Status ModifierUnset::log(mutablebson::Element logRoot) const {

        // We'd like to create an entry such as {$unset: {<fieldname>: 1}} under 'logRoot'.
        // We start by creating the {$unset: ...} Element.
        mutablebson::Document& doc = logRoot.getDocument();
        mutablebson::Element unsetElement = doc.makeElementObject("$unset");
        if (!unsetElement.ok()) {
            return Status(ErrorCodes::InternalError, "cannot create log entry for $unset mod");
        }

        // Then we create the {<fieldname>: <value>} Element. Note that <fieldname> must be a
        // dotted field, and not only the last part of that field. The rationale here is that
        // somoene picking up this log entry -- e.g., a secondary -- must be capable of doing
        // the same path find/creation that was done in the previous calls here.
        mutablebson::Element logElement = doc.makeElementInt(_fieldRef.dottedField(), 1);
        if (!logElement.ok()) {
            return Status(ErrorCodes::InternalError, "cannot create log details for $unset mod");
        }

        // Now, we attach the {<fieldname>: `} Element under the {$unset: ...} one.
        Status status = unsetElement.pushBack(logElement);
        if (!status.isOK()) {
            return status;
        }

        // And attach the result under the 'logRoot' Element provided.
        return logRoot.pushBack(unsetElement);
    }

    Status ModifierInc::log(mutablebson::Element logRoot) const {

        // We'd like to create an entry such as {$set: {<fieldname>: <value>}} under 'logRoot'.
        // We start by creating the {$set: ...} Element.
        mutablebson::Document& doc = logRoot.getDocument();
        mutablebson::Element setElement = doc.makeElementObject("$set");
        if (!setElement.ok()) {
            return Status(ErrorCodes::InternalError, "cannot append log entry for $set mod");
        }

        // Then we create the {<fieldname>: <value>} Element.
        mutablebson::Element logElement = doc.makeElementSafeNum(
            _fieldRef.dottedField(),
            _preparedState->newValue);

        if (!logElement.ok()) {
            return Status(ErrorCodes::InternalError, "cannot append details for $set mod");
        }

        // Now, we attach the {<fieldname>: <value>} Element under the {$set: ...} one.
        Status status = setElement.pushBack(logElement);
        if (!status.isOK()) {
            return status;
        }

        // And attach the result under the 'logRoot' Element provided.
        return logRoot.pushBack(setElement);
    }

Status ModifierSet::log(mutablebson::Element logRoot) const {

    // We'd like to create an entry such as {$set: {<fieldname>: <value>}} under 'logRoot'.
    // We start by creating the {$set: ...} Element.
    mutablebson::Document& doc = logRoot.getDocument();
    mutablebson::Element setElement = doc.makeElementObject("$set");
    if (!setElement.ok()) {
        return Status(ErrorCodes::InternalError, "cannot create log entry for $set mod");
    }

    // Then we create the {<fieldname>: <value>} Element. Note that we log the mod with a
    // dotted field, if it was applied over a dotted field. The rationale is that the
    // secondary may be in a different state than the primary and thus make different
    // decisions about creating the intermediate path in _fieldRef or not.
    mutablebson::Element logElement = doc.makeElementWithNewFieldName(_fieldRef.dottedField(),
                                      _val);
    if (!logElement.ok()) {
        return Status(ErrorCodes::InternalError, "cannot create details for $set mod");
    }

    // Now, we attach the {<fieldname>: <value>} Element under the {$set: ...} one.
    Status status = setElement.pushBack(logElement);
    if (!status.isOK()) {
        return status;
    }

    // And attach the result under the 'logRoot' Element provided.
    return logRoot.pushBack(setElement);
}

    Status ModifierPush::prepare(mutablebson::Element root,
                                 const StringData& matchedField,
                                 ExecInfo* execInfo) {

        _preparedState.reset(new PreparedState(&root.getDocument()));

        // If we have a $-positional field, it is time to bind it to an actual field part.
        if (_posDollar) {
            if (matchedField.empty()) {
                return Status(ErrorCodes::BadValue,
                              str::stream() << "The positional operator did not find the match "
                                               "needed from the query. Unexpanded update: "
                                            << _fieldRef.dottedField());
            }
            _fieldRef.setPart(_posDollar, matchedField);
        }

