fuzz: test for too many open txs in a flow

so as to avoid performance problems coming from this.

src/tests/fuzz/fuzz_applayerparserparse.c

@@ -59,6 +59,9 @@ int LLVMFuzzerInitialize(int *argc, char ***argv)
     return 0;
 }
 
+// arbitrary value
+#define ALPROTO_MAXTX 4096
+
 int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
 {
     Flow * f;
@@ -166,6 +169,31 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
             }
 
             AppLayerParserTransactionsCleanup(f);
+
+            if (f->alstate && f->alparser) {
+                // check if we have too many open transactions
+                const uint64_t total_txs = AppLayerParserGetTxCnt(f, f->alstate);
+                uint64_t min = 0;
+                AppLayerGetTxIterState state;
+                memset(&state, 0, sizeof(state));
+                uint64_t nbtx = 0;
+                AppLayerGetTxIteratorFunc IterFunc = AppLayerGetTxIterator(f->proto, f->alproto);
+                while (1) {
+                    AppLayerGetTxIterTuple ires =
+                            IterFunc(f->proto, f->alproto, f->alstate, min, total_txs, &state);
+                    if (ires.tx_ptr == NULL)
+                        break;
+                    min = ires.tx_id + 1;
+                    nbtx++;
+                    if (nbtx > ALPROTO_MAXTX) {
+                        printf("Too many open transactions for protocol %s\n",
+                                AppProtoToString(f->alproto));
+                        printf("Assertion failure: %s\n", AppProtoToString(f->alproto));
+                        fflush(stdout);
+                        abort();
+                    }
+                }
+            }
         }
         alsize -= alnext - albuffer + 4;
         albuffer = alnext + 4;