Core: don't return invalid cylinders from explicit_first_cylinder()

For reasons which I don't yet understand, when plotting a dive
whose first cylinder is not cylinder 0 and then plotting a dive
with only one cylinder, it can happen that for the latter
explicit_first_cylinder() returns an erroneous value.

This is due to the way in which we copy the dive to be plotted
to displayed_dive.

For now, make sure that no invalid cylinder is returned to avoid
crashes. This will have to be changed anyway, since this is very
fundamentally not thread-safe and inefficient.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

1 files changed, 6 insertions, 4 deletions

diff --git a/core/dive.c b/core/dive.c
index ca4b0b705..71b1038ec 100644
--- a/core/dive.c
+++ b/core/dive.c
@@ -865,17 +865,19 @@ static int same_rounded_pressure(pressure_t a, pressure_t b)
 /* Some dive computers (Cobalt) don't start the dive with cylinder 0 but explicitly
  * tell us what the first gas is with a gas change event in the first sample.
  * Sneakily we'll use a return value of 0 (or FALSE) when there is no explicit
- * first cylinder - in which case cylinder 0 is indeed the first cylinder */
+ * first cylinder - in which case cylinder 0 is indeed the first cylinder.
+ * We likewise return 0 if the event concerns a cylinder that doesn't exist. */
 int explicit_first_cylinder(const struct dive *dive, const struct divecomputer *dc)
 {
+	int res = 0;
 	if (dc) {
 		const struct event *ev = get_next_event(dc->events, "gaschange");
 		if (ev && ((dc->sample && ev->time.seconds == dc->sample[0].time.seconds) || ev->time.seconds <= 1))
-			return get_cylinder_index(dive, ev);
+			res = get_cylinder_index(dive, ev);
 		else if (dc->divemode == CCR)
-			return MAX(get_cylinder_idx_by_use(dive, DILUENT), 0);
+			res = MAX(get_cylinder_idx_by_use(dive, DILUENT), 0);
 	}
-	return 0;
+	return res < dive->cylinders.nr ? res : 0;
 }
 
 /* this gets called when the dive mode has changed (so OC vs. CC)