        // Locate the field name in 'root'. Note that we may not have all the parts in the path
        // in the doc -- which is fine. Our goal now is merely to reason about whether this mod
        // apply is a noOp or whether is can be in place. The remainin path, if missing, will
        // be created during the apply.
        Status status = pathsupport::findLongestPrefix(_fieldRef,
                                                       root,
                                                       &_preparedState->idxFound,
                                                       &_preparedState->elemFound);

        // FindLongestPrefix may say the path does not exist at all, which is fine here, or
        // that the path was not viable or otherwise wrong, in which case, the mod cannot
        // proceed.
        if (status.code() == ErrorCodes::NonExistentPath) {
            _preparedState->elemFound = root.getDocument().end();

        }
        else if (status.isOK()) {

            const bool destExists = (_preparedState->idxFound == (_fieldRef.numParts()-1));
            // If the path exists, we require the target field to be already an
            // array.
            if (destExists && _preparedState->elemFound.getType() != Array) {
                mb::Element idElem = mb::findFirstChildNamed(root, "_id");
                return Status(ErrorCodes::BadValue,
                              str::stream() << "The field '" << _fieldRef.dottedField() << "'"
                                            << " must be an array but is of type "
                                            << typeName(_preparedState->elemFound.getType())
                                            << " in document {" << idElem.toString() << "}");
            }
        }
        else {
            return status;
        }

        // We register interest in the field name. The driver needs this info to sort out if
        // there is any conflict among mods.
        execInfo->fieldRef[0] = &_fieldRef;

        return Status::OK();
    }

Status LogBuilder::addToSetsWithNewFieldName(StringData name, const mutablebson::Element val) {
    mutablebson::Element elemToSet = _logRoot.getDocument().makeElementWithNewFieldName(name, val);
    if (!elemToSet.ok())
        return Status(ErrorCodes::InternalError,
                      str::stream() << "Could not create new '" << name
                                    << "' element from existing element '"
                                    << val.getFieldName()
                                    << "' of type "
                                    << typeName(val.getType()));

    return addToSets(elemToSet);
}

    Status ModifierAddToSet::log(mb::Element logRoot) const {

        // TODO: This is copied more or less identically from $push. As a result, it copies the
        // behavior in $push that relies on 'apply' having been called unless this is a no-op.

        // TODO We can log just a positional set in several cases. For now, let's just log the
        // full resulting array.

        // We'd like to create an entry such as {$set: {<fieldname>: [<resulting aray>]}} under
        // 'logRoot'.  We start by creating the {$set: ...} Element.
        mb::Document& doc = logRoot.getDocument();
        mb::Element setElement = doc.makeElementObject("$set");
        if (!setElement.ok()) {
            return Status(ErrorCodes::InternalError, "cannot create log entry for $addToSet mod");
        }

        // Then we create the {<fieldname>:[]} Element, that is, an empty array.
        mb::Element logElement = doc.makeElementArray(_fieldRef.dottedField());
        if (!logElement.ok()) {
            return Status(ErrorCodes::InternalError, "cannot create details for $addToSet mod");
        }

        // Fill up the empty array.
        mb::Element curr = _preparedState->elemFound.leftChild();
        while (curr.ok()) {

            dassert(curr.hasValue());

            // We need to copy each array entry from the resulting document to the log
            // document.
            mb::Element currCopy = doc.makeElementWithNewFieldName(
                StringData(),
                curr.getValue());
            if (!currCopy.ok()) {
                return Status(ErrorCodes::InternalError, "could create copy element");
            }
            Status status = logElement.pushBack(currCopy);
            if (!status.isOK()) {
                return Status(ErrorCodes::BadValue, "could not append entry for $addToSet log");
            }
            curr = curr.rightSibling();
        }

        // Now, we attach the {<fieldname>: [<filled array>]} Element under the {$set: ...}
        // one.
        Status status = setElement.pushBack(logElement);
        if (!status.isOK()) {
            return status;
        }

        // And attach the result under the 'logRoot' Element provided.
        return logRoot.pushBack(setElement);
    }

Status ModifierObjectReplace::prepare(mutablebson::Element root,
                                      StringData matchedField,
                                      ExecInfo* execInfo) {
    _preparedState.reset(new PreparedState(&root.getDocument()));

    // objectSize checked by binaryEqual (optimization)
    BSONObj objOld = root.getDocument().getObject();
    if (objOld.binaryEqual(_val)) {
        _preparedState->noOp = true;
        execInfo->noOp = true;
    }

    return Status::OK();
}

    Status ModifierObjectReplace::prepare(mutablebson::Element root,
                                          const StringData& matchedField,
                                          ExecInfo* execInfo) {

