Add BLite.NetStandard21.Tests: coverage for netstandard2.1 conditional code paths

[Copilot]

BLite.Core and BLite.Bson multi-target net10.0;netstandard2.1 with numerous #if branches, but the only test project targets net10.0 exclusively — leaving every #else path untested.

New project

tests/BLite.NetStandard21.Tests/ — targets netstandard2.1 only, compiles against the netstandard2.1 builds of BLite.Core and BLite.Bson (no BLite.Shared, which is net10.0-only).

InternalsVisibleTo

Added BLite.NetStandard21.Tests to BLite.Core.csproj to expose internal types (PageFile, StorageEngine, SlottedPageHeader, TimeSeriesPage, PriorityQueue polyfill).

Test coverage — one file per conditional path

File	Branch exercised
PageFileNetStandardTests	ReadPageAsync #else: Seek + SemaphoreSlim lock
StorageEngineNetStandardTests	ReadPageAsync WAL fast paths returning default instead of ValueTask.CompletedTask
ObjectIdNetStandardTests	NewObjectId() random via new Random(), ToString() via BitConverter.ToString(...).Replace("-","")
BsonWriterNetStandardTests	WriteDoubleInternal via WriteInt64LittleEndian(BitConverter.DoubleToInt64Bits(v))
PageHeaderNetStandardTests	WriteTo via MemoryMarshal.Write(destination, ref copy) on both PageHeader and SlottedPageHeader
TimeSeriesPageNetStandardTests	SetLastTimestamp/SetEntryCount via MemoryMarshal.Write(..., ref value)
IndexKeyNetStandardTests	ComputeHashCode via foreach (var b in data) hash.Add(b)
PolyfillsNetStandardTests	Internal PriorityQueue<TElement,TPriority> min-heap polyfill (guarded #if NETSTANDARD2_1)

All test code uses only netstandard2.1-compatible APIs — no Random.Shared, Convert.ToHexString, RandomAccess, or ValueTask.CompletedTask.

Original prompt

Problema

Le librerie di produzione (BLite, BLite.Core, BLite.Bson) fanno multi-targeting su net10.0;netstandard2.1, ma l'unico progetto di test esistente — BLite.Tests — target solo net10.0. Questo significa che nessun test viene mai eseguito contro il runtime netstandard2.1, nonostante esistano numerosi path di codice condizionali (#if NETSTANDARD2_1, #if NET6_0_OR_GREATER, #if NET5_0_OR_GREATER, #if NET8_0_OR_GREATER, ecc.) che attivano implementazioni alternative.

Obiettivo

Creare un nuovo progetto di test dedicato tests/BLite.NetStandard21.Tests/ che:

1. Target esclusivamente netstandard2.1
2. Copra solo i path critici specifici di quel runtime (non duplicare i test già coperti da BLite.Tests)
3. Sia eseguibile con dotnet test -f netstandard2.1 (tramite un runtime compatibile, es. .NET 5/6/8/9)

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Path critici da coprire (tutti con codice condizionale nel sorgente)

1. PageFile.ReadPageAsync — path #else (Seek + SemaphoreSlim)

File: src/BLite.Core/Storage/PageFile.cs

Su net10.0 usa RandomAccess.ReadAsync (lock-free). Su netstandard2.1 usa il path #else:

// netstandard2.1: FileStream.Seek + ReadAsync — not lock-free, serialize under _lock.
await _lock.WaitAsync(cancellationToken).ConfigureAwait(false);
_fileStream.Seek(offset, SeekOrigin.Begin);
bytesRead = await _fileStream.ReadAsync(slice, cancellationToken).ConfigureAwait(false);

Test da scrivere:

• ReadPageAsync_SingleRead_ReturnsCorrectData — scrive una pagina, la rilegge async e verifica i dati
• ReadPageAsync_ConcurrentReads_DoNotDeadlock — 20 reader concorrenti sulla stessa pagina, verifica che non ci sia deadlock o corruzione (il SemaphoreSlim serializza, non deve bloccarsi)
• ReadPageAsync_WithCancellation_ThrowsOperationCanceledException — cancellation token già cancellato deve lanciare OperationCanceledException

2. StorageEngine.ReadPageAsync — ValueTask.CompletedTask polyfill

File: src/BLite.Core/Storage/StorageEngine.Pages.cs

Su netstandard2.1 non esiste ValueTask.CompletedTask, quindi usa return default;:

#if NET5_0_OR_GREATER
    return ValueTask.CompletedTask;
#else
    return default; // netstandard2.1
#endif

Test da scrivere:

• ReadPageAsync_FromWalCache_ReturnsCorrectly_OnNetStandard — scrive in WAL cache e legge async; il ValueTask restituito dal path default deve essere awaitable correttamente
• ReadPageAsync_FromWalIndex_ReturnsCorrectly_OnNetStandard — stessa verifica per il path WAL index

3. ObjectId.NewObjectId() — generazione random

File: src/BLite.Bson/ObjectId.cs

Su net6.0+ usa Random.Shared.NextInt64(). Su netstandard2.1:

var random = (long)((ulong)new Random().Next() << 32 | (ulong)(uint)new Random().Next());

Test da scrivere:

• ObjectId_NewObjectId_IsUnique — genera 1000 ObjectId e verifica che siano tutti distinti
• ObjectId_NewObjectId_HasCurrentTimestamp — verifica che il timestamp dell'ObjectId sia vicino a DateTimeOffset.UtcNow
• ObjectId_ToString_ReturnsHexString — verifica il path #else di ToString() (usa BitConverter.ToString + Replace("-", "") invece di Convert.ToHexString)

4. ObjectId.ToString() — Convert.ToHexString polyfill

File: src/BLite.Bson/ObjectId.cs

#if NET5_0_OR_GREATER
    return Convert.ToHexString(bytes).ToLowerInvariant();
#else
    return BitConverter.ToString(bytes.ToArray()).Replace("-", "").ToLowerInvariant();
#endif

Test da scrivere:

• ObjectId_RoundTrip_ToString_Parse — serializza un ObjectId a stringa e verifica che il formato sia 24 caratteri hex lowercase
• ObjectId_KnownValue_ToString_MatchesExpected — crea un ObjectId con bytes noti e verifica che ToString() produca il valore hex atteso

5. BsonBufferWriter.WriteDoubleInternal — BinaryPrimitives.WriteDoubleLittleEndian

File: src/BLite.Bson/BsonBufferWriter.cs

Su netstandard2.1 usa:

BinaryPrimitives.WriteInt64LittleEndian(span, BitConverter.DoubleToInt64Bits(value));

Test da scrivere:

• BsonWriter_WriteDouble_RoundTrip — scrive un double nel BSON e lo rilegge, verificando identità di valore
• BsonWriter_WriteDouble_SpecialValues — testa double.NaN, double.PositiveInfinity, double.NegativeInfinity, double.MaxValue, double.MinValue

6. PageHeader.WriteTo e SlottedPageHeader.WriteTo — MemoryMarshal.Write con ref vs in

File: src/BLite.Core/Storage/PageHeader.cs, src/BLite.Core/Storage/SlottedPage.cs

Su netstandard2.1 usa ref copy invece di in this:

#if NET5_0_OR_GREATER
    MemoryMarshal.Write(destination, in this);
#else
    var copy = this;
    MemoryMarshal.Write(destination, ref copy);
#endif

Test da scrivere:

• PageHeader_WriteTo_ReadFrom_RoundTrip — crea un PageHeader con valori noti, lo scrive su uno span e lo rilegge con ReadFrom, verificando ogni campo
• `SlottedPageHeader_WriteTo_ReadFrom...

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