gistfile1.cpp

/*
 * This is for writing C++ code that does the equivalent of JS try/finally.
 * Normal infallible C++ cleanup code should just go in a C++ destructor.
 * This is for cleanup that should be *skipped* in the unusual case that
 * the try block exits with an uncatchable error (OOM or timeout).
 *
 * Use it like this:
 *
 *  bool ok;
 *  ... YOUR TRY-BLOCK CODE HERE ...
 *  if (AutoFinallyBlock afb(cx, ok)) {
 *      bool cleanupOK;
 *      ... YOUR CLEANUP CODE HERE ...
 *      ok = afb.leave(closedOK);
 *  }
 *  return ok;
 *
 */
class AutoFinallyBlock {
  private:
    JSContext *cx;
    Value exception;

    enum { OK = JS_GENERIC_MAGIC, DONE = JS_THIS_POISON };

  public:
    AutoFinallyBlock(JSContext *cx, bool ok) : cx(cx), ok(ok) {
        if (ok) {
            /* Case 1: try-block exited successfully. */
            JS_ASSERT(!cx->isExceptionPending());
            exception.setMagic(OK);
        } else if (cx->isExceptionPending()) {
            /* Case 2: try-block threw an exception. */
            exception = cx->getPendingException();
            cx->clearPendingException();
        } else {
            /*
             * Case 3: try-block died with an uncatchable error. We're done
             * here; the caller will skip the finally block.
             */
            exception.setMagic(DONE);
        }
    }

    operator bool() const {
        return !exception.isParticularMagic(DONE);
    }

    void debugSetDone() {
#ifdef DEBUG
        exception.setMagic(DONE);
#endif
    }

    void leave(bool cleanupOK) {
        /*
         * There are four possible cases:
         *     was ok and now cleanupOK - do nothing, return true
         *     was ok but now !cleanupOK - do nothing, return false
         *     had exception and now cleanupOK - restore exception, return false
         *     had exception and now !cleanupOK - do nothing, return false
         *
         * This method MUST NOT be called if we had an uncatchable error.
         * The isMagic call here enforces that with an assertion.
         */
        if (exception.isMagic(JS_EVERYTHING_IS_FINE)) {
            debugSetDone();
            return cleanupOK;
        }
        if (cleanupOK)
            cx->setPendingException(exception);
        debugSetDone();
        return false;
    }

    ~AutoFinallyBlock() {
        JS_ASSERT(exception.isParticularMagic(DONE));
    }
};