        _preparedState.reset(new PreparedState(&root.getDocument()));
        return Status::OK();
    }

 PreparedState(mutablebson::Element root)
     : doc(root.getDocument())
     , fromElemFound(doc.end())
     , toIdxFound(0)
     , toElemFound(doc.end())
     , applyCalled(false){
 }

    Status ModifierCurrentDate::prepare(mutablebson::Element root,
                                StringData matchedField,
                                ExecInfo* execInfo) {

        _preparedState.reset(new PreparedState(root.getDocument()));

        // If we have a $-positional field, it is time to bind it to an actual field part.
        if (_pathReplacementPosition) {
            if (matchedField.empty()) {
                return Status(ErrorCodes::BadValue,
                              str::stream() << "The positional operator did not find the match "
                                               "needed from the query. Unexpanded update: "
                                            << _updatePath.dottedField());
            }
            _updatePath.setPart(_pathReplacementPosition, matchedField);
        }

        // Locate the field name in 'root'. Note that we may not have all the parts in the path
        // in the doc -- which is fine. Our goal now is merely to reason about whether this mod
        // apply is a noOp or whether is can be in place. The remaining path, if missing, will
        // be created during the apply.
        Status status = pathsupport::findLongestPrefix(_updatePath,
                                                root,
                                                &_preparedState->idxFound,
                                                &_preparedState->elemFound);

        // FindLongestPrefix may say the path does not exist at all, which is fine here, or
        // that the path was not viable or otherwise wrong, in which case, the mod cannot
        // proceed.
        if (status.code() == ErrorCodes::NonExistentPath) {
            _preparedState->elemFound = root.getDocument().end();
        }
        else if (!status.isOK()) {
            return status;
        }

        // We register interest in the field name. The driver needs this info to sort out if
        // there is any conflict among mods.
        execInfo->fieldRef[0] = &_updatePath;

        return Status::OK();
    }

Status ModifierUnset::prepare(mutablebson::Element root,
                              const StringData& matchedField,
                              ExecInfo* execInfo) {

    _preparedState.reset(new PreparedState(&root.getDocument()));

    // If we have a $-positional field, it is time to bind it to an actual field part.
    if (_posDollar) {
        if (matchedField.empty()) {
            return Status(ErrorCodes::BadValue,
                          str::stream() << "The positional operator did not find the match "
                          "needed from the query. Unexpanded update: "
                          << _fieldRef.dottedField());
        }
        _preparedState->boundDollar = matchedField.toString();
        _fieldRef.setPart(_posDollar, _preparedState->boundDollar);
    }


    // Locate the field name in 'root'. Note that if we don't have the full path in the
    // doc, there isn't anything to unset, really.
    Status status = pathsupport::findLongestPrefix(_fieldRef,
                    root,
                    &_preparedState->idxFound,
                    &_preparedState->elemFound);
    if (!status.isOK() ||
            _preparedState->idxFound != (_fieldRef.numParts() -1)) {
        execInfo->noOp = _preparedState->noOp = true;
        execInfo->fieldRef[0] = &_fieldRef;

        return Status::OK();
    }

    // If there is indeed something to unset, we register so, along with the interest in
    // the field name. The driver needs this info to sort out if there is any conflict
    // among mods.
    execInfo->fieldRef[0] = &_fieldRef;

    // The only way for an $unset to be inplace is for its target field to be the last one
    // of the object. That is, it is always the right child on its paths. The current
    // rationale is that there should be no holes in a BSONObj and, to be in place, no
    // field boundaries must change.
    //
    // TODO:
    // mutablebson::Element curr = _preparedState->elemFound;
    // while (curr.ok()) {
    //     if (curr.rightSibling().ok()) {
    //     }
    //     curr = curr.parent();
    // }

    return Status::OK();
}

 Status AuthorizationManager::getBSONForPrivileges(const PrivilegeVector& privileges,
                                                   mutablebson::Element resultArray) {
     for (PrivilegeVector::const_iterator it = privileges.begin();
             it != privileges.end(); ++it) {
         std::string errmsg;
         ParsedPrivilege privilege;
         if (!ParsedPrivilege::privilegeToParsedPrivilege(*it, &privilege, &errmsg)) {
             return Status(ErrorCodes::BadValue, errmsg);
         }
         resultArray.appendObject("privileges", privilege.toBSON());
     }
     return Status::OK();
 }

