Fixes #2803, #2553, #2597 & #2498 (#2940)

Fixes #2803
Fixes #2553
Fixes #2597 (Couldn't find a cleaner way to make this work, unfortunately. Not all procs-as-verbs can be used as procs.)
Fixes #2498
Fixes #2912

code/modules/multiz/structures.dm

@@ -43,7 +43,9 @@
 
 /obj/structure/ladder/attackby(obj/item/C as obj, mob/user as mob)
 	attack_hand(user)
-	return
+
+/obj/structure/ladder/attack_robot(mob/user)
+	attack_hand(user)
 
 /obj/structure/ladder/attack_hand(var/mob/M)
 	if(!M.may_climb_ladders(src))
@@ -102,6 +104,19 @@
 		return FALSE
 	return TRUE
 
+/mob/living/silicon/may_climb_ladders(ladder)
+	to_chat(src, "<span class='warning'>You're too heavy to climb [ladder]!</span>")
+	return FALSE
+
+/mob/living/silicon/robot/drone/may_climb_ladders(ladder)
+	if(!Adjacent(ladder))
+		to_chat(src, "<span class='warning'>You need to be next to \the [ladder] to start climbing.</span>")
+		return FALSE
+	if(incapacitated())
+		to_chat(src, "<span class='warning'>You are physically unable to climb \the [ladder].</span>")
+		return FALSE
+	return TRUE
+
 /mob/observer/ghost/may_climb_ladders(var/ladder)
 	return TRUE
 

code/modules/multiz/turf.dm

@@ -54,10 +54,32 @@
 		if (mover.Check_Shoegrip(FALSE) && mover.can_fall(below, src))
 			to_chat(mover, span("notice",
 				"You are stopped from falling off the edge by \the [mover.shoes] you're wearing!"))
-			return 0
+			return FALSE
 
 	return ..()
 
+// Add a falling atom by default. Even if it's not an atom that can actually fall.
+// SSfalling will check this on its own and remove if necessary. This is saner, as it
+// centralizes control to SSfalling.
+/turf/simulated/open/Entered(atom/movable/mover)
+	..()
+	ADD_FALLING_ATOM(mover)
+	update_icon()
+
+// Override to deny a climber exit if they're set to adhere to CLIMBER_NO_EXIT
+/turf/simulated/open/Exit(atom/movable/mover, atom/newloc)
+	var/flags = remove_climber(mover)
+
+	if (flags & CLIMBER_NO_EXIT)
+		ADD_FALLING_ATOM(mover)
+		return FALSE
+
+	return ..()
+
+// Remove from climbers just in case.
+/turf/simulated/open/Exited(atom/movable/mover, atom/newloc)
+	LAZYREMOVE(climbers, mover)
+
 /turf/simulated/open/Destroy()
 	SSopenturf.openspace_turfs -= src
 	SSopenturf.queued_turfs -= src
@@ -95,6 +117,42 @@
 
 	return FALSE
 
+/**
+ * Used to add a climber to the climbers list. Climbers do not fall down this specific tile.
+ *
+ * @param climber The atom to be added as a climber.
+ * @param flags Bitflags to control the status of the climber. Should always be non-0!
+ *
+ * @return TRUE if a climber was successfully added. FALSE if the climber is already
+ * present or an error occured.
+ */
+/turf/simulated/open/proc/add_climber(atom/climber, flags = CLIMBER_DEFAULT)
+	if (!flags)
+		PROCLOG_WEIRD("Attempted to add climber [climber] without flags.")
+		return FALSE
+
+	if (LAZYACCESS(climbers, climber))
+		return FALSE
+
+	LAZYINITLIST(climbers)
+	climbers[climber] = flags
+	return TRUE
+
+/**
+ * Used to remove a climber from the climbers list. Returns the flags the climber
+ * was assigned.
+ *
+ * @param climber The atom to be removed from climbers.
+ *
+ * @return The flags assigned to the climber if it was present in the list. 0 otherwise.
+ */
+/turf/simulated/open/proc/remove_climber(atom/climber)
+	. = 0
+
+	if (LAZYACCESS(climbers, climber))
+		. = climbers[climber]
+		LAZYREMOVE(climbers, climber)
+
 /turf/simulated/open/airless
 	oxygen = 0
 	nitrogen = 0
@@ -127,11 +185,6 @@
 /turf/simulated/open/update_dirt()
 	return 0
 
-/turf/simulated/open/Entered(atom/movable/mover)
-	..()
-	ADD_FALLING_ATOM(mover)
-	update_icon()
-
 // override to make sure nothing is hidden
 /turf/simulated/open/levelupdate()
 	for(var/obj/O in src)