crash importing large database dump into maas-test-db

[Jacopo Rota]

As per summary, here the steps to reproduce:

1) generate a large database with `SAMPLEDATA_MACHINES=250 make sampledata`
2) `sudo cp maasdb.dump /var/snap/maas-test-db/common`
3) `sudo snap run --shell maas-test-db.psql -c 'db-dump restore $SNAP_COMMON/maasdb.dump maassampledata'` -> crash

Here's the error
pg_restore: while PROCESSING TOC:
pg_restore: from TOC entry 4145; 2606 19907 CONSTRAINT maasserver_event maasserver_event_pkey ubuntu
pg_restore: error: could not execute query: ERROR: could not open shared memory segment "/PostgreSQL.1198490976": Permission denied
Command was: ALTER TABLE ONLY public.maasserver_event
    ADD CONSTRAINT maasserver_event_pkey PRIMARY KEY (id);
4:52 PM

The sample database dump (with 1000 machines) can be found in the attachments.

[Jacopo Rota (r00ta)]

The root cause is that `postgres` creates child processes and they do not have the `SNAP_INSTANCE_NAME` in the stack (see https://github.com/maas/snap-preload/blob/753db287d2980eaae0400076ab1d7a81c188b478/snap-preload.c#L79). We use LD_PRELOAD to change the `shm_path` at runtime for the snap https://github.com/maas/snap-preload/blob/main/snap-preload.c .

Debugging the issue, it is possible to see that the new path is constructed like SNAP_INSTANCE_NAME is not set
```
[pid 49217] openat(AT_FDCWD, "/dev/shm/snap..PostgreSQL.946200062", O_RDWR|O_CREAT|O_EXCL|O_NOFOLLOW|O_CLOEXEC, 0600) = -1 EACCES (Permission denied)
```

The LD_PRELOAD library is loaded properly by the child processes
```
(gdb) info sharedlibrary
From To Syms Read Shared Object Library
0x00007ff232e3e180 0x00007ff232e3e4da Yes (*) target:/snap/maas-test-db/131/usr/lib/snap-preload.so
```

A possible fix is to store the result of `secure_getenv("SNAP_INSTANCE_NAME");` in the heap instead of in the stack, because the heap is copied properly instead of the stack.

[Jacopo Rota (r00ta)]

See https://github.com/maas/snap-preload/pull/4