    Status ModifierObjectReplace::log(mutablebson::Element logRoot) const {

        // We'd like to create an entry such as {<object replacement>} under 'logRoot'.
        mutablebson::Document& doc = logRoot.getDocument();
        BSONObjIterator it(_val);
        while (it.more()) {
            BSONElement elem = it.next();
            Status status = doc.root().appendElement(elem);
            if (!status.isOK()) {
                return status;
            }
        }

        return Status::OK();
    }

    Status AuthorizationManager::getBSONForRole(RoleGraph* graph,
                                                const RoleName& roleName,
                                                mutablebson::Element result) {
        if (!graph->roleExists(roleName)) {
            return Status(ErrorCodes::RoleNotFound,
                          mongoutils::str::stream() << roleName.getFullName() <<
                                  "does not name an existing role");
        }
        std::string id = mongoutils::str::stream() << roleName.getDB() << "." << roleName.getRole();
        result.appendString("_id", id);
        result.appendString(ROLE_NAME_FIELD_NAME, roleName.getRole());
        result.appendString(ROLE_DB_FIELD_NAME, roleName.getDB());

        // Build privileges array
        mutablebson::Element privilegesArrayElement =
                result.getDocument().makeElementArray("privileges");
        result.pushBack(privilegesArrayElement);
        const PrivilegeVector& privileges = graph->getDirectPrivileges(roleName);
        Status status = getBSONForPrivileges(privileges, privilegesArrayElement);
        if (!status.isOK()) {
            return status;
        }

        // Build roles array
        mutablebson::Element rolesArrayElement = result.getDocument().makeElementArray("roles");
        result.pushBack(rolesArrayElement);
        for (RoleNameIterator roles = graph->getDirectSubordinates(roleName);
             roles.more();
             roles.next()) {

            const RoleName& subRole = roles.get();
            mutablebson::Element roleObj = result.getDocument().makeElementObject("");
            roleObj.appendString(ROLE_NAME_FIELD_NAME, subRole.getRole());
            roleObj.appendString(ROLE_DB_FIELD_NAME, subRole.getDB());
            rolesArrayElement.pushBack(roleObj);
        }

        return Status::OK();
    }

    Status ModifierAddToSet::prepare(mb::Element root,
                                     const StringData& matchedField,
                                     ExecInfo* execInfo) {

        _preparedState.reset(new PreparedState(root.getDocument()));

        // If we have a $-positional field, it is time to bind it to an actual field part.
        if (_posDollar) {
            if (matchedField.empty()) {
                return Status(ErrorCodes::BadValue, "matched field not provided");
            }
            _preparedState->boundDollar = matchedField.toString();
            _fieldRef.setPart(_posDollar, _preparedState->boundDollar);
        }

        // Locate the field name in 'root'.
        Status status = pathsupport::findLongestPrefix(_fieldRef,
                                                       root,
                                                       &_preparedState->idxFound,
                                                       &_preparedState->elemFound);

        // FindLongestPrefix may say the path does not exist at all, which is fine here, or
        // that the path was not viable or otherwise wrong, in which case, the mod cannot
        // proceed.
        if (status.code() == ErrorCodes::NonExistentPath) {
            _preparedState->elemFound = root.getDocument().end();
        } else if (!status.isOK()) {
            return status;
        }

        // We register interest in the field name. The driver needs this info to sort out if
        // there is any conflict among mods.
        execInfo->fieldRef[0] = &_fieldRef;

        //
        // in-place and no-op logic
        //

        // If the field path is not fully present, then this mod cannot be in place, nor is a
        // noOp.
        if (!_preparedState->elemFound.ok() ||
            _preparedState->idxFound < static_cast<int32_t>(_fieldRef.numParts() - 1)) {
            // If no target element exists, we will simply be creating a new array.
            _preparedState->addAll = true;
            return Status::OK();
        }

        // This operation only applies to arrays
        if (_preparedState->elemFound.getType() != mongo::Array)
            return Status(
                ErrorCodes::BadValue,
                "Cannot apply $addToSet to a non-array value");

        // If the array is empty, then we don't need to check anything: all of the values are
        // going to be added.
        if (!_preparedState->elemFound.hasChildren()) {
           _preparedState->addAll = true;
            return Status::OK();
        }

