Server: stop a team being archived while a vault is created in it

src/DodoSSH.Api/Features/Teams/TeamLock.cs

@@ -0,0 +1,27 @@
+using DodoSSH.Infrastructure;
+using Microsoft.EntityFrameworkCore;
+
+namespace DodoSSH.Api.Features.Teams;
+
+/// <summary>
+/// Serialises the writes that decide what a team owns, for the life of the transaction.
+/// </summary>
+/// <remarks>
+/// <para>
+/// Archiving a team refuses while it owns a vault, and creating a vault refuses on an archived team — but each
+/// used to check outside any transaction the other could see, so the two could pass each other: the archive
+/// counted no vaults, a create landed a moment later, and the team was archived with a live vault in it that
+/// nobody could reach. Both now take this lock inside their transaction and check again under it.
+/// </para>
+/// <para>
+/// The same advisory lock the sync push takes per vault, keyed apart so a team and a vault never share one.
+/// Requires <c>Multiplexing=false</c> on the connection, as that one does.
+/// </para>
+/// </remarks>
+internal static class TeamLock
+{
+    internal static Task<int> AcquireAsync(DodoDbContext database, Guid teamId, CancellationToken cancellationToken) =>
+        database.Database.ExecuteSqlAsync(
+            $"SELECT pg_advisory_xact_lock(hashtextextended({"team:" + teamId.ToString()}, 0))",
+            cancellationToken);
+}