fix(diagram): set an empty PlantUML operand id when there is none

dg_puml_operand set *label up front but *id only on its success paths, so an
opener without its closer (`Alice ->: x`, `[-> Alice: x`, `class [Unclosed`)
returned with *id unwritten. dg_puml_relation and dg_puml_decl then read an
uninitialized dg_str_t and passed it to dg_element, which crashed the index
worker on some stacks. The operand now starts with an empty id, so those lines
take the existing `!lid.n` / `!rid.n` exits and declarations make no element.

Fixes #2818

Signed-off-by: Rodion Kazennov <nefayran@gmail.com>

internal/cbm/diagram.c

@@ -1225,9 +1225,11 @@ static bool dg_word_at(dg_str_t s, const char *word) {
 }
 
 /* An operand of a PlantUML line: `[Comp]`, `(use case)`, `:actor:`, `"text"`,
- * an id, or `[*]`; its id in *id; the position after it (i when none). */
+ * an id, or `[*]`; its id in *id; the position after it (i and an empty *id
+ * when none, as for the diagram edge in `A ->: x` or `[-> A: x`). */
 static size_t dg_puml_operand(dg_str_t s, size_t i, dg_str_t *id, dg_str_t *label) {
     i = dg_skip_ws(s, i);
+    *id = dg_s(NULL, 0);
     *label = dg_s(NULL, 0);
     if (i + CBM_SZ_3 <= s.n && memcmp(s.p + i, "[*]", CBM_SZ_3) == 0) {
         *id = dg_s(s.p + i, CBM_SZ_3);

tests/test_diagrams.c

@@ -329,6 +329,36 @@ TEST(diagrams_plantuml) {
     PASS();
 }
 
+/* #2818: a sequence message to or from the diagram edge (`A ->: x`, `A <-: x`,
+ * `[-> A: x`) and a declaration with an unclosed bracket have no operand id;
+ * they used to read an uninitialized one and crash the index worker. */
+TEST(diagrams_plantuml_edge_messages) {
+    CBMFileResult *r = dg_file("@startuml\n"
+                               "participant Alice\n"
+                               "Alice ->: out\n"
+                               "Alice <-: in\n"
+                               "[-> Alice: from the edge\n"
+                               "Alice -> Bob : hello\n"
+                               "class [Unclosed\n"
+                               "@enduml\n",
+                               "docs/edge.puml");
+    ASSERT_NOT_NULL(r);
+    ASSERT_NOT_NULL(dg_def(r, "edge#diagram-1.Alice"));
+    ASSERT_NOT_NULL(dg_def(r, "edge#diagram-1.Bob"));
+    ASSERT_EQ(r->diagram_arrows.count, 1);
+    ASSERT_EQ(dg_arrows(r, "#diagram-1.Alice", "#diagram-1.Bob", "message", "hello"), 1);
+    int elements = 0;
+    for (int i = 0; i < r->defs.count; i++) {
+        const char *label = r->defs.items[i].label;
+        if (label && strcmp(label, "DiagramElement") == 0) {
+            elements++;
+        }
+    }
+    ASSERT_EQ(elements, 2);
+    cbm_free_result(r);
+    PASS();
+}
+
 TEST(diagrams_dot) {
     CBMFileResult *r =
         dg_file("digraph G {\n"
@@ -1101,6 +1131,7 @@ SUITE(diagrams) {
     RUN_TEST(diagrams_mermaid_kinds);
     RUN_TEST(diagrams_other_fences);
     RUN_TEST(diagrams_plantuml);
+    RUN_TEST(diagrams_plantuml_edge_messages);
     RUN_TEST(diagrams_dot);
     RUN_TEST(diagrams_d2);
     RUN_TEST(diagrams_drawio);