Github user paul-rogers commented on a diff in the pull request:

    https://github.com/apache/drill/pull/1121#discussion_r169203509
  
    --- Diff: 
exec/java-exec/src/main/java/org/apache/drill/exec/physical/impl/protocol/OperatorDriver.java
 ---
    @@ -0,0 +1,222 @@
    +/*
    + * 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 {
    +
    +    /**
    +     * Before the first call to next().
    +     */
    +
    +    START,
    +
    +    /**
    +     * The first call to next() has been made and schema (only)
    +     * was returned. On the subsequent call to next(), return any
    +     * data that might have accompanied that first batch.
    +     */
    +
    +    SCHEMA,
    +
    +    /**
    +     * The second call to next() has been made and there is more
    +     * data to deliver on subsequent calls.
    +     */
    +
    +    RUN,
    +
    +    /**
    +     * No more data to deliver.
    +     */
    +
    +    END,
    +
    +    /**
    +     * An error occurred. Operation was cancelled.
    +     */
    +
    +    FAILED,
    +
    +    /**
    +     * close() called and resources are released.
    +     */
    +
    +    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 opContext, OperatorExec opExec) {
    +    this.opContext = opContext;
    +    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;
    --- End diff --
    
    Added a Cancelled state. But, nothing ever reads that state. The point of 
FAILED is just to avoid confusion when calling `next()` after a failure or 
cancellation.
    
    We definitely *do not* want cancellation to move to the `CLOSED` state. 
This is a bug that exists in several operators. If `cancel()` closes the 
operator, then the operator is closed twice: once when the downstream operator 
says it wants no more rows, and a second time when the fragment executor does 
the real close.
    
    So, the `FAILED` state marks that a) we won't return any more rows, but 
that b) `close()` has not been called yet. The new `CANCELED` state has the 
same semantics.


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