Re: [Libguestfs] [libnbd PATCH v3 07/29] lib/utils: add async-signal-safe assert()

On 2/21/23 07:33, Laszlo Ersek wrote: > On 2/20/23 19:21, Laszlo Ersek wrote: >> On 2/15/23 21:57, Eric Blake wrote: >>> On Wed, Feb 15, 2023 at 03:11:36PM +0100, Laszlo Ersek wrote: > >>>> + >>>> + xwrite (STDERR_FILENO, file, strlen (file)); >>>> + xwrite (STDERR_FILENO, ":", 1); >>> >>> Presumably, if our first best-effort xwrite() fails to produce full >>> output, all later xwrite() will hopefully hit the same error condition >>> so that we aren't producing even more-mangled output where later >>> syscalls succeed despite missing context earlier in the overall >>> output. If it were something we truly wanted to worry about, the >>> solution would be pre-loading the entire message into a single buffer, >>> then calling xwrite() just once - but that's far more effort for >>> something we don't anticipate hitting in normal usage anyways. I'm >>> happy if you ignore this whole paragraph of mine. >> >> Any single buffer presents the problem of sizing the buffer >> appropriately, which we can't do in this context :) > > Actually, we *can* do better: > > https://pubs.opengroup.org/onlinepubs/9699919799/functions/writev.html > > What we're doing here is the textbook use case for scatter-gather (well, > in this case, gather). It's strange that it has taken me one night to > realize this (it occurred to me before falling asleep last night), given > that I heavily used scatter-gather in other, not-so-old, code. (Namely > the edk2 virtiofs driver.) > > I'll attempt to replace xwrite() with xwritev(). No, I won't -- writev() is not required to be async-signal-safe, so it's not good enough for NBD_INTERNAL_FORK_SAFE_ASSERT().