GH-1190: Reserve view slots for empty view vector values

[goutamadwant]

What's Changed

Fixes BaseVariableWidthViewVector.handleSafe so setSafe reserves a full 16-byte view slot for the target index even when the value length is zero.

Adds coverage for empty values written through the first view-buffer boundary for both ViewVarCharVector and ViewVarBinaryVector.

Tests

• mvn -pl vector -am -Dtest=TestVariableWidthViewVector#testSetSafeEmptyValueAtViewBufferBoundary -Dsurefire.failIfNoSpecifiedTests=false test
• mvn -pl vector -am -Dtest=TestVariableWidthViewVector -Dsurefire.failIfNoSpecifiedTests=false test

Closes #1190.

[mbutrovich]

Thanks @goutamadwant, this looks like what I was going to do! One coverage gap worth closing: handleSafe(index, 0) is also hit by setNull (1195) and fillEmpties via setValueCount (1022), so the bug was reachable without ever writing an empty string. The fix covers those too, but the test only exercises setSafe(i, new byte[0]). Suggest adding the two tests below. Non-blocking.

[mbutrovich]

Correct. Reserves the full slot regardless of payload; ((long) index + 1) avoids overflow. roundUpToMultipleOf16 is a no-op here but fine to keep.

[mbutrovich]

Only covers the empty-value path. setNull and fillEmpties/setValueCount reach the same handleSafe(index, 0). Suggest adding tests for those.

setValueCount trailing gap (silent trigger, no empty string written)

@ParameterizedTest
@MethodSource({"vectorCreatorProvider"})
public void testSetValueCountFillsEmptiesAtViewBufferBoundary(
    Function<BufferAllocator, BaseVariableWidthViewVector> vectorCreator) {
  try (final BaseVariableWidthViewVector vector = vectorCreator.apply(allocator)) {
    vector.allocateNew();
    final int valueCapacity = vector.getValueCapacity();
    vector.setSafe(valueCapacity - 1, "x".getBytes(StandardCharsets.UTF_8));
    // Leaves index valueCapacity unset: fillEmpties -> handleSafe(_, 0) on a full buffer.
    vector.setValueCount(valueCapacity + 1);
    assertTrue(vector.getValueCapacity() > valueCapacity);
    assertTrue(vector.isNull(valueCapacity));
  }
}

setNull at the boundary

@ParameterizedTest
@MethodSource({"vectorCreatorProvider"})
public void testSetNullAtViewBufferBoundary(
    Function<BufferAllocator, BaseVariableWidthViewVector> vectorCreator) {
  try (final BaseVariableWidthViewVector vector = vectorCreator.apply(allocator)) {
    vector.allocateNew();
    final int valueCapacity = vector.getValueCapacity();
    for (int i = 0; i <= valueCapacity; i++) {
      vector.setNull(i);
    }
    vector.setValueCount(valueCapacity + 1);
    assertTrue(vector.getValueCapacity() > valueCapacity);
    assertTrue(vector.isNull(valueCapacity));
  }
}