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[bug#34863] [WIP] syscalls: Add loop device interface.
From: |
Ludovic Courtès |
Subject: |
[bug#34863] [WIP] syscalls: Add loop device interface. |
Date: |
Mon, 18 Mar 2019 09:42:36 +0100 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux) |
Hello,
Danny Milosavljevic <address@hidden> skribis:
> 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.
Oh OK (too much mail!).
>> > + (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+"
>
> ?
Exactly.
>>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).
It’d be good to double-check. :-)
>> > +(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.
Tests always need to be enclosed in a ‘test-XYZ’ form. Otherwise it’s
code that’s evaluated as the top level and that’s not listed in the test
log.
So in this case, to check for a 'system-error exception, you could do, say:
(test-equal "foo"
ENOENT
(catch 'system-error
(lambda () … #f)
(lambda args
(system-error-errno args))))
There are examples of that in ‘tests/syscalls.scm’.
HTH!
Ludo’.