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From "Paul Rogers (JIRA)" <j...@apache.org>
Subject [jira] [Comment Edited] (DRILL-5490) RepeatedVarCharOutput.recordStart becomes corrupted on realloc
Date Tue, 09 May 2017 04:47:05 GMT

    [ https://issues.apache.org/jira/browse/DRILL-5490?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16002016#comment-16002016
] 

Paul Rogers edited comment on DRILL-5490 at 5/9/17 4:46 AM:
------------------------------------------------------------

The other place that {{recordStart}} is used is to get the first row of data for DRILL-951,
to extract the header. Consider {{CompliantTextReader.extractHeader()}}:

{code}
    HeaderOutputMutator hOutputMutator = new HeaderOutputMutator();
    TextOutput hOutput = new RepeatedVarCharOutput(hOutputMutator, getColumns(), true);
    this.allocate(hOutputMutator.fieldVectorMap);
    ...
    // extract first row only
    reader.parseNext();

    // grab the field names from output
    String [] fieldNames = ((RepeatedVarCharOutput)hOutput).getTextOutput();
{code}

The only way the above can works is that we never call {RepeatedVarCharOutput.startBatch()}},
so {{recordStart}} defaults to 0, and so the zero-record check accidentally works:

{code}
    if (this.recordStart != characterData) {
      throw new ExecutionSetupException("record text was requested before finishing record");
    }
{code}

It is likely, however, that an empty header line would not be caught and so the above exception
would never actually fire.


was (Author: paul-rogers):
The other place that {{recordStart}} is used is to get the first row of data for DRILL-951,
to extract the header. Here again, we are fortunate that one bug masks another. The {{RepeatedVarCharOutput}}
is never used with headers. Consider {{CompliantTextReader.setup()}}:

{code}
      if (settings.isHeaderExtractionEnabled()){
        //extract header and use that to setup a set of VarCharVectors
        String [] fieldNames = extractHeader();
        output = new FieldVarCharOutput(outputMutator, fieldNames, getColumns(), isStarQuery());
      } else {
        //simply use RepeatedVarCharVector
        output = new RepeatedVarCharOutput(outputMutator, getColumns(), isStarQuery());
      }
{code}

> RepeatedVarCharOutput.recordStart becomes corrupted on realloc
> --------------------------------------------------------------
>
>                 Key: DRILL-5490
>                 URL: https://issues.apache.org/jira/browse/DRILL-5490
>             Project: Apache Drill
>          Issue Type: Bug
>    Affects Versions: 1.10.0
>            Reporter: Paul Rogers
>            Priority: Minor
>
> The class {{RepeatedVarCharOutput}}, used to read text data into the {{columns}} array,
has a member called {{recordStart}} which is supposed to track the memory offset of the start
of the record data in the {{columns}} VarChar array. However, the logic for the member is
very wrong. First, it is initialized based on an uninitialized variable:
> {{code}}
>   public void startBatch() {
>     this.recordStart = characterDataOriginal;
>     ...
>     loadVarCharDataAddress();
>   }
>   private void loadVarCharDataAddress(){
>     ...
>     this.characterDataOriginal = buf.memoryAddress();
>     ...
>   }
> {{code}}
> Second, the class keeps track of actual memory addresses for the VarChar data. When data
would exceed the buffer, memory is reallocated using the following method. But, this method
*does not* adjust the {{recordStart}} member, which now points to a bogus memory address:
> {code}
>   private void expandVarCharData(){
>     vector.getDataVector().reAlloc();
>     long diff = characterData - characterDataOriginal;
>     loadVarCharDataAddress();
>     characterData += diff;
>   }
> {code}
> Fortunately, like so many bugs in this class, these bugs are offset by the fact that
the variable is never used in a way that cause obvious problems (else these issues would have
been found sooner.) The only real use is:
> {code}
>   @Override
>   public boolean rowHasData() {
>     return recordStart < characterData;
>   }
> {code}
> Which, it turns out, is used only at EOF in {{TextReader.parseRecord()}}:
> {code}
>     } catch(StreamFinishedPseudoException e) {
>       // if we've written part of a field or all of a field, we should send this row.
>       if (fieldsWritten == 0 && !output.rowHasData()) {
>         throw e;
>       }
>     }
> {code}
> Thus, the bug will cause a failure only if:
> * A batch is resized, and
> * The new address is lower than the original address, and
> * It is the last batch in the file.
> Because of the low probability of hitting the bug, the priority is set to Minor.



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