Code review comment: The most risky bug in this code is: Improper handling of indexes leading to potential `IndexOutOfBoundsException`.  You can modify the code like this: ```java              List<StatementBase> stmts = SqlParser.parse(request.getSql(), ctx.getSessionVariable().getSqlMode()); +            if (idx < 0 || idx >= stmts.size()) { +                throw new IllegalArgumentException("Invalid statement index: " + idx); +            }              StatementBase statement = stmts.get(idx);              StmtExecutor stmtExecutor = new StmtExecutor(ctx, statement);              stmtExecutor.setProxy();              stmtExecutor.execute(); ``` This modification ensures that the statement index (`idx`) falls within the valid range of statements parsed from the request SQL. If `idx` is out of bounds, an `IllegalArgumentException` is thrown, thereby preventing `IndexOutOfBoundsException` and providing a clearer error message.

Code under review:
public TMasterOpResult proxyExecute(TMasterOpRequest request) {
        ctx.setCurrentCatalog(request.catalog);
        if (ctx.getCurrentCatalog() == null) {
            
            
            
            TMasterOpResult result = new TMasterOpResult();
            ctx.getState().setError(
                    "Missing current catalog. You need to upgrade this Frontend to the same version as Leader Frontend.");
            result.setMaxJournalId(GlobalStateMgr.getCurrentState().getMaxJournalId());
            result.setPacket(getResultPacket());
            return result;
        }
        ctx.setDatabase(request.db);
        ctx.setQualifiedUser(request.user);
        ctx.setGlobalStateMgr(GlobalStateMgr.getCurrentState());
        ctx.getState().reset();
        if (request.isSetResourceInfo()) {
            ctx.getSessionVariable().setResourceGroup(request.getResourceInfo().getGroup());
        }
        if (request.isSetUser_ip()) {
            ctx.setRemoteIP(request.getUser_ip());
        }
        if (request.isSetTime_zone()) {
            ctx.getSessionVariable().setTimeZone(request.getTime_zone());
        }
        if (request.isSetStmt_id()) {
            ctx.setForwardedStmtId(request.getStmt_id());
        }
        if (request.isSetSqlMode()) {
            ctx.getSessionVariable().setSqlMode(request.sqlMode);
        }
        if (request.isSetEnableStrictMode()) {
            ctx.getSessionVariable().setEnableInsertStrict(request.enableStrictMode);
        }
        if (request.isSetCurrent_user_ident()) {
            UserIdentity currentUserIdentity = UserIdentity.fromThrift(request.getCurrent_user_ident());
            ctx.setCurrentUserIdentity(currentUserIdentity);
        }

        if (request.isSetUser_roles()) {
            List<Long> roleIds = request.getUser_roles().getRole_id_list();
            ctx.setCurrentRoleIds(new HashSet<>(roleIds));
        } else {
            ctx.setCurrentRoleIds(new HashSet<>());
        }

        if (request.isSetIsLastStmt()) {
            ctx.setIsLastStmt(request.isIsLastStmt());
        } else {
            
            
            
            ctx.setIsLastStmt(true);
        }

        if (request.isSetQuery_options()) {
            TQueryOptions queryOptions = request.getQuery_options();
            if (queryOptions.isSetMem_limit()) {
                ctx.getSessionVariable().setMaxExecMemByte(queryOptions.getMem_limit());
            }
            if (queryOptions.isSetQuery_timeout()) {
                ctx.getSessionVariable().setQueryTimeoutS(queryOptions.getQuery_timeout());
            }
            if (queryOptions.isSetLoad_mem_limit()) {
                ctx.getSessionVariable().setLoadMemLimit(queryOptions.getLoad_mem_limit());
            }
            if (queryOptions.isSetMax_scan_key_num()) {
                ctx.getSessionVariable().setMaxScanKeyNum(queryOptions.getMax_scan_key_num());
            }
            if (queryOptions.isSetMax_pushdown_conditions_per_column()) {
                ctx.getSessionVariable().setMaxPushdownConditionsPerColumn(
                        queryOptions.getMax_pushdown_conditions_per_column());
            }
        } else {
            
            if (request.isSetExecMemLimit()) {
                ctx.getSessionVariable().setMaxExecMemByte(request.getExecMemLimit());
            }
            if (request.isSetQueryTimeout()) {
                ctx.getSessionVariable().setQueryTimeoutS(request.getQueryTimeout());
            }
            if (request.isSetLoadMemLimit()) {
                ctx.getSessionVariable().setLoadMemLimit(request.loadMemLimit);
            }
        }

        if (request.isSetQueryId()) {
            ctx.setQueryId(UUIDUtil.fromTUniqueid(request.getQueryId()));
        }

        if (request.isSetForward_times()) {
            ctx.setForwardTimes(request.getForward_times());
        }

        ctx.setThreadLocalInfo();

        if (ctx.getCurrentUserIdentity() == null) {
            
            
            