        // For each value in the $each clause, compare it against the values in the array. If
        // the element is not present, record it as one to add.
        mb::Element eachIter = _val.leftChild();
        while (eachIter.ok()) {
            mb::Element where = mb::findElement(
                _preparedState->elemFound.leftChild(),
                mb::woEqualTo(eachIter, false));
            if (!where.ok()) {
                // The element was not found. Record the element from $each as one to be added.
                _preparedState->elementsToAdd.push_back(eachIter);
            }
            eachIter = eachIter.rightSibling();
        }

        // If we didn't find any elements to add, then this is a no-op, and therefore in place.
        if (_preparedState->elementsToAdd.empty()) {
            _preparedState->noOp = execInfo->noOp = true;
            execInfo->inPlace = true;
        }

        return Status::OK();
    }

    Status ModifierInc::prepare(mutablebson::Element root,
                                const StringData& matchedField,
                                ExecInfo* execInfo) {

        _preparedState.reset(new PreparedState(root.getDocument()));

        // If we have a $-positional field, it is time to bind it to an actual field part.
        if (_posDollar) {
            if (matchedField.empty()) {
                return Status(ErrorCodes::BadValue, "matched field not provided");
            }
            _preparedState->boundDollar = matchedField.toString();
            _fieldRef.setPart(_posDollar, _preparedState->boundDollar);
        }

        // Locate the field name in 'root'. Note that we may not have all the parts in the path
        // in the doc -- which is fine. Our goal now is merely to reason about whether this mod
        // apply is a noOp or whether is can be in place. The remaining path, if missing, will
        // be created during the apply.
        Status status = pathsupport::findLongestPrefix(_fieldRef,
                                                       root,
                                                       &_preparedState->idxFound,
                                                       &_preparedState->elemFound);

        // FindLongestPrefix may say the path does not exist at all, which is fine here, or
        // that the path was not viable or otherwise wrong, in which case, the mod cannot
        // proceed.
        if (status.code() == ErrorCodes::NonExistentPath) {
            _preparedState->elemFound = root.getDocument().end();
        }
        else if (!status.isOK()) {
            return status;
        }

        // We register interest in the field name. The driver needs this info to sort out if
        // there is any conflict among mods.
        execInfo->fieldRef[0] = &_fieldRef;

        // Capture the value we are going to write. At this point, there may not be a value
        // against which to operate, so the result will be simply _val.
        _preparedState->newValue = _val;

        //
        // in-place and no-op logic
        //
        // If the field path is not fully present, then this mod cannot be in place, nor is a
        // noOp.
        if (!_preparedState->elemFound.ok() ||
            _preparedState->idxFound < static_cast<int32_t>(_fieldRef.numParts() - 1)) {
            return Status::OK();
        }

        // If the value being $inc'ed is the same as the one already in the doc, than this is a
        // noOp.
        if (!_preparedState->elemFound.isNumeric())
            return Status(ErrorCodes::BadValue,
                          "invalid attempt to increment a non-numeric field");

        const SafeNum currentValue = _preparedState->elemFound.getValueSafeNum();

        // Update newValue w.r.t to the current value of the found element.
        _preparedState->newValue += currentValue;

        // If the result of the addition is invalid, we must return an error.
        if (!_preparedState->newValue.isValid())
            return Status(ErrorCodes::BadValue,
                          "Failed to increment current value");

        // If the values are identical (same type, same value), then this is a no-op, and
        // therefore in-place as well.
        if (_preparedState->newValue.isIdentical(currentValue)) {
            _preparedState->noOp = execInfo->noOp = true;
            _preparedState->inPlace = execInfo->inPlace = true;
            return Status::OK();
        }

        // If the types are the same, this can be done in place.
        //
        // TODO: Potentially, cases where $inc results in a mixed type of the same size could
        // be in-place as well, but we don't currently handle them.
        if (_preparedState->newValue.type() == currentValue.type()) {
            _preparedState->inPlace = execInfo->inPlace = true;
        }

        return Status::OK();
    }

    Status ModifierInc::prepare(mutablebson::Element root,
                                const StringData& matchedField,
                                ExecInfo* execInfo) {

        _preparedState.reset(new PreparedState(root.getDocument()));

