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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
// This header contains an interface `FailureLatch`, and some implementation
// helpers that may be used across a range of classes and functions to handle
// failures at any point during a process, and share that failure state so that
// the process may gracefully stop quickly and report the first error.
//
// It could be thought as a replacement for C++ exceptions, but it's less strong
// (cancellations may be delayed).
// Now, if possible, mozilla::Result may be a better option as C++ exceptions
// replacement, as it is more visible in all affected functions.
// Consider FailureLatch if failures may happen in different places, but where
// `return`ing this potential failure from all functions would be too arduous.
#ifndef mozilla_FailureLatch_h
#define mozilla_FailureLatch_h
#include <mozilla/Assertions.h>
#include <string>
namespace mozilla {
// ----------------------------------------------------------------------------
// Main interface
// ----------------------------------------------------------------------------
// Interface handling a failure latch (starting in a successful state, the first
// failure gets recorded, subsequent failures are ignored.)
class FailureLatch {
public:
virtual ~FailureLatch() = default;
// Can this ever fail? (This may influence how some code deals with
// failures, e.g., if infallible, OOMs should assert&crash.)
[[nodiscard]] virtual bool Fallible() const = 0;
// Set latch in its failed state because of an external cause.
// The first call sets the reason, subsequent calls are ignored.
virtual void SetFailure(std::string aReason) = 0;
// Has there been any failure so far?
[[nodiscard]] virtual bool Failed() const = 0;
// Return first failure string, may be null if not failed yet.
[[nodiscard]] virtual const char* GetFailure() const = 0;
// Retrieve the one source FailureLatch. It could reference `*this`!
// This may be used by dependent proxy FailureLatch'es to find where to
// redirect calls.
[[nodiscard]] virtual const FailureLatch& SourceFailureLatch() const = 0;
[[nodiscard]] virtual FailureLatch& SourceFailureLatch() = 0;
// Non-virtual helpers.
// Transfer any failure from another FailureLatch.
void SetFailureFrom(const FailureLatch& aOther) {
if (Failed()) {
return;
}
if (const char* otherFailure = aOther.GetFailure(); otherFailure) {
SetFailure(otherFailure);
}
}
};
// ----------------------------------------------------------------------------
// Concrete implementations
// ----------------------------------------------------------------------------
// Concrete infallible FailureLatch class.
// Any `SetFailure` leads to an assert-crash, so the final runtime result can
// always be assumed to be succesful.
class FailureLatchInfallibleSource final : public FailureLatch {
public:
[[nodiscard]] bool Fallible() const final { return false; }
void SetFailure(std::string aReason) final {
MOZ_RELEASE_ASSERT(false,
"SetFailure in infallible FailureLatchInfallibleSource");
}
[[nodiscard]] bool Failed() const final { return false; }
[[nodiscard]] const char* GetFailure() const final { return nullptr; }
[[nodiscard]] const ::mozilla::FailureLatch& SourceFailureLatch()
const final {
return *this;
}
[[nodiscard]] ::mozilla::FailureLatch& SourceFailureLatch() final {
return *this;
}
// Singleton FailureLatchInfallibleSource that may be used as default
// FailureLatch proxy.
static FailureLatchInfallibleSource& Singleton() {
static FailureLatchInfallibleSource singleton;
return singleton;
}
};
// Concrete FailureLatch class, intended to be intantiated as an object shared
// between classes and functions that are part of a long operation, so that
// failures can happen anywhere and be visible everywhere.
// Not thread-safe.
class FailureLatchSource final : public FailureLatch {
public:
[[nodiscard]] bool Fallible() const final { return true; }
void SetFailure(std::string aReason) final {
if (!mFailed) {
mFailed = true;
mReason = std::move(aReason);
}
}
[[nodiscard]] bool Failed() const final { return mFailed; }
[[nodiscard]] const char* GetFailure() const final {
return mFailed ? mReason.c_str() : nullptr;
}
[[nodiscard]] const FailureLatch& SourceFailureLatch() const final {
return *this;
}
[[nodiscard]] FailureLatch& SourceFailureLatch() final { return *this; }
private:
bool mFailed = false;
std::string mReason;
};
// ----------------------------------------------------------------------------
// Helper macros, to be used in FailureLatch-derived classes
// ----------------------------------------------------------------------------
// Classes deriving from FailureLatch can use this to forward virtual calls to
// another FailureLatch.
#define FAILURELATCH_IMPL_PROXY(FAILURELATCH_REF) \
[[nodiscard]] bool Fallible() const final { \
return static_cast<const ::mozilla::FailureLatch&>(FAILURELATCH_REF) \
.Fallible(); \
} \
void SetFailure(std::string aReason) final { \
static_cast<::mozilla::FailureLatch&>(FAILURELATCH_REF) \
.SetFailure(std::move(aReason)); \
} \
[[nodiscard]] bool Failed() const final { \
return static_cast<const ::mozilla::FailureLatch&>(FAILURELATCH_REF) \
.Failed(); \
} \
[[nodiscard]] const char* GetFailure() const final { \
return static_cast<const ::mozilla::FailureLatch&>(FAILURELATCH_REF) \
.GetFailure(); \
} \
[[nodiscard]] const FailureLatch& SourceFailureLatch() const final { \
return static_cast<const ::mozilla::FailureLatch&>(FAILURELATCH_REF) \
.SourceFailureLatch(); \
} \
[[nodiscard]] FailureLatch& SourceFailureLatch() final { \
return static_cast<::mozilla::FailureLatch&>(FAILURELATCH_REF) \
.SourceFailureLatch(); \
}
// Classes deriving from FailureLatch can use this to forward virtual calls to
// another FailureLatch through a pointer, unless it's null in which case act
// like an infallible FailureLatch.
#define FAILURELATCH_IMPL_PROXY_OR_INFALLIBLE(FAILURELATCH_PTR, CLASS_NAME) \
[[nodiscard]] bool Fallible() const final { \
return FAILURELATCH_PTR \
? static_cast<const ::mozilla::FailureLatch*>(FAILURELATCH_PTR) \
->Fallible() \
: false; \
} \
void SetFailure(std::string aReason) final { \
if (FAILURELATCH_PTR) { \
static_cast<::mozilla::FailureLatch*>(FAILURELATCH_PTR) \
->SetFailure(std::move(aReason)); \
} else { \
MOZ_RELEASE_ASSERT(false, "SetFailure in infallible " #CLASS_NAME); \
} \
} \
[[nodiscard]] bool Failed() const final { \
return FAILURELATCH_PTR \
? static_cast<const ::mozilla::FailureLatch*>(FAILURELATCH_PTR) \
->Failed() \
: false; \
} \
[[nodiscard]] const char* GetFailure() const final { \
return FAILURELATCH_PTR \
? static_cast<const ::mozilla::FailureLatch*>(FAILURELATCH_PTR) \
->GetFailure() \
: nullptr; \
} \
[[nodiscard]] const FailureLatch& SourceFailureLatch() const final { \
return FAILURELATCH_PTR \
? static_cast<const ::mozilla::FailureLatch*>(FAILURELATCH_PTR) \
->SourceFailureLatch() \
: ::mozilla::FailureLatchInfallibleSource::Singleton(); \
} \
[[nodiscard]] FailureLatch& SourceFailureLatch() final { \
return FAILURELATCH_PTR \
? static_cast<::mozilla::FailureLatch*>(FAILURELATCH_PTR) \
->SourceFailureLatch() \
: ::mozilla::FailureLatchInfallibleSource::Singleton(); \
}
} // namespace mozilla
#endif /* mozilla_FailureLatch_h */