            TMasterOpResult result = new TMasterOpResult();
            ctx.getState().setError(
                    "Missing current user identity. You need to upgrade this Frontend to the same version as Leader Frontend.");
            result.setMaxJournalId(GlobalStateMgr.getCurrentState().getMaxJournalId());
            result.setPacket(getResultPacket());
            return result;
        }

        try {
            
            if (request.isSetModified_variables_sql()) {
                LOG.info("Set session variables first: {}", request.modified_variables_sql);

                StatementBase statement = SqlParser.parseSingleStatement(request.modified_variables_sql,
                        ctx.getSessionVariable().getSqlMode());
                StmtExecutor stmtExecutor = new StmtExecutor(ctx, statement);
                stmtExecutor.setProxy();
                stmtExecutor.execute();
            }
            
            int idx = request.isSetStmtIdx() ? request.getStmtIdx() : 0;

            List<StatementBase> stmts = SqlParser.parse(request.getSql(), ctx.getSessionVariable());
            StatementBase statement = stmts.get(idx);
            StmtExecutor stmtExecutor = new StmtExecutor(ctx, statement);
            stmtExecutor.setProxy();
            stmtExecutor.execute();
        } catch (IOException e) {
            
            LOG.warn("Process one query failed because IOException: ", e);
            ctx.getState().setError("StarRocks process failed: " + e.getMessage());
        } catch (Throwable e) {
            
            
            LOG.warn("Process one query failed because unknown reason: ", e);
            ctx.getState().setError("Unexpected exception: " + e.getMessage());
        }

        
        if (executor != null && executor.getIsForwardToLeaderOrInit(false)) {
            return executor.getLeaderOpExecutor().getResult();
        }

        
        
        TMasterOpResult result = new TMasterOpResult();
        result.setMaxJournalId(GlobalStateMgr.getCurrentState().getMaxJournalId());
        
        
        if (!ctx.getIsLastStmt()
                && ctx.getState().getStateType() != QueryState.MysqlStateType.ERR) {
            ctx.getState().serverStatus |= MysqlServerStatusFlag.SERVER_MORE_RESULTS_EXISTS;
        }
        result.setPacket(getResultPacket());
        result.setState(ctx.getState().getStateType().toString());
        if (executor != null) {
            if (executor.getProxyResultSet() != null) {  
                result.setResultSet(executor.getProxyResultSet().tothrift());
            } else if (executor.getProxyResultBuffer() != null) {  
                result.setChannelBufferList(executor.getProxyResultBuffer());
            }

            String resourceGroupName = ctx.getAuditEventBuilder().build().resourceGroup;
            if (StringUtils.isNotEmpty(resourceGroupName)) {
                result.setResource_group_name(resourceGroupName);
            }

            PQueryStatistics audit = executor.getQueryStatisticsForAuditLog();
            if (audit != null) {
                result.setAudit_statistics(AuditStatisticsUtil.toThrift(audit));
            }
        }
        return result;
    }

Line commented on: ctx.getState().setError("StarRocks process failed: " + e.getMessage());

Revised code:
public TMasterOpResult proxyExecute(TMasterOpRequest request) {
        ctx.setCurrentCatalog(request.catalog);
        if (ctx.getCurrentCatalog() == null) {
            
            
            
            TMasterOpResult result = new TMasterOpResult();
            ctx.getState().setError(
                    "Missing current catalog. You need to upgrade this Frontend to the same version as Leader Frontend.");
            result.setMaxJournalId(GlobalStateMgr.getCurrentState().getMaxJournalId());
            result.setPacket(getResultPacket());
            return result;
        }
        ctx.setDatabase(request.db);
        ctx.setQualifiedUser(request.user);
        ctx.setGlobalStateMgr(GlobalStateMgr.getCurrentState());
        ctx.getState().reset();
        if (request.isSetResourceInfo()) {
            ctx.getSessionVariable().setResourceGroup(request.getResourceInfo().getGroup());
        }
        if (request.isSetUser_ip()) {
            ctx.setRemoteIP(request.getUser_ip());
        }
        if (request.isSetTime_zone()) {
            ctx.getSessionVariable().setTimeZone(request.getTime_zone());
        }
        if (request.isSetStmt_id()) {
            ctx.setForwardedStmtId(request.getStmt_id());
        }
        if (request.isSetSqlMode()) {
            ctx.getSessionVariable().setSqlMode(request.sqlMode);
        }
        if (request.isSetEnableStrictMode()) {
            ctx.getSessionVariable().setEnableInsertStrict(request.enableStrictMode);
        }
        if (request.isSetCurrent_user_ident()) {
            UserIdentity currentUserIdentity = UserIdentity.fromThrift(request.getCurrent_user_ident());
            ctx.setCurrentUserIdentity(currentUserIdentity);
        }

        if (request.isSetUser_roles()) {
            List<Long> roleIds = request.getUser_roles().getRole_id_list();
            ctx.setCurrentRoleIds(new HashSet<>(roleIds));
        } else {
            ctx.setCurrentRoleIds(new HashSet<>());
        }

        if (request.isSetIsLastStmt()) {
            ctx.setIsLastStmt(request.isIsLastStmt());
        } else {
            
