simplified Decode As entry if the next protocol requires manual selection

There's a number of protocols whose payload contains yet another
protocol but no criterion to figure out what this next protocol is.

Define a new global function register_decode_as_next_proto() to register
a Decode As entry for this scenario so the user can manually select the
next protocol.

A lot of the housekeeping that is normally required for Decode As is not
applicable to such a scenario. Provide simple data structures and
functions to cover this, make them internal to epan/decode_as.c and
allow them to be shared by multiple of the new simplified Decode As
entries.

(For now, the mechanism is based on an FT_UINT32 dissectore table where
all entries are linked to number 0. We should eventually come up with a
better mechanism.)

Change-Id: I3f81e331d7d04cfdfe9a58732d881652d77fabe2
Reviewed-on: https://code.wireshark.org/review/22376
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Petri-Dish: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>

debian/libwireshark0.symbols

@@ -1266,6 +1266,7 @@ libwireshark.so.0 libwireshark0 #MINVER#
  register_custom_dissector_table@Base 1.99.8
  register_custom_dissector_table@Base 1.99.8
  register_decode_as@Base 1.12.0~rc1
+ register_decode_as_next_proto@Base 2.5.0
  register_depend_dissector@Base 2.1.0
  register_dissector@Base 2.1.0
  register_dissector_table@Base 1.9.1

epan/decode_as.c

@@ -55,6 +55,41 @@ void register_decode_as(decode_as_t* reg)
     decode_as_list = g_list_append(decode_as_list, reg);
 }
 
+static void next_proto_prompt(packet_info *pinfo _U_, gchar *result)
+{
+    g_snprintf(result, MAX_DECODE_AS_PROMPT_LEN, "Next level protocol as");
+}
+
+static gpointer next_proto_value(packet_info *pinfo _U_)
+{
+    return 0;
+}
+
+static build_valid_func next_proto_values[] = { next_proto_value };
+static decode_as_value_t next_proto_da_values =
+                        { next_proto_prompt, 1, next_proto_values };
+
+void register_decode_as_next_proto(
+        const char *name, const gchar *title, const gchar *table_name)
+{
+    decode_as_t *da;
+    dissector_table_t dt;
+
+    dt = find_dissector_table(table_name);
+    DISSECTOR_ASSERT(IS_FT_UINT(dissector_table_get_type(dt)));
+
+    da = wmem_new0(wmem_epan_scope(), decode_as_t);
+    da->name = wmem_strdup(wmem_epan_scope(), name);
+    da->title = wmem_strdup(wmem_epan_scope(), title);
+    da->table_name = wmem_strdup(wmem_epan_scope(), table_name);
+    da->num_items = 1;
+    da->values = &next_proto_da_values;
+    da->populate_list = decode_as_default_populate_list;
+    da->reset_value = decode_as_default_reset;
+    da->change_value = decode_as_default_change;
+
+    register_decode_as(da);
+}
 
 struct decode_as_default_populate
 {

epan/decode_as.h

@@ -85,6 +85,14 @@ typedef struct decode_as_s {
 /** register a "Decode As".  A copy of the decode_as_t will be maintained by the decode_as module */
 WS_DLL_PUBLIC void register_decode_as(decode_as_t* reg);
 
+/** Register a "Decode As" entry for the special case where there is no
+    indication for the next protocol (such as port number etc.).
+    For now, this will use a uint32 dissector table internally and
+    assign all registered protocols to 0. The framework to do this can
+    be kept internal to epan. */
+WS_DLL_PUBLIC void register_decode_as_next_proto(
+      const char *name, const gchar *title, const gchar *table_name);
+
 /* Walk though the dissector table and provide dissector_handle_t for each item in the table */
 WS_DLL_PUBLIC void decode_as_default_populate_list(const gchar *table_name, decode_as_add_to_list_func add_to_list, gpointer ui_element);
 /* Clear a FT_UINT32 value from dissector table list */