GNU bug report logs - #36402
Installer: null pointer exception during partitioning

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From: Mathieu Othacehe <m.othacehe <at> gmail.com>
To: Ludovic Courtès <ludo <at> gnu.org>
Cc: 36402 <at> debbugs.gnu.org, Juan <r5jm <at> protonmail.com>
Subject: bug#36402: installation error
Date: Wed, 04 Sep 2019 14:31:35 +0200

Hey Ludo,

> Please share whatever you gather before you get depressed.  ;-)

Thanks a lot for your help, it was really useful! I used valgrind a lot
to understand what was happening.

Ok so here's a little summary:

* Using ped_device_get Parted returns new devices. It may return already
existing devices if the given path is already known. Users are
responsible for their destruction calling ped_device_destroy.

* Using ped_disk_new Parted returns new disks from devices. Users are
responsible for their destruction calling ped_disk_destroy.

* A disk contains partitions. A user can remove partitions without
deleting them. It is also possible to delete them calling
ped_partition_destroy. On disk destruction, all the partitions
associated are destroyed.

Here's how memory is managed in Guile-Parted:

* ped_device_destroy is set a finalizer for device pointers.

* ped_disk_destroy is set a finalizer for disk pointers. Device object
associated to disks are recorded in a weak key hash table, to ensure
that the lifetime of a disk is shorted than that of its device.

* No finalizer is set for partition pointers. However, disk associated
to partitions are recorded in a weak key hash table, to ensure that the
lifetime of a partition is shorter that that of its disk. The user can
access ped_disk_remove_partition function from Parted but cannot acces
ped_partition_destroy function. Partition destruction is done by Parted
of disk destruction.

And here is what was going wrong:

ped_device_get and ped_device_get_next can return pointers to already
existing device object. So set-pointer-finalizer! was possibly called
multiple times on the same device pointer, resulting in calling
ped_device_destroy multiple times on the same device pointer.

To prevent that, I created a weak value hash table to make sure that one
<device> object maps to exactly one device pointer, and that the pointer
finalizer is set only once. See commit b35839b.

I also added (gc) calls at various locations in the tests in commit
728fd01.

WDYT?

Thanks,

Mathieu

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