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[bug#34863] [WIP] syscalls: Add loop device interface.
From: |
Danny Milosavljevic |
Subject: |
[bug#34863] [WIP] syscalls: Add loop device interface. |
Date: |
Sat, 16 Mar 2019 12:17:09 +0100 |
Hi Ludo :)
On Sat, 16 Mar 2019 11:29:17 +0100
Ludovic Courtès <address@hidden> wrote:
> What will be the use for this? I prefer to make sure we only add code
> that is actually going to be used. :-)
See "boot multiple Gnu/Linux Distributions from one USB key" on the guix-devel
list. This would make it possible to loop-mount stuff at boot.
> Like I wrote, a record may be more appropriate than an alist here.
> Also, no need to repeat ‘lo-’ in the parameter names.
Sure.
> > +(define (allocate-new-loop-device control-file)
> > + "Allocates a new loop device and returns an FD for it.
> > +CONTROL-FILE should be an open file \"/dev/loop-control\".
>
> Nitpick: s/an FD/a file descriptor/
> s/an open file/an open port for/
>
> > + (open-io-file (string-append "/dev/loop" (number->string ret))))
>
> I didn’t know about ‘open-io-file’ and indeed, it’s undocumented. So
> I’d suggest using ‘open-file’ instead to be on the safe side.
Do you mean
open-file ... "r+"
?
>Note that BACKING-FILE, the port, can be closed when it’s GC’d, which as
>a side effect would close its associated file descriptor. Is this OK or
>does the FD have to remain open for the lifetime of the loopback device?
I don't know, but guess it's okay for it to be closed again (the
"losetup" process doesn't keep running for long either and the loop device
is fine).
> > +(let ((loop-device (allocate-new-loop-device (open-io-file
> > "/dev/loop-control"))))
> > + (set-loop-device-backing-file loop-device (open-input-file
> > "tests/syscalls.scm"))
> > + (set-loop-device-status loop-device (get-loop-device-status
> > loop-device)))
>
> You’re missing a ‘test-assert’ or similar.
What would I be asserting? I found no function to test whether an
exception was raised or not (or to just assert that no exception was
raised). So I resorted to that.
>Also, isn’t ‘loop-device’ a
> number? Then the ‘set-loop-device-*’ calls fail with wrong-type-arg,
> no?
It's actually a port now and the comment in allocate-new-loop-device is
outdated.
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