tor-browser

Exception.h (8119B)

---

/* -*- 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
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#ifndef js_Exception_h
#define js_Exception_h

#include "mozilla/Attributes.h"
#include "mozilla/Maybe.h"

#include "jstypes.h"

#include "js/RootingAPI.h"  // JS::{Handle,Rooted}
#include "js/TypeDecls.h"
#include "js/Value.h"  // JS::Value, JS::Handle<JS::Value>

class JSErrorReport;

namespace JS {
enum class ExceptionStackBehavior : bool {
 // Do not capture any stack.
 DoNotCapture,

 // Capture the current JS stack when setting the exception. It may be
 // retrieved by JS::GetPendingExceptionStack.
 Capture
};
}  // namespace JS

extern JS_PUBLIC_API bool JS_IsExceptionPending(JSContext* cx);

// [SMDOC] Out Of Memory (OOM) Handling
//
// Many functions in SpiderMonkey can throw exceptions, and sometimes
// the exception thrown is out of memory. Unlike other exceptions,
// this is not an object, but rather the literal string "out of memory".
//
// **Out of Memory handling in SpiderMonkey is best-effort!**
//
// While the developers of SpiderMonkey do attempt to convert various scenarios
// into OutOfMemory calls, such that embedders can attempt to do some sort of
// recovery, we do not guarantee this in all cases.
//
// There are some places where attempting to convey OOM is challenging
// or where it would leave the engine in a state with invariants
// no longer holding. In those cases **the process will crash**.
//
// An example, though not comprehensive, signal of this to a curious
// reader would be AutoEnterOOMUnsafeRegion, which flags various
// places developers have indicated that crashing is better than
// throwing OOM.
//
// Currently we endeavour to always throw out-of-memory when we
// encounter GC heap limits. We also will sometimes throw OOM
// exceptions for things which are not really OOM: For example
// our executable code limits.
//
// It is important to not rely on OOM generation in SpiderMonkey
// as your only reliability measure, as it is not guaranteed.

// Check for pending out of memory exception.
extern JS_PUBLIC_API bool JS_IsThrowingOutOfMemory(JSContext* cx);

// Try and get the pending exception. This can return false
// if there is no pending exception -or- if there is a problem
// attempting to produce the exception (for example if wrapping
// the exception fails.)
extern JS_PUBLIC_API bool JS_GetPendingException(JSContext* cx,
                                                JS::MutableHandleValue vp);

extern JS_PUBLIC_API void JS_SetPendingException(
   JSContext* cx, JS::HandleValue v,
   JS::ExceptionStackBehavior behavior = JS::ExceptionStackBehavior::Capture);

extern JS_PUBLIC_API void JS_ClearPendingException(JSContext* cx);

/**
* If the given object is an exception object, the exception will have (or be
* able to lazily create) an error report struct, and this function will return
* the address of that struct.  Otherwise, it returns nullptr. The lifetime
* of the error report struct that might be returned is the same as the
* lifetime of the exception object.
*/
extern JS_PUBLIC_API JSErrorReport* JS_ErrorFromException(JSContext* cx,
                                                         JS::HandleObject obj);

namespace JS {

// When propagating an exception up the call stack, we store the underlying
// reason on the JSContext as one of the following enum values.
//
// TODO: Track uncatchable exceptions explicitly.
enum class ExceptionStatus {
 // No exception status.
 None,

 // Used by debugger when forcing an early return from a frame. This uses
 // exception machinery, but at the right time is turned back into a normal
 // non-error completion.
 ForcedReturn,

 // Throwing a (catchable) exception. Certain well-known exceptions are
 // explicitly tracked for convenience.
 Throwing,
 OutOfMemory,
 OverRecursed,
};

// Returns true if the status is a catchable exception. Formerly this was
// indicated by the `JSContext::throwing` flag.
static MOZ_ALWAYS_INLINE bool IsCatchableExceptionStatus(
   ExceptionStatus status) {
 return status >= ExceptionStatus::Throwing;
}

