unregister AE_READABLE from the read pipe in backgroundSaveDoneHandlerSocket (#8991)

In diskless replication, we create a read pipe for the RDB, between the child and the parent.
When we close this pipe (fd), the read handler also needs to be removed from the event loop (if it still registered).
Otherwise, next time we will use the same fd, the registration will be fail (panic), because
we will use EPOLL_CTL_MOD (the fd still register in the event loop), on fd that already removed from epoll_ctl

src/rdb.c

@@ -2684,6 +2684,7 @@ static void backgroundSaveDoneHandlerSocket(int exitcode, int bysignal) {
     }
     if (server.rdb_child_exit_pipe!=-1)
         close(server.rdb_child_exit_pipe);
+    aeDeleteFileEvent(server.el, server.rdb_pipe_read, AE_READABLE);
     close(server.rdb_pipe_read);
     server.rdb_child_exit_pipe = -1;
     server.rdb_pipe_read = -1;

tests/integration/replication.tcl

@@ -771,6 +771,45 @@ test "diskless replication child being killed is collected" {
     }
 }
 
+test "diskless replication read pipe cleanup" {
+    # In diskless replication, we create a read pipe for the RDB, between the child and the parent.
+    # When we close this pipe (fd), the read handler also needs to be removed from the event loop (if it still registered).
+    # Otherwise, next time we will use the same fd, the registration will be fail (panic), because
+    # we will use EPOLL_CTL_MOD (the fd still register in the event loop), on fd that already removed from epoll_ctl
+    start_server {tags {"repl"}} {
+        set master [srv 0 client]
+        set master_host [srv 0 host]
+        set master_port [srv 0 port]
+        set master_pid [srv 0 pid]
+        $master config set repl-diskless-sync yes
+        $master config set repl-diskless-sync-delay 0
+
+        # put enough data in the db, and slowdown the save, to keep the parent busy at the read process
+        $master config set rdb-key-save-delay 100000
+        $master debug populate 20000 test 10000
+        $master config set rdbcompression no
+        start_server {} {
+            set replica [srv 0 client]
+            set loglines [count_log_lines 0]
+            $replica config set repl-diskless-load swapdb
+            $replica replicaof $master_host $master_port
+
+            # wait for the replicas to start reading the rdb
+            wait_for_log_messages 0 {"*Loading DB in memory*"} $loglines 800 10
+
+            set loglines [count_log_lines 0]
+            # send FLUSHALL so the RDB child will be killed
+            $master flushall
+
+            # wait for another RDB child process to be started
+            wait_for_log_messages -1 {"*Background RDB transfer started by pid*"} $loglines 800 10
+
+            # make sure master is alive
+            $master ping
+        }
+    }
+}
+
 test {replicaof right after disconnection} {
     # this is a rare race condition that was reproduced sporadically by the psync2 unit.
     # see details in #7205