[tarantool-patches] [PATCH 7/6] swim: make swim_upsert_member returning two values

[Vladislav Shpilevoy]

Here is one new patch. It appeared to be usefull both
for the failure detection and for the dissemination.

================================================================

swim_upsert_member is a function to add new members or update
existing ones using info received from other cluster
participants. At this moment it is quite simple and
straightforward: either create a new member and return it, or
update an existing member and return it.

But things are going to change in failure detection and
dissemination components. A couple of examples showing that a
member should be returned separately from success/error status:

  * To prevent undead members the failure detection forbids to
    add dead members. Otherwise a dead member would be added and
    removed back and forth by different components infinitely.
    Upsert for such members should return NULL, but it is not an
    error - it is a normal function of the protocol;

  * When the dissemination component receives a UUID update of
    an existing member, but with too old incarnation, it does not
    apply that update. And can not return that member from upsert
    because of UUID being different from the one in the
    'request'. In such a case it should be ok to return NULL, but
    do not deem it an error.

Part of #3234
---

diff --git a/src/lib/swim/swim.c b/src/lib/swim/swim.c
index df34ce247..1b623fc27 100644
--- a/src/lib/swim/swim.c
+++ b/src/lib/swim/swim.c
@@ -657,22 +657,32 @@ swim_update_member_addr(struct swim *swim, struct swim_member *member,
 /**
  * Update or create a member by its definition, received from a
  * remote instance.
- * @retval NULL Error.
- * @retval New member, or updated old member.
+ * @param swim SWIM instance to upsert into.
+ * @param def Member definition to build a new member or update an
+ *        existing one.
+ * @param[out] result A result member: a new, or an updated, or
+ *        NULL in case of nothing has changed. For example, @a def
+ *        was too old.
+ *
+ * @retval 0 Success. Member is added, or updated. Or nothing has
+ *         changed but not always it is an error.
+ * @retval -1 Error.
  */
-static struct swim_member *
-swim_upsert_member(struct swim *swim, const struct swim_member_def *def)
+static int
+swim_upsert_member(struct swim *swim, const struct swim_member_def *def,
+		   struct swim_member **result)
 {
 	struct swim_member *member = swim_find_member(swim, &def->uuid);
 	if (member == NULL) {
-		member = swim_new_member(swim, &def->addr, &def->uuid,
-					 def->status);
-		return member;
+		*result = swim_new_member(swim, &def->addr, &def->uuid,
+					  def->status);
+		return *result != NULL ? 0 : -1;
 	}
 	struct swim_member *self = swim->self;
 	if (member != self)
 		swim_update_member_addr(swim, member, &def->addr);
-	return member;
+	*result = member;
+	return 0;
 }
 
 /** Decode an anti-entropy message, update member table. */
@@ -686,9 +696,10 @@ swim_process_anti_entropy(struct swim *swim, const char **pos, const char *end)
 		return -1;
 	for (uint64_t i = 0; i < size; ++i) {
 		struct swim_member_def def;
+		struct swim_member *member;
 		if (swim_member_def_decode(&def, pos, end, prefix) != 0)
 			return -1;
-		if (swim_upsert_member(swim, &def) == NULL) {
+		if (swim_upsert_member(swim, &def, &member) != 0) {
 			/*
 			 * Not a critical error. Other members
 			 * still can be updated.