            
            
            ctx.setIsLastStmt(true);
        }

        if (request.isSetQuery_options()) {
            TQueryOptions queryOptions = request.getQuery_options();
            if (queryOptions.isSetMem_limit()) {
                ctx.getSessionVariable().setMaxExecMemByte(queryOptions.getMem_limit());
            }
            if (queryOptions.isSetQuery_timeout()) {
                ctx.getSessionVariable().setQueryTimeoutS(queryOptions.getQuery_timeout());
            }
            if (queryOptions.isSetLoad_mem_limit()) {
                ctx.getSessionVariable().setLoadMemLimit(queryOptions.getLoad_mem_limit());
            }
            if (queryOptions.isSetMax_scan_key_num()) {
                ctx.getSessionVariable().setMaxScanKeyNum(queryOptions.getMax_scan_key_num());
            }
            if (queryOptions.isSetMax_pushdown_conditions_per_column()) {
                ctx.getSessionVariable().setMaxPushdownConditionsPerColumn(
                        queryOptions.getMax_pushdown_conditions_per_column());
            }
        } else {
            
            if (request.isSetExecMemLimit()) {
                ctx.getSessionVariable().setMaxExecMemByte(request.getExecMemLimit());
            }
            if (request.isSetQueryTimeout()) {
                ctx.getSessionVariable().setQueryTimeoutS(request.getQueryTimeout());
            }
            if (request.isSetLoadMemLimit()) {
                ctx.getSessionVariable().setLoadMemLimit(request.loadMemLimit);
            }
        }

        if (request.isSetQueryId()) {
            ctx.setQueryId(UUIDUtil.fromTUniqueid(request.getQueryId()));
        }

        if (request.isSetForward_times()) {
            ctx.setForwardTimes(request.getForward_times());
        }

        ctx.setThreadLocalInfo();

        if (ctx.getCurrentUserIdentity() == null) {
            
            
            
            TMasterOpResult result = new TMasterOpResult();
            ctx.getState().setError(
                    "Missing current user identity. You need to upgrade this Frontend to the same version as Leader Frontend.");
            result.setMaxJournalId(GlobalStateMgr.getCurrentState().getMaxJournalId());
            result.setPacket(getResultPacket());
            return result;
        }

        StmtExecutor executor = null;
        try {
            
            if (request.isSetModified_variables_sql()) {
                LOG.info("Set session variables first: {}", request.modified_variables_sql);

                StatementBase statement = SqlParser.parseSingleStatement(request.modified_variables_sql,
                        ctx.getSessionVariable().getSqlMode());
                executor = new StmtExecutor(ctx, statement);
                executor.setProxy();
                executor.execute();
            }
            
            int idx = request.isSetStmtIdx() ? request.getStmtIdx() : 0;

            List<StatementBase> stmts = SqlParser.parse(request.getSql(), ctx.getSessionVariable());
            StatementBase statement = stmts.get(idx);
            executor = new StmtExecutor(ctx, statement);
            executor.setProxy();
            executor.execute();
        } catch (IOException e) {
            
            LOG.warn("Process one query failed because IOException: ", e);
            ctx.getState().setError("StarRocks process failed: " + e.getMessage());
        } catch (Throwable e) {
            
            
            LOG.warn("Process one query failed because unknown reason: ", e);
            ctx.getState().setError("Unexpected exception: " + e.getMessage());
        }

        
        if (executor != null && executor.getIsForwardToLeaderOrInit(false)) {
            return executor.getLeaderOpExecutor().getResult();
        }

        
        
        TMasterOpResult result = new TMasterOpResult();
        result.setMaxJournalId(GlobalStateMgr.getCurrentState().getMaxJournalId());
        
        
        if (!ctx.getIsLastStmt()
                && ctx.getState().getStateType() != QueryState.MysqlStateType.ERR) {
            ctx.getState().serverStatus |= MysqlServerStatusFlag.SERVER_MORE_RESULTS_EXISTS;
        }
        result.setPacket(getResultPacket());
        result.setState(ctx.getState().getStateType().toString());
        if (executor != null) {
            if (executor.getProxyResultSet() != null) {  
                result.setResultSet(executor.getProxyResultSet().tothrift());
            } else if (executor.getProxyResultBuffer() != null) {  
                result.setChannelBufferList(executor.getProxyResultBuffer());
            }

            String resourceGroupName = ctx.getAuditEventBuilder().build().resourceGroup;
            if (StringUtils.isNotEmpty(resourceGroupName)) {
                result.setResource_group_name(resourceGroupName);
            }

            PQueryStatistics audit = executor.getQueryStatisticsForAuditLog();
            if (audit != null) {
                result.setAudit_statistics(AuditStatisticsUtil.toThrift(audit));
            }
        }
        return result;
    }