runmodes: introduce unknown engine mode

Querying an engine mode with an unknown value signals a bug when
the engine mode has not been determined but is already queried by
other functions.

Ticket: #5959

src/runmode-unittests.c

@@ -232,6 +232,7 @@ void RunUnittests(int list_unittests, const char *regex_arg)
 #ifdef UNITTESTS
     /* Initializations for global vars, queues, etc (memsets, mutex init..) */
     GlobalsInitPreConfig();
+    EngineModeSetIDS();
 
 #ifdef HAVE_LUAJIT
     if (LuajitSetupStatesPool() != 0) {

src/suricata.c

@@ -168,7 +168,7 @@ int run_mode = RUNMODE_UNKNOWN;
 
 /** Engine mode: inline (ENGINE_MODE_IPS) or just
   * detection mode (ENGINE_MODE_IDS by default) */
-static enum EngineMode g_engine_mode = ENGINE_MODE_IDS;
+static enum EngineMode g_engine_mode = ENGINE_MODE_UNKNOWN;
 
 /** Host mode: set if box is sniffing only
  * or is a router */
@@ -208,13 +208,24 @@ int SuriHasSigFile(void)
     return (suricata.sig_file != NULL);
 }
 
+int EngineModeIsUnknown(void)
+{
+    return (g_engine_mode == ENGINE_MODE_UNKNOWN);
+}
+
 int EngineModeIsIPS(void)
 {
+#ifdef DEBUG
+    BUG_ON(g_engine_mode == ENGINE_MODE_UNKNOWN);
+#endif
     return (g_engine_mode == ENGINE_MODE_IPS);
 }
 
 int EngineModeIsIDS(void)
 {
+#ifdef DEBUG
+    BUG_ON(g_engine_mode == ENGINE_MODE_UNKNOWN);
+#endif
     return (g_engine_mode == ENGINE_MODE_IDS);
 }
 
@@ -2666,13 +2677,18 @@ int PostConfLoadedSetup(SCInstance *suri)
 
     MacSetRegisterFlowStorage();
 
-    SetMasterExceptionPolicy();
-
     LiveDeviceFinalize(); // must be after EBPF extension registration
 
     RunModeEngineIsIPS(
             suricata.run_mode, suricata.runmode_custom_mode, suricata.capture_plugin_name);
 
+    if (EngineModeIsUnknown()) { // if still uninitialized the set the default
+        SCLogInfo("Setting engine mode to IDS mode by default");
+        EngineModeSetIDS();
+    }
+
+    SetMasterExceptionPolicy();
+
     AppLayerSetup();
 
     /* Suricata will use this umask if provided. By default it will use the

src/suricata.h

@@ -99,12 +99,14 @@ enum {
 
 /* Engine is acting as */
 enum EngineMode {
+    ENGINE_MODE_UNKNOWN,
     ENGINE_MODE_IDS,
     ENGINE_MODE_IPS,
 };
 
 void EngineModeSetIPS(void);
 void EngineModeSetIDS(void);
+int EngineModeIsUnknown(void);
 int EngineModeIsIPS(void);
 int EngineModeIsIDS(void);