test: create each SQL Server test database once, as rows arriving together would race it

test/CodoMetis.TypeKit.EntityFrameworkCore.Tests/SqlServer.cs

@@ -32,7 +32,10 @@ public sealed class SqlServer : IAsyncLifetime
             .WithWaitStrategy(Wait.ForUnixContainer().UntilMessageIsLogged("SQL Server is now ready for client connections"))
             .Build();
 
-    private readonly ConcurrentDictionary<string, Task<string>> _databases = new();
+    // Lazy, because GetOrAdd may run its factory more than once for callers that arrive together, and two of them
+    // would drop and create the same database under each other: with the project's rows run in parallel, the first
+    // rows of a sweep did, "Database 'typekit_queries' already exists" (#251).
+    private readonly ConcurrentDictionary<string, Lazy<Task<string>>> _databases = new();
 
     private string ConnectionString(string database) => new SqlConnectionStringBuilder
     {
@@ -44,7 +47,7 @@ public sealed class SqlServer : IAsyncLifetime
     }.ConnectionString;
 
     /// <summary>A database of its own on the server, created empty the first time it is asked for, and its connection string.</summary>
-    public Task<string> DatabaseAsync(string name) => _databases.GetOrAdd(name, CreateAsync);
+    public Task<string> DatabaseAsync(string name) => _databases.GetOrAdd(name, key => new Lazy<Task<string>>(() => CreateAsync(key))).Value;
 
     private async Task<string> CreateAsync(string name)
     {

test/CodoMetis.TypeKit.EntityFrameworkCore.Tests/SqlServerFixtureTests.cs

@@ -0,0 +1,31 @@
+using Microsoft.Data.SqlClient;
+
+namespace CodoMetis.TypeKit.EntityFrameworkCore.Tests;
+
+/// <summary>
+/// A guard on the harness: a database many callers ask for at once is created once, as the rows of a sweep ask for
+/// it when they run in parallel (#251). Created more than once, one caller drops it under another, "Database
+/// 'typekit_queries' already exists", or after another has returned, and every row reading it fails.
+/// </summary>
+[Collection(SqlServerCollection.Name)]
+[Trait("Needs", "Docker")]
+public sealed class SqlServerFixtureTests(SqlServer server)
+{
+    [Fact]
+    public async Task A_database_asked_for_by_many_callers_at_once_is_created_once()
+    {
+        var name = $"typekit_fixture_{Guid.NewGuid():N}";
+
+        var answers = await Task.WhenAll(Enumerable.Range(0, 32).Select(_ => Task.Run(() => server.DatabaseAsync(name))));
+
+        answers.Distinct().ShouldHaveSingleItem();
+
+        // A creation that lost the race could still drop the database after the winner returned; give it the time.
+        await Task.Delay(TimeSpan.FromSeconds(2), TestContext.Current.CancellationToken);
+        await using var connection = new SqlConnection(answers[0]);
+        await connection.OpenAsync(TestContext.Current.CancellationToken);
+        await using var command = connection.CreateCommand();
+        command.CommandText = "SELECT DB_NAME()";
+        (await command.ExecuteScalarAsync(TestContext.Current.CancellationToken)).ShouldBe(name);
+    }
+}