Restore old client 'processCommandAndResetClient' to fix false dead client indicator (#8715)

'processCommandAndResetClient' returns 1 if client is dead. It does it
by checking if serve.current_client is NULL. On script timeout, Redis will re-enter
'processCommandAndResetClient' and when finish we will set server.current_client
to NULL. This will cause later to falsely return 1 and think that the client that
sent the timed-out script is dead (Redis to stop reading from the client buffer).

src/networking.c

@@ -1990,12 +1990,20 @@ void commandProcessed(client *c) {
  * of processing the command, otherwise C_OK is returned. */
 int processCommandAndResetClient(client *c) {
     int deadclient = 0;
+    client *old_client = server.current_client;
     server.current_client = c;
     if (processCommand(c) == C_OK) {
         commandProcessed(c);
     }
     if (server.current_client == NULL) deadclient = 1;
-    server.current_client = NULL;
+    /*
+     * Restore the old client, this is needed because when a script
+     * times out, we will get into this code from processEventsWhileBlocked.
+     * Which will cause to set the server.current_client. If not restored
+     * we will return 1 to our caller which will falsely indicate the client
+     * is dead and will stop reading from its buffer.
+     */
+    server.current_client = old_client;
     /* performEvictions may flush slave output buffers. This may
      * result in a slave, that may be the active client, to be
      * freed. */

tests/unit/scripting.tcl

@@ -640,6 +640,43 @@ start_server {tags {"scripting"}} {
         assert_match {*killed by user*} $res        
     }
 
+    test {Timedout script does not cause a false dead client} {
+        set rd [redis_deferring_client]
+        r config set lua-time-limit 10
+
+        # senging (in a pipeline):
+        # 1. eval "while 1 do redis.call('ping') end" 0
+        # 2. ping
+        set buf "*3\r\n\$4\r\neval\r\n\$33\r\nwhile 1 do redis.call('ping') end\r\n\$1\r\n0\r\n"
+        append buf "*1\r\n\$4\r\nping\r\n"
+        $rd write $buf
+        $rd flush
+
+        wait_for_condition 50 100 {
+            [catch {r ping} e] == 1
+        } else {
+            fail "Can't wait for script to start running"
+        }
+        catch {r ping} e
+        assert_match {BUSY*} $e
+
+        r script kill
+        wait_for_condition 50 100 {
+            [catch {r ping} e] == 0
+        } else {
+            fail "Can't wait for script to be killed"
+        }
+        assert_equal [r ping] "PONG"
+
+        catch {$rd read} res
+        assert_match {*killed by user*} $res
+
+        set res [$rd read]
+        assert_match {*PONG*} $res        
+
+        $rd close
+    }
+
     test {Timedout script link is still usable after Lua returns} {
         r config set lua-time-limit 10
         r eval {for i=1,100000 do redis.call('ping') end return 'ok'} 0