grammar: add repetition threshold check

The change introduces a maximum repetition threshold to avoid
excessive rule expansion during grammar parsing. When parsing
repetition patterns like {m,n}, the parser now calculates the
potential number of rules that would be generated and throws an error
if the product of previous rules and new rules exceeds the threshold.

A test case was added to verify the threshold is properly enforced for
deeply nested repetition patterns that would otherwise cause hangs.

src/llama-grammar.cpp

@@ -455,6 +455,7 @@ const char * llama_grammar_parser::parse_sequence(
         bool               is_nested) {
     size_t last_sym_start = rule.size();
     const char * pos = src;
+    uint64_t n_prev_rules = 1;
 
     // use UINT64_MAX as the empty value because we aligned to the proper uint64_t type so -1 can't be used
     // (though it's technically the same as -1 now)
@@ -482,6 +483,18 @@ const char * llama_grammar_parser::parse_sequence(
         //            S'     ::= S |
 
         llama_grammar_rule prev_rule(rule.begin() + last_sym_start, rule.end());
+        // Calculate the total number of rules that will be generated by this repetition
+        uint64_t total_rules = 1; // Start with 1 for the original rule
+        if (!no_max && max_times > 0) {
+            total_rules = max_times;
+        } else if (min_times > 0) {
+            total_rules = min_times;
+        }
+
+        if (n_prev_rules * total_rules >= MAX_REPETITION_THRESHOLD) {
+            throw std::runtime_error("number of rules that are going to be repeated multiplied by the new repetition exceeds sane defaults, please reduce the number of repetitions or rule complexity");
+        }
+
         if (min_times == 0) {
             rule.resize(last_sym_start);
         } else {
@@ -509,12 +522,15 @@ const char * llama_grammar_parser::parse_sequence(
         if (n_opt > 0) {
             rule.push_back({LLAMA_GRETYPE_RULE_REF, last_rec_rule_id});
         }
+        n_prev_rules *= total_rules;
+        GGML_ASSERT(n_prev_rules >= 1);
     };
 
     while (*pos) {
         if (*pos == '"') { // literal string
             pos++;
             last_sym_start = rule.size();
+            n_prev_rules = 1;
             while (*pos != '"') {
                 if (!*pos) {
                     throw std::runtime_error("unexpected end of input");
@@ -532,6 +548,7 @@ const char * llama_grammar_parser::parse_sequence(
                 start_type = LLAMA_GRETYPE_CHAR_NOT;
             }
             last_sym_start = rule.size();
+            n_prev_rules = 1;
             while (*pos != ']') {
                 if (!*pos) {
                     throw std::runtime_error("unexpected end of input");
@@ -562,6 +579,7 @@ const char * llama_grammar_parser::parse_sequence(
             auto token_pair = parse_token(vocab, pos);
             const char * token_end  = token_pair.second;
             last_sym_start = rule.size();
+            n_prev_rules = 1;
             rule.push_back({type, token_pair.first});
             pos = parse_space(token_end, is_nested);
         } else if (is_word_char(*pos)) { // rule reference
@@ -569,12 +587,15 @@ const char * llama_grammar_parser::parse_sequence(
             uint32_t ref_rule_id = get_symbol_id(pos, name_end - pos);
             pos = parse_space(name_end, is_nested);
             last_sym_start = rule.size();
+            n_prev_rules = 1;
             rule.push_back({LLAMA_GRETYPE_RULE_REF, ref_rule_id});
         } else if (*pos == '(') { // grouping
             // parse nested alternates into synthesized rule
             pos = parse_space(pos + 1, true);
+            uint32_t n_rules_before = symbol_ids.size();
             uint32_t sub_rule_id = generate_symbol_id(rule_name);
             pos = parse_alternates(pos, rule_name, sub_rule_id, true);
+            n_prev_rules = std::max(1u, (uint32_t)symbol_ids.size() - n_rules_before);
             last_sym_start = rule.size();
             // output reference to synthesized rule
             rule.push_back({LLAMA_GRETYPE_RULE_REF, sub_rule_id});
@@ -584,6 +605,7 @@ const char * llama_grammar_parser::parse_sequence(
             pos = parse_space(pos + 1, is_nested);
         } else if (*pos == '.') { // any char
             last_sym_start = rule.size();
+            n_prev_rules = 1;
             rule.push_back({LLAMA_GRETYPE_CHAR_ANY, 0});
             pos = parse_space(pos + 1, is_nested);
         } else if (*pos == '*') {

tests/test-grammar-integration.cpp

@@ -802,19 +802,6 @@ static void test_quantifiers() {
             "yy"
         }
     );
-    test_grammar(
-        "hang",
-        // Grammar
-        R"""(
-            root ::= (((((([^x]*){0,99}){0,99}){0,99}){0,99}){0,99}){0,99}
-        )""",
-        // Passing strings
-        {
-        },
-        // Failing strings
-        {
-        }
-    );
 }
 
 static void test_failure_missing_root() {

tests/test-grammar-parser.cpp

@@ -145,6 +145,10 @@ int main()
         root ::= "a"{,}"
     )""");
 
+    verify_failure(R"""(
+        root ::= (((((([^x]*){0,99}){0,99}){0,99}){0,99}){0,99}){0,99}
+    )""");
+
     verify_failure(R"""(
         root ::= "a"{,10}"
     )""");