GetRangeAsync returns incorrect range with ...OrEqual key selectors.

[george-polevoy]

Following query can never return the last element with KeySelector.LastLessOrEqual is uses, it is off by 1 or 2 elements.
If KeySelector FirstGreaterOrEqual is used for the beginExclusive parameter, the last element is off by 2 elements

Example:

var     beginRepr = new byte[8];
var     endRepr   = new byte[8];
UInt64? result    = null;
await db.ReadAsync(async tr =>
{
    BinaryPrimitives.WriteUInt64BigEndian(beginRepr, 0);
    BinaryPrimitives.WriteUInt64BigEndian(endRepr, 7);
    var begin = KeySelector.FirstGreaterThan(TuPack.Pack(("SampleTs", beginRepr)));
    var end   = KeySelector.LastLessOrEqual(TuPack.Pack(("SampleTs", endRepr)));
    var range = await tr.GetRangeAsync(begin, end, FdbRangeOptions.KeysOnly);
    if (!range.IsEmpty)
    {
        var bytes = TuPack.Unpack(range.Last().Key.Span).Get<byte[]>(1);
        result = BinaryPrimitives.ReadUInt64BigEndian(bytes);
        // result = 6 (5, if FirstGreaterOrEqual)
    }
...

This is how I'm writing the values:

BinaryPrimitives. WriteUInt64BigEndian(intRepr,x);
tr.Set(TuPack.Pack(("SampleTs", intRepr)), Encoding.UTF8.GetBytes(x.ToString()));

    <PackageVersion Include="FoundationDB.Client" Version="7.3.2" />
    <PackageVersion Include="FoundationDB.Client.Native" Version="7.3.68" />

[KrzysFR]

Sorry, I'm out of office so I can't easily test things, but in my experience the most common issue with key selectors is forgetting that the end selector will select the key in the database at which the range read will STOP. Meaning that if you "select" an end key, the range will read everything that is before. If you need this key to be included, you need to select a key after. In you case, you could use FirstGreatherThan(....) instead.

Btw, you can use Slice.FromXYZ(...) to quickly create binary keys, instead of having to juggle with byte[] and BinaryPrimitives. There is a lot of overloads that will create keys of any size, little or big endian, fixed or variable size, utf8 strings, etc...