Add UUID version 7 as the default guid generator

src/EFCore.PG/ValueGeneration/Internal/NpgsqlUuid7ValueGenerator.cs

@@ -0,0 +1,107 @@
+using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
+
+namespace Npgsql.EntityFrameworkCore.PostgreSQL.ValueGeneration.Internal;
+
+/// <summary>
+///     This API supports the Entity Framework Core infrastructure and is not intended to be used
+///     directly from your code. This API may change or be removed in future releases.
+/// </summary>
+public class NpgsqlUuid7ValueGenerator : ValueGenerator<Guid>
+{
+    /// <summary>
+    ///     This API supports the Entity Framework Core infrastructure and is not intended to be used
+    ///     directly from your code. This API may change or be removed in future releases.
+    /// </summary>
+    public override bool GeneratesTemporaryValues => false;
+
+    /// <summary>
+    ///     This API supports the Entity Framework Core infrastructure and is not intended to be used
+    ///     directly from your code. This API may change or be removed in future releases.
+    /// </summary>
+    public override Guid Next(EntityEntry entry) => BorrowedFromNet9.CreateVersion7(timestamp: DateTimeOffset.UtcNow);
+
+    // Code borrowed from .NET 9 should be removed as soon as the target framework includes such code
+    #region Borrowed from .NET 9
+
+#pragma warning disable IDE0007 // Use implicit type -- Avoid changes to code borrowed from BCL
+
+    // https://github.com/dotnet/runtime/blob/f402418aaed508c1d77e41b942e3978675183bfc/src/libraries/System.Private.CoreLib/src/System/Guid.cs
+    internal static class BorrowedFromNet9
+    {
+        private const byte Variant10xxMask = 0xC0;
+        private const byte Variant10xxValue = 0x80;
+
+        private const ushort VersionMask = 0xF000;
+        private const ushort Version7Value = 0x7000;
+
+        /// <summary>Creates a new <see cref="Guid" /> according to RFC 9562, following the Version 7 format.</summary>
+        /// <returns>A new <see cref="Guid" /> according to RFC 9562, following the Version 7 format.</returns>
+        /// <remarks>
+        ///     <para>This uses <see cref="DateTimeOffset.UtcNow" /> to determine the Unix Epoch timestamp source.</para>
+        ///     <para>This seeds the rand_a and rand_b sub-fields with random data.</para>
+        /// </remarks>
+        public static Guid CreateVersion7() => CreateVersion7(DateTimeOffset.UtcNow);
+
+        /// <summary>Creates a new <see cref="Guid" /> according to RFC 9562, following the Version 7 format.</summary>
+        /// <param name="timestamp">The date time offset used to determine the Unix Epoch timestamp.</param>
+        /// <returns>A new <see cref="Guid" /> according to RFC 9562, following the Version 7 format.</returns>
+        /// <exception cref="ArgumentOutOfRangeException"><paramref name="timestamp" /> represents an offset prior to <see cref="DateTimeOffset.UnixEpoch" />.</exception>
+        /// <remarks>
+        ///     <para>This seeds the rand_a and rand_b sub-fields with random data.</para>
+        /// </remarks>
+        public static Guid CreateVersion7(DateTimeOffset timestamp)
+        {
+            // NewGuid uses CoCreateGuid on Windows and Interop.GetCryptographicallySecureRandomBytes on Unix to get
+            // cryptographically-secure random bytes. We could use Interop.BCrypt.BCryptGenRandom to generate the random
+            // bytes on Windows, as is done in RandomNumberGenerator, but that's measurably slower than using CoCreateGuid.
+            // And while CoCreateGuid only generates 122 bits of randomness, the other 6 bits being for the version / variant
+            // fields, this method also needs those bits to be non-random, so we can just use NewGuid for efficiency.
+            var result = Guid.NewGuid();
+
+            // 2^48 is roughly 8925.5 years, which from the Unix Epoch means we won't
+            // overflow until around July of 10,895. So there isn't any need to handle
+            // it given that DateTimeOffset.MaxValue is December 31, 9999. However, we
+            // can't represent timestamps prior to the Unix Epoch since UUIDv7 explicitly
+            // stores a 48-bit unsigned value, so we do need to throw if one is passed in.
+
+            var unix_ts_ms = timestamp.ToUnixTimeMilliseconds();
+            ArgumentOutOfRangeException.ThrowIfNegative(unix_ts_ms, nameof(timestamp));
+
+            ref var resultClone = ref Unsafe.As<Guid, GuidDoppleganger>(ref result); // Deviation from BLC: Reinterpret Guid as our own type so that we can manipulate its private fields
+
+            Unsafe.AsRef(in resultClone._a) = (int)(unix_ts_ms >> 16);
+            Unsafe.AsRef(in resultClone._b) = (short)unix_ts_ms;
+
+            Unsafe.AsRef(in resultClone._c) = (short)(resultClone._c & ~VersionMask | Version7Value);
+            Unsafe.AsRef(in resultClone._d) = (byte)(resultClone._d & ~Variant10xxMask | Variant10xxValue);
+
+            return result;
+        }
+    }
+
+    /// <summary>
+    /// Used to manipulate the private fields of a <see cref="Guid"/> like its internal methods do, by treating a <see cref="Guid"/> as a <see cref="GuidDoppleganger"/>.
+    /// </summary>
+    [StructLayout(LayoutKind.Sequential)]
+    internal readonly struct GuidDoppleganger
+    {
+#pragma warning disable IDE1006 // Naming Styles -- Avoid further changes to code borrowed from BCL when working with the current type
+        internal readonly int _a;   // Do not rename (binary serialization)
+        internal readonly short _b; // Do not rename (binary serialization)
+        internal readonly short _c; // Do not rename (binary serialization)
+        internal readonly byte _d;  // Do not rename (binary serialization)
+        internal readonly byte _e;  // Do not rename (binary serialization)
+        internal readonly byte _f;  // Do not rename (binary serialization)
+        internal readonly byte _g;  // Do not rename (binary serialization)
+        internal readonly byte _h;  // Do not rename (binary serialization)
+        internal readonly byte _i;  // Do not rename (binary serialization)
+        internal readonly byte _j;  // Do not rename (binary serialization)
+        internal readonly byte _k;  // Do not rename (binary serialization)
+#pragma warning restore IDE1006 // Naming Styles
+    }
+
+#pragma warning restore IDE0007 // Use implicit type
+
+    #endregion
+}

