mLastError = rv;
    NS_IMETHOD ReadSegments(nsWriteSegmentFun aWriter, void *aClosure,                            PRUint32 aCount, PRUint32 *aBytesRead) {        nsresult rv = mSource->ReadSegments(aWriter, aClosure, aCount, aBytesRead);        if (NS_SUCCEEDED(rv))            mBytesRead += *aBytesRead;        return rv;    }