Ticket 49257 - Reject dbcachesize updates while auto cache sizing is enabled

Description:  We should reject updates to nsslapd-dbcachesize while auto cache sizing
              is in effect.  This is because at startup we would overwrite the
              manually set dbcache size anyway.  It would never take effect, so it
              should be rejected.

https://pagure.io/389-ds-base/issue/49257

Reviewed by: tbordaz & firstyear(Thanks!!)

(cherry picked from commit 910a4ce2b5a19fa8de939519ac865a56feb7b626)

ldap/servers/slapd/back-ldbm/ldbm_config.c

@@ -420,7 +420,7 @@ static int ldbm_config_dbcachesize_set(void *arg, void *value, char *errorbuf, i
         /* Stop the user configuring a stupidly small cache */
         /* min: 8KB (page size) * def thrd cnts (threadnumber==20). */
 #define DBDEFMINSIZ     500000
-        /* We allow a value of 0, because the autotuting in start.c will
+        /* We allow a value of 0, because the autotuning in start.c will
          * register that, and trigger the recalculation of the dbcachesize as
          * needed on the next start up.
          */
@@ -443,7 +443,18 @@ static int ldbm_config_dbcachesize_set(void *arg, void *value, char *errorbuf, i
                 return LDAP_UNWILLING_TO_PERFORM;
             }
         }
+
         if (CONFIG_PHASE_RUNNING == phase) {
+            if (val > 0 && li->li_cache_autosize) {
+                /* We are auto-tuning the cache, so this change would be overwritten - return an error */
+                slapi_create_errormsg(errorbuf, SLAPI_DSE_RETURNTEXT_SIZE,
+                    "Error: \"nsslapd-dbcachesize\" can not be updated while \"nsslapd-cache-autosize\" is set "
+                    "in \"cn=config,cn=ldbm database,cn=plugins,cn=config\".");
+                slapi_log_err(SLAPI_LOG_ERR, "ldbm_config_dbcachesize_set",
+                    "\"nsslapd-dbcachesize\" can not be set while \"nsslapd-cache-autosize\" is set "
+                    "in \"cn=config,cn=ldbm database,cn=plugins,cn=config\".\n");
+                return LDAP_UNWILLING_TO_PERFORM;
+            }
             li->li_new_dbcachesize = val;
             if (val == 0) {
                 slapi_log_err(SLAPI_LOG_NOTICE, "ldbm_config_dbcachesize_set", "cache size reset to 0, will be autosized on next startup.\n");