        // If we have a $-positional field, it is time to bind it to an actual field part.
        if (_posDollar) {
            if (matchedField.empty()) {
                return Status(ErrorCodes::BadValue,
                              str::stream() << "The positional operator did not find the match "
                                               "needed from the query. Unexpanded update: "
                                            << _fieldRef.dottedField());
            }
            _fieldRef.setPart(_posDollar, matchedField);
        }

        // Locate the field name in 'root'. Note that we may not have all the parts in the path
        // in the doc -- which is fine. Our goal now is merely to reason about whether this mod
        // apply is a noOp or whether is can be in place. The remaining path, if missing, will
        // be created during the apply.
        Status status = pathsupport::findLongestPrefix(_fieldRef,
                                                       root,
                                                       &_preparedState->idxFound,
                                                       &_preparedState->elemFound);

        // FindLongestPrefix may say the path does not exist at all, which is fine here, or
        // that the path was not viable or otherwise wrong, in which case, the mod cannot
        // proceed.
        if (status.code() == ErrorCodes::NonExistentPath) {
            _preparedState->elemFound = root.getDocument().end();
        }
        else if (!status.isOK()) {
            return status;
        }

        // We register interest in the field name. The driver needs this info to sort out if
        // there is any conflict among mods.
        execInfo->fieldRef[0] = &_fieldRef;

        // Capture the value we are going to write. At this point, there may not be a value
        // against which to operate, so the result will be simply _val.
        _preparedState->newValue = _val;

        //
        // in-place and no-op logic
        //
        // If the field path is not fully present, then this mod cannot be in place, nor is a
        // noOp.
        if (!_preparedState->elemFound.ok() ||
            _preparedState->idxFound < (_fieldRef.numParts() - 1)) {

            // For multiplication, we treat ops against missing as yielding zero. We take
            // advantage here of the promotion rules for SafeNum; the expression below will
            // always yield a zero of the same type of operand that the user provided
            // (e.g. double).
            if (_mode == MODE_MUL)
                _preparedState->newValue *= SafeNum(static_cast<int>(0));

            return Status::OK();
        }

        // If the value being $inc'ed is the same as the one already in the doc, than this is a
        // noOp.
        if (!_preparedState->elemFound.isNumeric()) {
            mb::Element idElem = mb::findFirstChildNamed(root, "_id");
            return Status(
                ErrorCodes::BadValue,
                str::stream() << "Cannot apply "
                              << (_mode == MODE_INC ? "$inc" : "$mul")
                              << " to a value of non-numeric type. {"
                              << idElem.toString()
                              << "} has the field '" <<  _preparedState->elemFound.getFieldName()
                              << "' of non-numeric type "
                              << typeName(_preparedState->elemFound.getType()));
        }
        const SafeNum currentValue = _preparedState->elemFound.getValueSafeNum();

        // Update newValue w.r.t to the current value of the found element.
        if (_mode == MODE_INC)
            _preparedState->newValue += currentValue;
        else
            _preparedState->newValue *= currentValue;

        // If the result of the addition is invalid, we must return an error.
        if (!_preparedState->newValue.isValid()) {
            mb::Element idElem = mb::findFirstChildNamed(root, "_id");
            return Status(ErrorCodes::BadValue,
                          str::stream() << "Failed to apply $inc operations to current value ("
                                        << currentValue.debugString() << ") for document {"
                                        << idElem.toString() << "}");
        }

        // If the values are identical (same type, same value), then this is a no-op.
        if (_preparedState->newValue.isIdentical(currentValue)) {
            _preparedState->noOp = execInfo->noOp = true;
            return Status::OK();
        }

        return Status::OK();
//.........这里部分代码省略.........

    Status ModifierPush::prepare(mutablebson::Element root,
                                 const StringData& matchedField,
                                 ExecInfo* execInfo) {

        _preparedState.reset(new PreparedState(&root.getDocument()));

        // If we have a $-positional field, it is time to bind it to an actual field part.
        if (_posDollar) {
            if (matchedField.empty()) {
                return Status(ErrorCodes::BadValue, "matched field not provided");
            }
            _preparedState->boundDollar = matchedField.toString();
            _fieldRef.setPart(_posDollar, _preparedState->boundDollar);
        }

        // Locate the field name in 'root'. Note that we may not have all the parts in the path
        // in the doc -- which is fine. Our goal now is merely to reason about whether this mod
        // apply is a noOp or whether is can be in place. The remainin path, if missing, will
        // be created during the apply.
        Status status = pathsupport::findLongestPrefix(_fieldRef,
                                                       root,
                                                       &_preparedState->idxFound,
                                                       &_preparedState->elemFound);

