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Subject: [Qemu-trivial] block/curl: Remedy a crashing bug completing AIOCBs
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In QEMU master, attempting to read a cached block from a HTTP (or otherwise)
mounted ISO causes an assert to be triggered, killing the entire QEMU process.
It looks like this:

hw/ide/pci.c:314: bmdma_cmd_writeb: Assertion `bm->bus->dma->aiocb == 
((void *)0)' failed.

The following two patches add flush capability to the curl backend, and avoid
triggering the assert by finishing the AIOCB in a QEMUBH callback, rather than
directly in curl_aio_readv().