I just got bit by code that looks somewhat like this:

SyncHostResponse handler;
file->Open(m_filename, m_flags, m_perms, &handler);
handler.WaitForResponse();

(here, file is a XrdCl::File*; actual code is slightly more complex, but the above suffices). For a long time, this code worked fine - the Open was the first call to XrdCl, so it never failed. However, an unrelated commit added a prior call to XrdCl::FileSystem::Query. If that ended in a fatal error, the call to file->Open failed immediately -- and the WaitForResponse() hangs indefinitely as there is no callback.

We should mark the relevant XrdCl::File and XrdCl::FileSystem asynchronous calls with the attribute warn_unused_result where available; this would have resulted in a compiler warning, which would have allowed me to avoid a deadlock.

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