        // FindLongestPrefix may say the path does not exist at all, which is fine here, or
        // that the path was not viable or otherwise wrong, in which case, the mod cannot
        // proceed.
        if (status.code() == ErrorCodes::NonExistentPath) {
            _preparedState->elemFound = root.getDocument().end();

        }
        else if (status.isOK()) {

            const bool destExists = (_preparedState->idxFound == (_fieldRef.numParts()-1));
            // If the path exists, we require the target field to be already an
            // array.
            if (destExists && _preparedState->elemFound.getType() != Array) {
                return Status(ErrorCodes::BadValue, "can only $push into arrays");
            }

            // If the $sort clause is being used, we require all the items in the array to be
            // objects themselves (as opposed to base types). This is a temporary restriction
            // that can be lifted once we support full sort semantics in $push.
            if (_sortPresent && destExists) {
                mutablebson::Element curr = _preparedState->elemFound.leftChild();
                while (curr.ok()) {
                    if (curr.getType() != Object) {
                        return Status(ErrorCodes::BadValue,
                                      "$push with sort requires object arrays");
                    }
                    curr = curr.rightSibling();
                }
            }

        }
        else {
            return status;
        }

        // We register interest in the field name. The driver needs this info to sort out if
        // there is any conflict among mods.
        execInfo->fieldRef[0] = &_fieldRef;

        return Status::OK();
    }

Status ModifierBit::prepare(mutablebson::Element root,
                            StringData matchedField,
                            ExecInfo* execInfo) {
    _preparedState.reset(new PreparedState(root.getDocument()));

    // If we have a $-positional field, it is time to bind it to an actual field part.
    if (_posDollar) {
        if (matchedField.empty()) {
            return Status(ErrorCodes::BadValue,
                          str::stream() << "The positional operator did not find the match "
                          "needed from the query. Unexpanded update: "
                          << _fieldRef.dottedField());
        }
        _fieldRef.setPart(_posDollar, matchedField);
    }

    // Locate the field name in 'root'.
    Status status = pathsupport::findLongestPrefix(
                        _fieldRef, root, &_preparedState->idxFound, &_preparedState->elemFound);


    // FindLongestPrefix may say the path does not exist at all, which is fine here, or
    // that the path was not viable or otherwise wrong, in which case, the mod cannot
    // proceed.
    if (status.code() == ErrorCodes::NonExistentPath) {
        _preparedState->elemFound = root.getDocument().end();
    } else if (!status.isOK()) {
        return status;
    }

    // We register interest in the field name. The driver needs this info to sort out if
    // there is any conflict among mods.
    execInfo->fieldRef[0] = &_fieldRef;

    //
    // in-place and no-op logic
    //

    // If the field path is not fully present, then this mod cannot be in place, nor is a
    // noOp.
    if (!_preparedState->elemFound.ok() || _preparedState->idxFound < (_fieldRef.numParts() - 1)) {
        // If no target element exists, the value we will write is the result of applying
        // the operation to a zero-initialized integer element.
        _preparedState->newValue = apply(SafeNum(static_cast<int>(0)));
        return Status::OK();
    }

    if (!_preparedState->elemFound.isIntegral()) {
        mb::Element idElem = mb::findElementNamed(root.leftChild(), "_id");
        return Status(ErrorCodes::BadValue,
                      str::stream() << "Cannot apply $bit to a value of non-integral type."
                      << idElem.toString() << " has the field "
                      << _preparedState->elemFound.getFieldName()
                      << " of non-integer type "
                      << typeName(_preparedState->elemFound.getType()));
    }

    const SafeNum currentValue = _preparedState->elemFound.getValueSafeNum();

    // Apply the op over the existing value and the mod value, and capture the result.
    _preparedState->newValue = apply(currentValue);

    if (!_preparedState->newValue.isValid()) {
        // TODO: Include list of ops, if that is easy, at some future point.
        return Status(ErrorCodes::BadValue,
                      str::stream() << "Failed to apply $bit operations to current value: "
                      << currentValue.debugString());
    }
    // If the values are identical (same type, same value), then this is a no-op.
    if (_preparedState->newValue.isIdentical(currentValue)) {
        _preparedState->noOp = execInfo->