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