage is identical to std::error_code:
> if (TypedError Err = foo())
>   return Err;
> and (2) the boilerplate for catching errors is usually easily contained in
> a handful of utility functions, and tends not to crowd the rest of your
> source code. My initial experiments with this scheme involved updating many
> source lines, but did not add much code at all beyond the new error classes
> that were introduced.
> I believe that this scheme addresses many of the shortcomings of
> std::error_code while maintaining the strengths:
> 1. Context - Custom error classes enable the user to attach as much
> contextual information as desired.
> 2. Difficult to drop - The 'checked' flag in TypedError ensures that it
> can't be dropped, it must be explicitly "handled", even if that only
> involves catching the error and doing nothing.
> 3. Debugger friendly - You can set a breakpoint on any custom error
> class's constructor to catch that error being created. Since the error
> class hierarchy is rooted you can break on
> TypedErrorInfoBase::TypedErrorInfoBase to catch any error being raised.
> 4. Lightweight - Because TypedError instances are just a pointer and a
> checked-bit, move-constructing it is very cheap. We may also want to
> consider ignoring the 'checked' bit in release mode, at which point
> TypedError should be as cheap as std::error_code.
> 5. Explicit - TypedError is represented explicitly in the APIs, the same
> as std::error_code.
> 6. Does not require C++ RTTI - The custom RTTI system does not rely on any
> standard C++ RTTI features.
> This scheme also has one attribute that I haven't seen in previous error
> handling systems (though my experience in this area is limited): Errors are
> not copyable, due to ownership semantics of TypedError. I think this
> actually neatly captures the idea that there is a chain of responsibility
> for dealing with any given error. Responsibility may be transferred (e.g.
> by returning it to a caller), but it cannot be duplicated as it doesn't
> generally make sense for multiple people to report or attempt to recover
> from the same error.
> I've tested this prototype out by threading it through the object-creation
> APIs of libObject and using custom error classes to report errors in MachO
> headers. My initial experience is that this has enabled much richer error
> messages than are possible with std::error_code.
> To enable interaction with APIs that still use std::error_code I have
> added a custom ECError class that wraps a std::error_code, and can be
> converted back to a std::error_code using the typedErrorToErrorCode
> function. For now, all custom error code classes should (and do, in the
> prototype) derive from this utility class. In my experiments, this has made
> it easy to thread TypedError selectively through parts of the API.
> Eventually my hope is that TypedError could replace std::error_code for
> user-facing APIs, at which point custom errors would no longer need to
> derive from ECError, and ECError could be relegated to a utility for
> interacting with other codebases that still use std::error_code.
> So - I look forward to hearing your thoughts. :)
> Cheers,
> Lang.
> Attached files:
> typed_error.patch - Adds include/llvm/Support/TypedError.h (also adds
> anchor() method to lib/Support/ErrorHandling.cpp).
> error_demo.tgz - Stand-alone program demo'ing basic use of the TypedError
> API.
> libobject_typed_error_demo.patch - Threads TypedError through the
> binary-file creation methods (createBinary, createObjectFile, etc).
> Proof-of-concept for how TypedError can be integrated into an existing
> system.
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
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