// This class encapsulates a (pending) exception and the corresponding optional
// SavedFrame stack object captured when the pending exception was set
// on the JSContext. This fuzzily correlates with a `throw` statement in JS,
// although arbitrary JSAPI consumers or VM code may also set pending exceptions
// via `JS_SetPendingException`.
//
// This is not the same stack as `e.stack` when `e` is an `Error` object.
// (That would be JS::ExceptionStackOrNull).
class MOZ_STACK_CLASS ExceptionStack {
 Rooted<Value> exception_;
 Rooted<JSObject*> stack_;

 friend JS_PUBLIC_API bool GetPendingExceptionStack(
     JSContext* cx, JS::ExceptionStack* exceptionStack);

 void init(HandleValue exception, HandleObject stack) {
   exception_ = exception;
   stack_ = stack;
 }

public:
 explicit ExceptionStack(JSContext* cx) : exception_(cx), stack_(cx) {}

 ExceptionStack(JSContext* cx, HandleValue exception, HandleObject stack)
     : exception_(cx, exception), stack_(cx, stack) {}

 HandleValue exception() const { return exception_; }

 // |stack| can be null.
 HandleObject stack() const { return stack_; }
};

/**
* Save and later restore the current exception state of a given JSContext.
* This is useful for implementing behavior in C++ that's like try/catch
* or try/finally in JS.
*
* Typical usage:
*
*     bool ok = JS::Evaluate(cx, ...);
*     AutoSaveExceptionState savedExc(cx);
*     ... cleanup that might re-enter JS ...
*     return ok;
*/
class JS_PUBLIC_API AutoSaveExceptionState {
private:
 JSContext* context;
 ExceptionStatus status;
 RootedValue exceptionValue;
 RootedObject exceptionStack;

public:
 /*
  * Take a snapshot of cx's current exception state. Then clear any current
  * pending exception in cx.
  */
 explicit AutoSaveExceptionState(JSContext* cx);

 /*
  * If neither drop() nor restore() was called, restore the exception
  * state only if no exception is currently pending on cx.
  */
 ~AutoSaveExceptionState();

 /*
  * Discard any stored exception state.
  * If this is called, the destructor is a no-op.
  */
 void drop();

 /*
  * Replace cx's exception state with the stored exception state. Then
  * discard the stored exception state. If this is called, the
  * destructor is a no-op.
  */
 void restore();
};

// Get the current pending exception value and stack.
// This function asserts that there is a pending exception.
// If this function returns false, then retrieving the current pending exception
// failed and might have been overwritten by a new exception.
extern JS_PUBLIC_API bool GetPendingExceptionStack(
   JSContext* cx, JS::ExceptionStack* exceptionStack);

// Similar to GetPendingExceptionStack, but also clears the current
// pending exception.
extern JS_PUBLIC_API bool StealPendingExceptionStack(
   JSContext* cx, JS::ExceptionStack* exceptionStack);

// Set both the exception value and its associated stack on the context as
// the current pending exception.
extern JS_PUBLIC_API void SetPendingExceptionStack(
   JSContext* cx, const JS::ExceptionStack& exceptionStack);

/**
* If the given object is an exception object (or an unwrappable
* cross-compartment wrapper for one), return the stack for that exception, if
* any.  Will return null if the given object is not an exception object
* (including if it's null or a security wrapper that can't be unwrapped) or if
* the exception has no stack.
*/
extern JS_PUBLIC_API JSObject* ExceptionStackOrNull(JS::HandleObject obj);

/**
* If the given object is an exception object, return the error cause for that
* exception, if any, or mozilla::Nothing.
*/
extern JS_PUBLIC_API mozilla::Maybe<JS::Value> GetExceptionCause(JSObject* exc);

}  // namespace JS

#endif  // js_Exception_h