fix(postgresql): seed Npgsql's global type mapper before the test suite runs (weasel#398)

src/Weasel.Postgresql.Tests/CommandExtensionsTests.cs

@@ -21,11 +21,42 @@ public void add_first_parameter()
         param.Value.ShouldBe("a");
         param.ParameterName.ShouldBe("p0");
 
+        // AddParameter was given no explicit type, so this asserts *Npgsql's* inference from
+        // the value rather than anything Weasel stamped on. That inference reads Npgsql's
+        // process-global type mapper and answers Unknown until the mapper has been seeded,
+        // which TestSetup does once at module load. Do not delete that warm-up. weasel#398.
         param.NpgsqlDbType.ShouldBe(NpgsqlDbType.Text);
 
         command.Parameters.ShouldContain(param);
     }
 
+    [Fact]
+    public void add_parameter_honors_an_explicit_type()
+    {
+        var command = new NpgsqlCommand();
+
+        var param = command.AddParameter("a", NpgsqlDbType.Varchar);
+
+        // The explicit-type path is the one Weasel actually controls, so unlike the test
+        // above it holds no matter what state Npgsql's global type mapper is in.
+        param.NpgsqlDbType.ShouldBe(NpgsqlDbType.Varchar);
+    }
+
+    [Fact]
+    public void add_parameter_without_a_type_defers_to_npgsql_rather_than_weasels_mapping()
+    {
+        var command = new NpgsqlCommand();
+
+        var param = command.AddParameter(new DateTime(2026, 7, 30, 12, 0, 0, DateTimeKind.Utc));
+
+        // Weasel must NOT start stamping its own CLR-type mapping onto untyped parameters.
+        // Weasel maps DateTime to "timestamp without time zone" for every value, while
+        // Npgsql resolves per value: a Kind=Utc DateTime is "timestamp with time zone", and
+        // writing one as "timestamp without time zone" throws at execution time. weasel#398.
+        param.NpgsqlDbType.ShouldBe(NpgsqlDbType.TimestampTz);
+        Instance.ToParameterType(typeof(DateTime)).ShouldBe(NpgsqlDbType.Timestamp);
+    }
+
     [Fact]
     public void add_second_parameter()
     {

src/Weasel.Postgresql.Tests/TestSetup.cs

@@ -1,4 +1,5 @@
 using System.Runtime.CompilerServices;
+using Npgsql;
 
 namespace Weasel.Postgresql.Tests;
 
@@ -23,5 +24,36 @@ internal static void Initialize()
         {
             PostgresqlProvider.Instance.UseCaseSensitiveQualifiedNames = useCaseSensitiveQualifiedNames;
         }
+
+        WarmUpNpgsqlTypeInference();
+    }
+
+    /// <summary>
+    /// Populate Npgsql's process-global type mapper before any test runs.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// When a parameter is added without an explicit type, Weasel leaves
+    /// <see cref="NpgsqlParameter.NpgsqlDbType" /> unset and lets Npgsql infer it from the
+    /// value. That getter answers out of Npgsql's process-global type mapper, which stays
+    /// empty until the process constructs its first <see cref="NpgsqlDataSource" /> or
+    /// <see cref="NpgsqlConnection" /> with a connection string — until then it reports
+    /// <c>NpgsqlDbType.Unknown</c> for every value.
+    /// </para>
+    /// <para>
+    /// So a test asserting an inferred parameter type silently depends on some *other*
+    /// test in the same process having built a data source first. Under xUnit v3's parallel
+    /// collections that is a race, and it is exactly what made
+    /// <c>CommandExtensionsTests.add_first_parameter</c> fail in three of the four Postgres
+    /// CI jobs and then pass on an unchanged re-run. See weasel#398.
+    /// </para>
+    /// <para>
+    /// Constructing a builder is enough to seed the global mapper; it opens no sockets and
+    /// needs no connection string, so it is safe to do unconditionally at load.
+    /// </para>
+    /// </remarks>
+    private static void WarmUpNpgsqlTypeInference()
+    {
+        _ = new NpgsqlDataSourceBuilder();
     }
 }