bug#42548: Cuirass 504 errors

[Mathieu Othacehe <othacehe@gnu.org>]

Hey,

> A second issue is caused when a build product download is started, then
> aborted. In that case, sendfile throws an exception or enters an endless
> loop.

Ok, so I found a couple of errors here. First, I noticed that it was not
possible to download simultaneously two build products, because the
first download was blocking the whole process.

This is solved by: 6ad9c602697ffe33c8fbb09ccd796b74bf600223. In short,
current-fiber was set to #f, both in the context of the caller and the
spawned thread. So I think the get-message operating was blocking the
whole thread instead of suspending the current fiber. But if someone
else could take a look it would be nice :).

Second issue, sendfile may throw EPIPE or ECONNRESET if the client
disconnects before the end of the transfer. I think, besides the dirty
backtrace, it was not harmful. But anyway, its better to catch this as
we are doing in "guix publish", see:
0955a11abd9e27c96a1375cca6a1c97869b5780a.

I fear it won't be enough to fix the 504 errors, but at least it's a
start.

Thanks,

Mathieu