Hi,

A file size dependent bug + a double-free bug.
Pay attention to the file sizes (these are thresholds).
Below the threshold everything seems fine.

I found, empirically(!), that `1360 * #Packet` is a good starting point to find
the threshold :D

=== Observation 1

$ dd if=/dev/zero of=/tmp/x bs=1 count=$((1360*12)) # count=16320
$ ./run poke
(poke) .file /tmp/x
(poke) deftype Packet = struct { int i; long j; };
(poke) defvar p = Packet[] @ 0#B;
Segmentation fault (core dumped)


=== Observation 2

$ dd if=/dev/zero of=/tmp/x bs=1 count=$((1360*16)) # count=21760
$ ./run poke
(poke) .file /tmp/x
(poke) deftype Packet = struct { long i; long j; };
(poke) defvar p = Packet[] @ 0#B;
Segmentation fault (core dumped)


=== Observation 3

$ dd if=/dev/zero of=/tmp/x bs=1 count=$(((1360 - 1)*24)) # count=32616
$ ./run poke
(poke) .file /tmp/x
(poke) deftype Packet = struct { long i; long j; long k; };
(poke) defvar p = Packet[] @ 0#B;
Segmentation fault (core dumped)


=== Observation 4

$ dd if=/dev/zero of=/tmp/x bs=1 count=$(((1360 + 2)*8)) # count=10896
$ ./run poke
(poke) .file /tmp/x
(poke) deftype Packet = struct { long i; };
(poke) defvar p = Packet[] @ 0#B;
Segmentation fault (core dumped)


=== Observation 5

New bug: double-free

$ dd if=/dev/zero of=/tmp/x bs=1 count=$(((1360 - 1)*24 - 1)) # count=32615
$ ./run poke
(poke) .file /tmp/x
(poke) deftype Packet = struct { long i; long j; long k; };
(poke) defvar p = Packet[] @ 0#B;
free(): invalid pointer
Aborted (core dumped)

Here's the backtrace:

#0  0x00007fcb3d047355 in raise () from /usr/lib/libc.so.6
#1  0x00007fcb3d030853 in abort () from /usr/lib/libc.so.6
#2  0x00007fcb3d08a878 in __libc_message () from /usr/lib/libc.so.6
#3  0x00007fcb3d091d3a in malloc_printerr () from /usr/lib/libc.so.6
#4  0x00007fcb3d093f92 in free_check () from /usr/lib/libc.so.6
#5  0x00007fcb3d433a4d in jitter_stack_finalize_backing (backing=0x558d18da6ff0) at ../../jitter/jitter/jitter-stack.c:73
#6  0x00007fcb3d4238e8 in pvm_state_finalize (jitter_state=0x558d18da6fd0) at ../../libpoke/pvm.jitter:689
#7  0x00007fcb3d417b44 in pvm_shutdown (apvm=0x558d18da6fd0) at ../../libpoke/pvm.c:150
#8  0x00007fcb3d3d38da in pk_compiler_free (pkc=0x558d18dab9f0) at ../../libpoke/libpoke.c:75
#9  0x0000558d183d369a in finalize () at ../../poke/poke.c:289
#10 0x0000558d183d313d in main (argc=<optimized out>, argv=<optimized out>) at ../../poke/poke.c:686



Thanks