waitForBytesWritten() can emit signals, most importantly readyRead(), *before* even writing data. Anything can happen in slots connected to these signals - which includes writing out some data, which can make a subsequent attempt to write data in waitForBytesWritten() fail(*), so waitForBytesWritten() fails. Recognize when anything invoked from the current call wrote out some data and return success instead. It is not possible to deal correctly with all shenanigans in signal handlers, but this seems quite reasonable. Also fix QAbstractSocket build with QABSTRACTSOCKET_DEBUG defined. (*) for at least two reasons: nothing to write anymore or OS write buffer full Pick-to: 6.10 6.9 6.8 6.5 Change-Id: Ibf4de93d5e7dc2f88b675de410b216674faa20ad Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>