1 files changed, 67 insertions, 0 deletions
diff --git a/internal/validate/checks.go b/internal/validate/checks.go
index 9d09eef..5eb4288 100644
--- a/internal/validate/checks.go
+++ b/internal/validate/checks.go
@@ -736,6 +736,7 @@ func validateDropTables(s Source) []Issue {
 	var issues []Issue
 	dropIDs := dropTableIDSet(s.DataDir)
 	itemIDs := s.Items.IDSet()
+	issues = append(issues, validateDropTableCycles(s, dropIDs)...)
 
 	for id := range dropIDs {
 		dt, err := behavior.LoadDropTable(s.DataDir, id)
@@ -838,6 +839,72 @@ func validateDropTables(s Source) []Issue {
 	return issues
 }
 
+// validateDropTableCycles reports every table that participates in a
+// reference cycle. ResolveDrop recurses at runtime, so a self- or
+// mutual-reference would otherwise overflow the goroutine stack and kill the
+// process; a cycle is therefore a hard startup error.
+func validateDropTableCycles(s Source, dropIDs map[string]bool) []Issue {
+	edges := make(map[string][]string, len(dropIDs))
+	for id := range dropIDs {
+		dt, err := behavior.LoadDropTable(s.DataDir, id)
+		if err != nil {
+			continue // reported by the main pass
+		}
+		for _, d := range dt.Drops {
+			if d.Table != "" && dropIDs[d.Table] {
+				edges[id] = append(edges[id], d.Table)
+			}
+		}
+	}
+	const (
+		white = 0
+		gray  = 1
+		black = 2
+	)
+	color := make(map[string]int, len(dropIDs))
+	reported := make(map[string]bool)
+	var stack []string
+	var issues []Issue
+	var visit func(id string)
+	visit = func(id string) {
+		color[id] = gray
+		stack = append(stack, id)
+		for _, next := range edges[id] {
+			switch color[next] {
+			case gray:
+				start := 0
+				for i, m := range stack {
+					if m == next {
+						start = i
+						break
+					}
+				}
+				for _, m := range stack[start:] {
+					if reported[m] {
+						continue
+					}
+					reported[m] = true
+					issues = append(issues, Issue{
+						Level:   "ERROR",
+						Type:    "integrity",
+						Message: fmt.Sprintf("Drop table %q participates in a reference cycle; drop-table recursion would overflow the stack", m),
+					})
+				}
+			case white:
+				visit(next)
+			}
+		}
+		stack = stack[:len(stack)-1]
+		color[id] = black
+	}
+	for id := range dropIDs {
+		if color[id] == white {
+			visit(id)
+		}
+	}
+	return issues
+}
+
 func validateCourses(s Source) []Issue {
 	var issues []Issue
 	roomIndex := s.World.RoomIndex()