Make use of a single listNode pointer for blocking utility lists (#919)

Saves some memory (one pointer) in the client struct.

Since a client cannot be blocked multiple times, we can assume
it will be held in only one extra utility list, so it is ok to maintain
a union of these listNode references. 

Signed-off-by: Ran Shidlansik <ranshid@amazon.com>
Signed-off-by: ranshid <88133677+ranshid@users.noreply.github.com>
Signed-off-by: Binbin <binloveplay1314@qq.com>
Co-authored-by: Viktor Söderqvist <viktor.soderqvist@est.tech>
Co-authored-by: Binbin <binloveplay1314@qq.com>

src/blocked.c

@@ -81,7 +81,7 @@ void initClientBlockingState(client *c) {
     c->bstate.numreplicas = 0;
     c->bstate.numlocal = 0;
     c->bstate.reploffset = 0;
-    c->bstate.client_waiting_acks_list_node = NULL;
+    c->bstate.generic_blocked_list_node = NULL;
     c->bstate.module_blocked_handle = NULL;
     c->bstate.async_rm_call_handle = NULL;
 }
@@ -194,8 +194,9 @@ void unblockClient(client *c, int queue_for_reprocessing) {
         if (moduleClientIsBlockedOnKeys(c)) unblockClientWaitingData(c);
         unblockClientFromModule(c);
     } else if (c->bstate.btype == BLOCKED_POSTPONE) {
-        listDelNode(server.postponed_clients, c->postponed_list_node);
-        c->postponed_list_node = NULL;
+        serverAssert(c->bstate.postponed_list_node);
+        listDelNode(server.postponed_clients, c->bstate.postponed_list_node);
+        c->bstate.postponed_list_node = NULL;
     } else if (c->bstate.btype == BLOCKED_SHUTDOWN) {
         /* No special cleanup. */
     } else {
@@ -613,7 +614,8 @@ void blockPostponeClient(client *c) {
     c->bstate.timeout = 0;
     blockClient(c, BLOCKED_POSTPONE);
     listAddNodeTail(server.postponed_clients, c);
-    c->postponed_list_node = listLast(server.postponed_clients);
+    serverAssert(c->bstate.postponed_list_node == NULL);
+    c->bstate.postponed_list_node = listLast(server.postponed_clients);
     /* Mark this client to execute its command */
     c->flag.pending_command = 1;
 }

src/networking.c

@@ -212,7 +212,6 @@ client *createClient(connection *conn) {
     c->peerid = NULL;
     c->sockname = NULL;
     c->client_list_node = NULL;
-    c->postponed_list_node = NULL;
     c->io_read_state = CLIENT_IDLE;
     c->io_write_state = CLIENT_IDLE;
     c->nwritten = 0;

src/server.h

@@ -1010,15 +1010,22 @@ typedef struct blockingState {
                            * is > timeout then the operation timed out. */
     int unblock_on_nokey; /* Whether to unblock the client when at least one of the keys
                              is deleted or does not exist anymore */
+    union {
+        listNode *client_waiting_acks_list_node; /* list node in server.clients_waiting_acks list. */
+        listNode *postponed_list_node;           /* list node in server.postponed_clients */
+        listNode *generic_blocked_list_node;     /* generic placeholder for blocked clients utility lists.
+                                                    Since a client cannot be blocked multiple times, we can assume
+                                                    it will be held in only one extra utility list, so it is ok to maintain
+                                                    a union of these listNode references. */
+    };
 
     /* BLOCKED_LIST, BLOCKED_ZSET and BLOCKED_STREAM or any other Keys related blocking */
     dict *keys; /* The keys we are blocked on */
 
     /* BLOCKED_WAIT and BLOCKED_WAITAOF */
-    int numreplicas;                         /* Number of replicas we are waiting for ACK. */
-    int numlocal;                            /* Indication if WAITAOF is waiting for local fsync. */
-    long long reploffset;                    /* Replication offset to reach. */
-    listNode *client_waiting_acks_list_node; /* list node in server.clients_waiting_acks list. */
+    int numreplicas;      /* Number of replicas we are waiting for ACK. */
+    int numlocal;         /* Indication if WAITAOF is waiting for local fsync. */
+    long long reploffset; /* Replication offset to reach. */
 
     /* BLOCKED_MODULE */
     void *module_blocked_handle; /* ValkeyModuleBlockedClient structure.
@@ -1321,7 +1328,6 @@ typedef struct client {
     sds peerid;                          /* Cached peer ID. */
     sds sockname;                        /* Cached connection target address. */
     listNode *client_list_node;          /* list node in client list */
-    listNode *postponed_list_node;       /* list node within the postponed list */
     void *module_blocked_client;         /* Pointer to the ValkeyModuleBlockedClient associated with this
                                           * client. This is set in case of module authentication before the
                                           * unblocked client is reprocessed to handle reply callbacks. */