Github user ppadma commented on a diff in the pull request:

    https://github.com/apache/drill/pull/1121#discussion_r168324742
  
    --- Diff: 
exec/java-exec/src/main/java/org/apache/drill/exec/physical/impl/protocol/OperatorDriver.java
 ---
    @@ -0,0 +1,183 @@
    +/*
    + * Licensed to the Apache Software Foundation (ASF) under one
    + * or more contributor license agreements.  See the NOTICE file
    + * distributed with this work for additional information
    + * regarding copyright ownership.  The ASF licenses this file
    + * to you under the Apache License, Version 2.0 (the
    + * "License"); you may not use this file except in compliance
    + * with the License.  You may obtain a copy of the License at
    + *
    + * http://www.apache.org/licenses/LICENSE-2.0
    + *
    + * Unless required by applicable law or agreed to in writing, software
    + * distributed under the License is distributed on an "AS IS" BASIS,
    + * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    + * See the License for the specific language governing permissions and
    + * limitations under the License.
    + */
    +package org.apache.drill.exec.physical.impl.protocol;
    +
    +import org.apache.drill.common.exceptions.UserException;
    +import org.apache.drill.exec.ops.OperatorContext;
    +import org.apache.drill.exec.record.RecordBatch.IterOutcome;
    +
    +/**
    + * State machine that drives the operator executable. Converts
    + * between the iterator protocol and the operator executable protocol.
    + * Implemented as a separate class in anticipation of eventually
    + * changing the record batch (iterator) protocol.
    + */
    +
    +public class OperatorDriver {
    +  public enum State { START, SCHEMA, RUN, END, FAILED, CLOSED }
    +
    +  private OperatorDriver.State state = State.START;
    +
    +  /**
    +   * Operator context. The driver "owns" the context and is responsible
    +   * for closing it.
    +   */
    +
    +  private final OperatorContext opContext;
    +  private final OperatorExec operatorExec;
    +  private final BatchAccessor batchAccessor;
    +  private int schemaVersion;
    +
    +  public OperatorDriver(OperatorContext opServicees, OperatorExec opExec) {
    +    this.opContext = opServicees;
    +    this.operatorExec = opExec;
    +    batchAccessor = operatorExec.batchAccessor();
    +  }
    +
    +  /**
    +   * Get the next batch. Performs initialization on the first call.
    +   * @return the iteration outcome to send downstream
    +   */
    +
    +  public IterOutcome next() {
    +    try {
    +      switch (state) {
    +      case START:
    +        return start();
    +      case RUN:
    +        return doNext();
    +       default:
    +        OperatorRecordBatch.logger.debug("Extra call to next() in state " 
+ state + ": " + operatorLabel());
    +        return IterOutcome.NONE;
    +      }
    +    } catch (UserException e) {
    +      cancelSilently();
    +      state = State.FAILED;
    +      throw e;
    +    } catch (Throwable t) {
    +      cancelSilently();
    +      state = State.FAILED;
    +      throw UserException.executionError(t)
    +        .addContext("Exception thrown from", operatorLabel())
    +        .build(OperatorRecordBatch.logger);
    +    }
    +  }
    +
    +  /**
    +   * Cancels the operator before reaching EOF.
    +   */
    +
    +  public void cancel() {
    +    try {
    +      switch (state) {
    +      case START:
    +      case RUN:
    +        cancelSilently();
    +        break;
    +      default:
    +        break;
    +      }
    +    } finally {
    +      state = State.FAILED;
    +    }
    +  }
    +
    + /**
    +   * Start the operator executor. Bind it to the various contexts.
    +   * Then start the executor and fetch the first schema.
    +   * @return result of the first batch, which should contain
    +   * only a schema, or EOF
    +   */
    +
    +  private IterOutcome start() {
    +    state = State.SCHEMA;
    +    if (operatorExec.buildSchema()) {
    +      schemaVersion = batchAccessor.schemaVersion();
    +      state = State.RUN;
    +      return IterOutcome.OK_NEW_SCHEMA;
    +    } else {
    +      state = State.END;
    +      return IterOutcome.NONE;
    +    }
    +  }
    +
    +  /**
    +   * Fetch a record batch, detecting EOF and a new schema.
    +   * @return the <tt>IterOutcome</tt> for the above cases
    +   */
    +
    +  private IterOutcome doNext() {
    +    if (! operatorExec.next()) {
    +      state = State.END;
    +      return IterOutcome.NONE;
    +    }
    +    int newVersion = batchAccessor.schemaVersion();
    +    if (newVersion != schemaVersion) {
    +      schemaVersion = newVersion;
    +      return IterOutcome.OK_NEW_SCHEMA;
    +    }
    +    return IterOutcome.OK;
    +  }
    +
    +  /**
    +   * Implement a cancellation, and ignore any exception that is
    +   * thrown. We're already in trouble here, no need to keep track
    +   * of additional things that go wrong.
    +   */
    +
    +  private void cancelSilently() {
    +    try {
    +      if (state == State.SCHEMA || state == State.RUN) {
    +        operatorExec.cancel();
    +      }
    +    } catch (Throwable t) {
    +      // Ignore; we're already in a bad state.
    +      OperatorRecordBatch.logger.error("Exception thrown from cancel() for 
" + operatorLabel(), t);
    +    }
    +  }
    +
    +  private String operatorLabel() {
    +    return operatorExec.getClass().getCanonicalName();
    +  }
    +
    +  public void close() {
    +    if (state == State.CLOSED) {
    --- End diff --
    
    should we check for  FAILED and END state also here ?


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