Debug check fails when checking whether to write snapshot after final simulation step when output list is used

Porting issue here from slack, with instructions for reproducible example.

Configure with:

./configure --with-hydro-dimension=1 --enable-debug --enable-debugging-checks

Reproducible example:

swiftsim/examples/RadiativeTransferTests/CosmoAdvection_1D

(yes, it's an RT example, but the debug check fail also occurs without RT.)

run with

../../../swift --hydro --cosmo rt_advection1D_medium_redshift.yml

Run crashes after final step:

[00001.8] engine_print_stats: Saving statistics at a=1.405296e-01
[00001.8] engine_dump_snapshot: Dumping snapshot at a=1.407294e-01
      61   9.276920e-01    0.1398616    6.1499254   9.757966e-03   50   50         1000            0            0            0            0                 8.487     24             0.101
[00001.8] cosmology.c:cosmology_get_delta_time():1294: ti_end must be >= ti_start

gdb backtrace:

[00001.8] cosmology.c:cosmology_get_delta_time():1294: ti_end must be >= ti_start

Thread 1 "swift" received signal SIGABRT, Aborted.
0x00007ffff4c969fc in pthread_kill () from /lib/x86_64-linux-gnu/libc.so.6
(gdb) bt
#0  0x00007ffff4c969fc in pthread_kill () from /lib/x86_64-linux-gnu/libc.so.6
#1  0x00007ffff4c42476 in raise () from /lib/x86_64-linux-gnu/libc.so.6
#2  0x00007ffff4c287f3 in abort () from /lib/x86_64-linux-gnu/libc.so.6
#3  0x0000000000483bd7 in cosmology_get_delta_time (c=<optimised out>, ti_start=<optimised out>, ti_end=<optimised out>) at cosmology.c:1294
#4  0x000000000044c7d4 in engine_io_check_snapshot_triggers (e=e@entry=0x7ffffff9a638) at engine_io.c:1270
#5  0x0000000000435f0a in engine_step (e=e@entry=0x7ffffff9a638) at engine.c:2465
#6  0x0000000000408283 in main (argc=<optimised out>, argv=<optimised out>) at swift.c:1720
(gdb) frame 3
#3  0x0000000000483bd7 in cosmology_get_delta_time (c=<optimised out>, ti_start=<optimised out>, ti_end=<optimised out>) at cosmology.c:1294
1294	  if (ti_end < ti_start) error("ti_end must be >= ti_start");
(gdb) p ti_end
$1 = <optimised out>
(gdb) p ti_start
$2 = <optimised out>
(gdb) frame 4
#4  0x000000000044c7d4 in engine_io_check_snapshot_triggers (e=e@entry=0x7ffffff9a638) at engine_io.c:1270
1270	    time_to_next_snap = cosmology_get_delta_time(e->cosmology, e->ti_current,
(gdb) l
1265	  const int with_cosmology = (e->policy & engine_policy_cosmology);
1266	
1267	  /* Time until the next snapshot */
1268	  double time_to_next_snap;
1269	  if (e->policy & engine_policy_cosmology) {
1270	    time_to_next_snap = cosmology_get_delta_time(e->cosmology, e->ti_current,
1271	                                                 e->ti_next_snapshot);
1272	  } else {
1273	    time_to_next_snap = (e->ti_next_snapshot - e->ti_current) * e->time_base;
1274	  }
(gdb) p e->ti_current
$3 = 139611588448485376
(gdb) p e->ti_next_snapshot 
$4 = -1

Stan's analysis:

What I believe is happening is that when using a snapshot file, and you have reached the last snapshot, the ti_next_snapshot is explicitly set to -1. However, when running with debugging checks, if ti_next_snapshot is negative, it throws an error. I don't remember the exact files/lines where it happens, but the -1 is deliberately set in one of the files that checks for snapshot times in a snapshot list.