src/EFCore.PG/ValueGeneration/Internal/NpgsqlValueGeneratorSelector.cs

@@ -104,6 +104,6 @@ public override bool TrySelect(IProperty property, ITypeBase typeBase, out Value
         => property.ClrType.UnwrapNullableType() == typeof(Guid)
             ? property.ValueGenerated == ValueGenerated.Never || property.GetDefaultValueSql() is not null
                 ? new TemporaryGuidValueGenerator()
-                : new GuidValueGenerator()
+                : new NpgsqlUuid7ValueGenerator()
             : base.FindForType(property, typeBase, clrType);
 }

test/EFCore.PG.Tests/NpgsqlValueGeneratorSelectorTest.cs

@@ -21,7 +21,7 @@
         AssertGenerator<TemporaryByteValueGenerator>("NullableByte");
         AssertGenerator<TemporaryDecimalValueGenerator>("Decimal");
         AssertGenerator<StringValueGenerator>("String");
-        AssertGenerator<GuidValueGenerator>("Guid");
+        AssertGenerator<NpgsqlUuid7ValueGenerator>("Guid");
         AssertGenerator<BinaryValueGenerator>("Binary");
     }
 
@@ -128,7 +128,7 @@
         AssertGenerator<NpgsqlSequenceHiLoValueGenerator<long>>("NullableLong", setSequences: true);
         AssertGenerator<NpgsqlSequenceHiLoValueGenerator<short>>("NullableShort", setSequences: true);
         AssertGenerator<StringValueGenerator>("String", setSequences: true);
-        AssertGenerator<GuidValueGenerator>("Guid", setSequences: true);
+        AssertGenerator<NpgsqlUuid7ValueGenerator>("Guid", setSequences: true);
         AssertGenerator<BinaryValueGenerator>("Binary", setSequences: true);
     }
 
@@ -210,4 +210,19 @@
         public override bool GeneratesTemporaryValues
             => false;
     }
+
+    [Fact]
+    public void NpgsqlUuid7ValueGenerator_creates_uuidv7()
+    {
+        var dtoNow = DateTimeOffset.UtcNow;
+        var net9Internal = Guid.CreateVersion7(dtoNow);
+        var custom = NpgsqlUuid7ValueGenerator.BorrowedFromNet9.CreateVersion7(dtoNow);
+        var bytenet9 = net9Internal.ToByteArray().AsSpan(0, 6);
+        var bytecustom = custom.ToByteArray().AsSpan(0, 6);
+        Assert.Equal(bytenet9, bytecustom);
+        Assert.Equal(7, net9Internal.Version);
+        Assert.Equal(net9Internal.Version, custom.Version);
+        Assert.InRange(net9Internal.Variant, 8, 0xB);
+        Assert.InRange(custom.Variant, 8, 0xB);
+    }
 }