fflush stdout after responses partial backport from (#9136)

2. Make redis-cli flush stdout when printing a reply
  This was needed in order to fix a hung in redis-cli test that uses
  --replica.
   Note that printf does flush when there's a newline, but fwrite does not.

3. fix the redis-cli --replica test which used to pass previously
   because it didn't really care what it read, and because redis-cli
   used printf to print these other things to stdout.

4. improve redis-cli --replica test to run with both diskless and disk-based.

Co-authored-by: Oran Agra <oran@redislabs.com>
Co-authored-by: Viktor Söderqvist <viktor@zuiderkwast.se>
(cherry picked from commit 1eb4baa5b8)
(cherry picked from commit 8884971223)

src/redis-cli.c

@@ -1275,6 +1275,7 @@ static int cliReadReply(int output_raw_strings) {
             }
         }
         fwrite(out,sdslen(out),1,stdout);
+        fflush(stdout);
         sdsfree(out);
     }
     freeReplyObject(reply);

tests/integration/redis-cli.tcl

@@ -247,9 +247,9 @@ start_server {tags {"cli"}} {
         test_redis_cli_rdb_dump
     }
 
-    test "Connecting as a replica" {
+    proc test_redis_cli_repl {} {
         set fd [open_cli "--replica"]
-        wait_for_condition 500 500 {
+        wait_for_condition 500 100 {
             [string match {*slave0:*state=online*} [r info]]
         } else {
             fail "redis-cli --replica did not connect"
@@ -258,14 +258,30 @@ start_server {tags {"cli"}} {
         for {set i 0} {$i < 100} {incr i} {
            r set test-key test-value-$i
         }
-        r client kill type slave
-        catch {
-            assert_match {*SET*key-a*} [read_cli $fd]
+
+        wait_for_condition 500 100 {
+            [string match {*test-value-99*} [read_cli $fd]]
+        } else {
+            fail "redis-cli --replica didn't read commands"
         }
 
-        close_cli $fd
+        fconfigure $fd -blocking true
+        r client kill type slave
+        catch { close_cli $fd } err
+        assert_match {*Server closed the connection*} $err
     }
 
+    test "Connecting as a replica" {
+        # Disk-based master
+        assert_match "OK" [r config set repl-diskless-sync no]
+        test_redis_cli_repl
+
+        # Disk-less master
+        assert_match "OK" [r config set repl-diskless-sync yes]
+        assert_match "OK" [r config set repl-diskless-sync-delay 0]
+        test_redis_cli_repl
+    } {}
+
     test "Piping raw protocol" {
         set cmds [tmpfile "cli_cmds"]
         set cmds_fd [open $cmds "w"]