SQLFeatureStore: Fix lost updates in multi-part transactions

[MrSnyder]

Prior to this patch, multi-part transactions against the SQLFeatureStore could lead to lost updates. Here's an example of such a transaction (in the form of a WFS 2.0 transaction):

<?xml version="1.0"?>
<wfs:Transaction xmlns:fes="http://www.opengis.net/fes/2.0" xmlns:wfs="http://www.opengis.net/wfs/2.0" xmlns:aixm="http://www.aixm.aero/schema/5.1" xmlns:gml="http://www.opengis.net/gml/3.2" version="2.0.0" service="WFS">
  <wfs:Update typeName="aixm:Airspace">
    <wfs:Property>
      <wfs:ValueReference action="insertBefore">aixm:timeSlice[1]</wfs:ValueReference>
      <wfs:Value>
        <aixm:AirportHeliportTimeSlice gml:id="TEST_11111111-1111-1111-1111-111111111111_3">
          <gml:validTime>
            <gml:TimePeriod gml:id="TEST_11111111-1111-1111-1111-111111111111_4">
              <gml:beginPosition>2015-01-02T00:00:00Z</gml:beginPosition>
              <gml:endPosition>2015-01-03T00:00:00Z</gml:endPosition>
            </gml:TimePeriod>
          </gml:validTime>
          <aixm:interpretation>TEMPDELTA</aixm:interpretation>
          <aixm:sequenceNumber>1</aixm:sequenceNumber>
        </aixm:AirportHeliportTimeSlice>
      </wfs:Value>
    </wfs:Property>
    <fes:Filter>
      <fes:ResourceId rid="uuid.11111111-1111-1111-1111-111111111111"/>
    </fes:Filter>
  </wfs:Update>
  <wfs:Update typeName="aixm:Airspace">
    <wfs:Property>
      <wfs:ValueReference action="insertBefore">aixm:timeSlice[1]</wfs:ValueReference>
      <wfs:Value>
        <aixm:AirportHeliportTimeSlice gml:id="TEST_11111111-1111-1111-1111-111111111111_5">
          <gml:validTime>
            <gml:TimePeriod gml:id="TEST_11111111-1111-1111-1111-111111111111_6">
              <gml:beginPosition>2015-01-06T00:00:00Z</gml:beginPosition>
              <gml:endPosition>2015-01-07T00:00:00Z</gml:endPosition>
            </gml:TimePeriod>
          </gml:validTime>
          <aixm:interpretation>TEMPDELTA</aixm:interpretation>
          <aixm:sequenceNumber>2</aixm:sequenceNumber>
        </aixm:AirportHeliportTimeSlice>
      </wfs:Value>
    </wfs:Property>
    <fes:Filter>
      <fes:ResourceId rid="uuid.11111111-1111-1111-1111-111111111111"/>
    </fes:Filter>
  </wfs:Update>
</wfs:Transaction>

For this example, the first update was lost and only the second one was applied. This has been observed for BLOB mode.

The cause for the problem was that the SQLFeatureStore always acquired a new JDBC connection, even when being used by an SQLFeatureStoreTransaction object as part of the transaction. After the patch, the SQLFeatureStore uses a ThreadLocal object to determine if the current thread has an active transaction and uses it, if this is the case.

It may not be ideal to limit the use of an SQLFeatureStoreTransaction instance to a single thread, but IMHO, there is no realistic use case for using it from multiple threads. Therefore, I consider the patch to be "good enough", unless somebody wants to fully correct and refactor the design of SQLFeatureStore and SQLFeatureStoreTransaction (which would involve a lot of changes).

[tfr42]

Thank you for your contribution. Your pull requests has been accepted by the TMC.
This pull request will be part of the 3.4-pre18 release.