[b2b_logic] fix crash on tracing expired b2b tuples

When a tuple expires via timer, be sure to provide a processing context, otherwise the tracer (if enabled) will simply crash.
As part of the fix, two new helper functions were added for pushing and poping new processing contexts - these are to be reused by some prev similar fix on uac_registrant.
Related to #3110
Many thanks to @vtzan for helping with the troubleshooting and testing

(cherry picked from commit 2a3eb1e)

context.c

@@ -143,6 +143,45 @@ int ensure_global_context(void)
 }
 
 
+#define CTX_STACK_SIZE  8
+static context_p ctx_stack[CTX_STACK_SIZE];
+static unsigned int ctx_stack_idx = 0;
+
+int push_new_global_context(void)
+{
+	if (ctx_stack_idx==CTX_STACK_SIZE) {
+		LM_ERR("too many stacked contexts (%d)\n",ctx_stack_idx);
+		return -1;
+	}
+
+	ctx_stack[ctx_stack_idx++] = current_processing_ctx;
+
+	current_processing_ctx = context_alloc(CONTEXT_GLOBAL);
+	if (!current_processing_ctx) {
+		LM_ERR("oom\n");
+		current_processing_ctx = ctx_stack[--ctx_stack_idx];
+		return -1;
+	}
+
+	memset(current_processing_ctx, 0, context_size(CONTEXT_GLOBAL));
+	return 0;
+}
+
+int pop_pushed_global_context(void)
+{
+	if (ctx_stack_idx==0) {
+		LM_ERR("nothing to pop form the stack\n");
+		return -1;
+	}
+
+	context_destroy(CONTEXT_GLOBAL, current_processing_ctx);
+	context_free(current_processing_ctx);
+
+	current_processing_ctx = ctx_stack[--ctx_stack_idx];
+	return 0;
+}
+
+
 void clear_global_context(void)
 {
 	if (current_processing_ctx) {

context.h

@@ -86,6 +86,12 @@ context_p context_alloc(enum osips_context type);
 int ensure_global_context(void);
 
 
+/* pushes or pops a context level from internal stack
+ */
+int push_new_global_context(void);
+int pop_pushed_global_context(void);
+
+
 /* free and reset the global context */
 void clear_global_context(void);
 

modules/b2b_logic/b2b_logic.c

@@ -682,11 +682,12 @@ void b2bl_db_timer_update(unsigned int ticks, void* param)
 	b2b_logic_dump(0);
 }
 
-static void term_entity(b2bl_entity_id_t *entity, int hash_index)
+static void term_entity(b2bl_entity_id_t *entity, int hash_index, str *key)
 {
 	str bye = {BYE, BYE_LEN};
 	b2b_req_data_t req_data;
 	b2b_rpl_data_t rpl_data;
+	struct b2b_context *ctx;
 
 	if (entity->type == B2B_SERVER &&
 		entity->state != B2BL_ENT_CONFIRMED) {
@@ -710,12 +711,22 @@ static void term_entity(b2bl_entity_id_t *entity, int hash_index)
 				entity->key.s);
 		b2bl_htable[hash_index].locked_by = -1;
 	} else {
-		memset(&req_data, 0, sizeof(b2b_req_data_t));
-		PREP_REQ_DATA(entity);
-		req_data.method =&bye;
-		b2bl_htable[hash_index].locked_by = process_no;
-		b2b_api.send_request(&req_data);
-		b2bl_htable[hash_index].locked_by = -1;
+		if ( key && ( !push_new_global_context() ||
+		(ctx=b2b_api.get_context())==NULL ||
+		pkg_str_dup(&ctx->b2bl_key, key)==0 )
+		) {
+			LM_ERR("preparing ctx for request failed, entity [%.*s]\n",
+				entity->key.len, entity->key.s);
+		} else {
+			memset(&req_data, 0, sizeof(b2b_req_data_t));
+			PREP_REQ_DATA(entity);
+			req_data.method =&bye;
+			b2bl_htable[hash_index].locked_by = process_no;
+			b2b_api.send_request(&req_data);
+			b2bl_htable[hash_index].locked_by = -1;
+			if (key)
+				pop_pushed_global_context();
+		}
 	}
 }
 
@@ -741,10 +752,10 @@ void b2bl_clean(unsigned int ticks, void* param)
 				if(tuple->bridge_entities[0] && tuple->bridge_entities[1] && !tuple->to_del)
 				{
 					if(!tuple->bridge_entities[0]->disconnected)
-						term_entity(tuple->bridge_entities[0], i);
+						term_entity(tuple->bridge_entities[0], i, tuple->key);
 
 					if(!tuple->bridge_entities[1]->disconnected)
-						term_entity(tuple->bridge_entities[1], i);
+						term_entity(tuple->bridge_entities[1], i, tuple->key);
 				}
 				b2bl_delete(tuple, i, 1, tuple->repl_flag != TUPLE_REPL_RECV);
 			}
@@ -1206,12 +1217,12 @@ static mi_response_t *mi_b2b_terminate_call(const mi_params_t *params,
 	if(tuple->bridge_entities[0] && tuple->bridge_entities[1] && !tuple->to_del)
 	{
 		if(!tuple->bridge_entities[0]->disconnected) {
-			term_entity(tuple->bridge_entities[0], hash_index);
+			term_entity(tuple->bridge_entities[0], hash_index, &key);
 			tuple->bridge_entities[0]->disconnected = 1;
 		}
 
 		if(!tuple->bridge_entities[1]->disconnected) {
-			term_entity(tuple->bridge_entities[1], hash_index);
+			term_entity(tuple->bridge_entities[1], hash_index, &key);
 			tuple->bridge_entities[1]->disconnected = 1;
 		}
 	}