Bug#868900: iso-scan: should be able to copy the ISO into RAM before mount

[Colin Watson <cjwatson@debian.org>]

On Wed, Jul 19, 2017 at 05:19:21PM +0200, Raphael Hertzog wrote:
> When you use an ISO image stored on a disk, it makes it impossible to
> re-use that disk in the installation process (e.g. for automatic
> partitioning).
> 
> In order to avoid this limitation, I would like iso-scan to be able
> to copy the ISO image into RAM and to mount it from there. The attached
> patch implements precisely when the debconf question "iso-scan/copy_iso_to_ram"
> returns true.
> 
> It would be nice to see this merged (the patch applies cleanly on top
> of the one in #868859, but you can also merge it separately if you drop
> the changelog entry).
> 
> Let me know if there's anything to improve before it's ready to be merged
> (notably on the debconf template, etc.).

This generally seems like a nice feature to have.  A couple of general
UX comments:

 * Would it be worth doing an explicit check of how much free memory is
   available and either warning the user or just refusing outright to do
   the copy-to-RAM operation if they don't have enough (size of image
   plus fudge factor)?  As it is, the user really doesn't have much
   information to go on when answering this question; they'd have to
   switch to a shell just to find out how much free memory they have.
   Normally this sort of test would be very unreliable, but in the
   context of d-i I think it could be made somewhat reasonable.

 * We should warn the user that, if they opt to copy the image into RAM,
   then they mustn't reboot between partitioning and the end of the
   installation as their installation media may be made unusable.
   (Wordsmithing this into something comprehensible is left as an
   exercise for the reader.)

diff --git a/debian/iso-scan.postinst b/debian/iso-scan.postinst
index e34f329..1109ba5 100755
--- a/debian/iso-scan.postinst
+++ b/debian/iso-scan.postinst
@@ -203,9 +203,26 @@ use_this_iso () {
        local iso_device=$2
        local RET

+       db_input low iso-scan/copy_iso_to_ram || true
+       db_go
+       db_get iso-scan/copy_iso_to_ram
+
        mount -t auto -o ro $iso_device /hd-media 2>/dev/null
        cd /hd-media
-       mount -t iso9660 -o loop,ro,exec $iso_to_try /cdrom 2>/dev/null
+
+       if [ "$RET" = false ]; then
+               # Direct mount
+               log "Mounting /hd-media/$iso_to_try on /cdrom"
+               mount -t iso9660 -o loop,ro,exec $iso_to_try /cdrom 2>/dev/null
+       else
+               # We copy the ISO to RAM before mounting it
+               log "Copying /hd-media/$iso_to_try to /installer.iso"
+               cp $iso_to_try /installer.iso
+               log "Mounting /installer.iso on /cdrom"
+               mount -t iso9660 -o loop,ro,exec /installer.iso /cdrom
+2>/dev/null
+               cd /
+               umount /hd-media
+       fi

        analyze_cd

The directory-changing here is messy, and left me wondering whether
analyze_cd might depend on the current directory, which is now different
depending on which option you select.  I don't think it does, but I had
to read that function to make sure.  I'd suggest simplifying this
section of code by mounting or copying by absolute path, i.e.
"/hd-media/$iso_to_try", rather than relying on it being in the current
directory; you then won't need either "cd /hd-media" or "cd /" in this
function.

-- 
Colin Watson                                       [cjwatson@debian.org]