Better messages when redis-cli migration meet value check failed

The clusterManagerCompareKeysValues is introduced in
143bfa1, which calls
DEBUG DIGEST-VALUE to check whether the value of the
source node and the target node are consistent.

However, the DEBUG DIGEST-VALUE command is not supported
in older version, such as version 4, and the node will return
unknown subcommand. Or the DEBUG command can be disabled
in version 7, and the node will return DEBUG not allowed.

In these cases, we need to output friendly message to
allow users to proceed to the next step, instead of just
outputing `Value check failed!`.

Unknown subcommand example:
```
*** Target key exists
*** Checking key values on both nodes...
Node 127.0.0.1:30001 replied with error:
ERR unknown subcommand or wrong number of arguments for 'DIGEST-VALUE'. Try DEBUG HELP.
Node 127.0.0.1:30003 replied with error:
ERR unknown subcommand or wrong number of arguments for 'DIGEST-VALUE'. Try DEBUG HELP.
*** Value check failed!
DEBUG DIGEST-VALUE command is not supported.
You can relaunch the command with --cluster-replace option to force key overriding.
```

DEBUG not allowed example:
```
*** Target key exists
*** Checking key values on both nodes...
Node 127.0.0.1:30001 replied with error:
ERR DEBUG command not allowed. If the enable-debug-command option is ...
Node 127.0.0.1:30003 replied with error:
ERR DEBUG command not allowed. If the enable-debug-command option is ...
*** Value check failed!
DEBUG command is not allowed.
You can turn on the enable-debug-command option.
Or you can relaunch the command with --cluster-replace option to force key overriding.
```

Signed-off-by: Binbin <binloveplay1314@qq.com>

src/valkey-cli.c

@@ -4655,10 +4655,16 @@ static int clusterManagerSetSlotOwner(clusterManagerNode *owner, int slot, int d
 /* Get the hash for the values of the specified keys in *keys_reply for the
  * specified nodes *n1 and *n2, by calling DEBUG DIGEST-VALUE command
  * on both nodes. Every key with same name on both nodes but having different
- * values will be added to the *diffs list. Return 0 in case of reply
- * error. */
+ * values will be added to the *diffs list.
+ *
+ * DEBUG DIGEST-VALUE currently will only return two errors:
+ * 1. Unknown subcommand. This happened in older server versions.
+ * 2. DEBUG command not allowed. This happened when we disable enable-debug-command.
+ *
+ * Return 0 and set the error message in case of reply error. */
 static int
-clusterManagerCompareKeysValues(clusterManagerNode *n1, clusterManagerNode *n2, redisReply *keys_reply, list *diffs) {
+clusterManagerCompareKeysValues(clusterManagerNode *n1, clusterManagerNode *n2, redisReply *keys_reply, list *diffs,
+                                char **n1_err, char **n2_err) {
     size_t i, argc = keys_reply->elements + 2;
     static const char *hash_zero = "0000000000000000000000000000000000000000";
     char **argv = zcalloc(argc * sizeof(char *));
@@ -4678,18 +4684,32 @@ clusterManagerCompareKeysValues(clusterManagerNode *n1, clusterManagerNode *n2,
     redisReply *r1 = NULL, *r2 = NULL;
     redisAppendCommandArgv(n1->context, argc, (const char **)argv, argv_len);
     success = (redisGetReply(n1->context, &_reply1) == REDIS_OK);
-    if (!success) goto cleanup;
+    if (!success) {
+        fprintf(stderr, "Error getting DIGEST-VALUE from %s:%d, error: %s\n", n1->ip, n1->port, n1->context->errstr);
+        exit(1);
+    }
     r1 = (redisReply *)_reply1;
     redisAppendCommandArgv(n2->context, argc, (const char **)argv, argv_len);
     success = (redisGetReply(n2->context, &_reply2) == REDIS_OK);
-    if (!success) goto cleanup;
+    if (!success) {
+        fprintf(stderr, "Error getting DIGEST-VALUE from %s:%d, error: %s\n", n2->ip, n2->port, n2->context->errstr);
+        exit(1);
+    }
     r2 = (redisReply *)_reply2;
     success = (r1->type != REDIS_REPLY_ERROR && r2->type != REDIS_REPLY_ERROR);
     if (r1->type == REDIS_REPLY_ERROR) {
+        if (n1_err != NULL) {
+            *n1_err = zmalloc((r1->len + 1) * sizeof(char));
+            valkey_strlcpy(*n1_err, r1->str, r1->len + 1);
+        }
         CLUSTER_MANAGER_PRINT_REPLY_ERROR(n1, r1->str);
         success = 0;
     }
     if (r2->type == REDIS_REPLY_ERROR) {
+        if (n2_err != NULL) {
+            *n2_err = zmalloc((r2->len + 1) * sizeof(char));
+            valkey_strlcpy(*n2_err, r2->str, r2->len + 1);
+        }
         CLUSTER_MANAGER_PRINT_REPLY_ERROR(n2, r2->str);
         success = 0;
     }
@@ -4875,10 +4895,26 @@ static int clusterManagerMigrateKeysInSlot(clusterManagerNode *source,
                     if (!do_replace) {
                         clusterManagerLogWarn("*** Checking key values on "
                                               "both nodes...\n");
+                        char *source_err = NULL;
+                        char *target_err = NULL;
                         list *diffs = listCreate();
-                        success = clusterManagerCompareKeysValues(source, target, reply, diffs);
+                        success = clusterManagerCompareKeysValues(source, target, reply, diffs, &source_err, &target_err);
                         if (!success) {
                             clusterManagerLogErr("*** Value check failed!\n");
+                            const char *debug_not_allowed = "ERR DEBUG command not allowed.";
+                            if ((source_err && !strncmp(source_err, debug_not_allowed, 30)) ||
+                                (target_err && !strncmp(target_err, debug_not_allowed, 30))) {
+                                clusterManagerLogErr("DEBUG command is not allowed.\n"
+                                                     "You can turn on the enable-debug-command option.\n"
+                                                     "Or you can relaunch the command with --cluster-replace "
+                                                     "option to force key overriding.\n");
+                            } else if (source_err || target_err) {
+                                clusterManagerLogErr("DEBUG DIGEST-VALUE command is not supported.\n"
+                                                     "You can relaunch the command with --cluster-replace "
+                                                     "option to force key overriding.\n");
+                            }
+                            if (source_err) zfree(source_err);
+                            if (target_err) zfree(target_err);
                             listRelease(diffs);
                             goto next;
                         }