[Improvement-10617] Add comment in slot check (#10618)

(cherry picked from commit 247ca4ae8a)

dolphinscheduler-master/src/main/java/org/apache/dolphinscheduler/server/master/runner/MasterSchedulerService.java

@@ -202,8 +202,9 @@ public class MasterSchedulerService extends BaseDaemonThread {
         for (final Command command : commands) {
             masterPrepareExecService.execute(() -> {
                 try {
-                    // todo: this check is not safe, the slot may change after command transform.
+                    // Note: this check is not safe, the slot may change after command transform.
-                    // slot check again
+                    // We use the database transaction in `handleCommand` so that we can guarantee the command will always be executed
+                    // by only one master
                     SlotCheckState slotCheckState = slotCheck(command);
                     if (slotCheckState.equals(SlotCheckState.CHANGE) || slotCheckState.equals(SlotCheckState.INJECT)) {
                         logger.info("Master handle command {} skip, slot check state: {}", command.getId(), slotCheckState);

dolphinscheduler-service/src/main/java/org/apache/dolphinscheduler/service/process/ProcessServiceImpl.java

@@ -137,6 +137,7 @@ import org.apache.commons.collections.CollectionUtils;
 import org.apache.commons.lang.StringUtils;
 import org.apache.commons.lang.math.NumberUtils;
 
+import java.sql.SQLIntegrityConstraintViolationException;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Date;
@@ -350,7 +351,8 @@ public class ProcessServiceImpl implements ProcessService {
     }
 
     /**
-     * save error command, and delete original command
+     * Save error command, and delete original command. If the given command has already been moved into error command,
+     * will throw {@link SQLIntegrityConstraintViolationException ).
      *
      * @param command command
      * @param message message