Hi, As reported in https://bugzilla.redhat.com/show_bug.cgi?id=1740193, our curl block driver can spontaneously hang. This becomes visible e.g. when reading compressed qcow2 images:
$ qemu-img convert -p -O raw -n \ https://download.cirros-cloud.net/0.4.0/cirros-0.4.0-x86_64-disk.img \ null-co:// (Hangs at 74.21 %, usually.) A more direct way is: $ qemu-img bench -f raw http://download.qemu.org/qemu-4.1.0.tar.xz \ -d 1 -S 524288 -c 2 (Which simply performs two requests, and the second one hangs. You can use any HTTP resource (probably FTP, too) you’d like that is at least 1 MB in size.) It turns out that this is because cURL 7.59.0 has added a protective feature against some misuse we had in our code: curl_multi_add_handle() must not be called from within a cURL callback, but in some cases we did. As of 7.59.0, this fails, our new request is not registered and the I/O request stalls. This is fixed by patch 5. Patch 6 makes us check for curl_multi_add_handle()’s return code, because if we had done that before, debugging would have been much simpler. On the way to fixing it, I had a look over the whole cURL code and found a suspicious QLIST_FOREACH_SAFE() loop that actually does not seem very safe at all. I think this may lead to crashes, although I have never seen any myself. https://bugzilla.redhat.com/show_bug.cgi?id=1744602#c5 shows one in exactly the function in question, so I think it actually is a problem. This is fixed by patch 4, patches 1 through 3 prepare for it. Max Reitz (6): curl: Keep pointer to the CURLState in CURLSocket curl: Keep *socket until the end of curl_sock_cb() curl: Pass CURLSocket to curl_multi_{do,read}() curl: Report only ready sockets curl: Handle success in multi_check_completion curl: Check curl_multi_add_handle()'s return code block/curl.c | 129 +++++++++++++++++++++++++-------------------------- 1 file changed, 63 insertions(+), 66 deletions(-) -- 2.21.0
