Desktop: probe for the TPM once, off the UI thread, instead of on every unlock

tests/DodoSSH.Client.App.Tests/ProbedDeviceKeyStoreTests.cs

@@ -0,0 +1,56 @@
+using DodoSSH.Client.App.Platform;
+using DodoSSH.Client.Session;
+using NSubstitute;
+
+namespace DodoSSH.Client.App.Tests;
+
+/// <summary>
+/// That the slow probe choosing a device key store runs once, and not on the thread that asked.
+/// </summary>
+/// <remarks>
+/// On Windows the probe creates and deletes a TPM key, and it used to run synchronously on the UI thread at
+/// startup and again after every unlock. See <see cref="ProbedDeviceKeyStore"/>.
+/// </remarks>
+public sealed class ProbedDeviceKeyStoreTests
+{
+    private static CancellationToken Token => TestContext.Current.CancellationToken;
+
+    [Fact]
+    public async Task TheChoiceIsMadeOnce_OffTheCallersThread()
+    {
+        var asking = Environment.CurrentManagedThreadId;
+        var probes = 0;
+        var probedOn = 0;
+
+        var inner = Substitute.For<IDeviceKeyStore>();
+
+        var store = new ProbedDeviceKeyStore(() =>
+        {
+            Interlocked.Increment(ref probes);
+            probedOn = Environment.CurrentManagedThreadId;
+            return inner;
+        });
+
+        (await store.IsAvailableAsync(Token)).ShouldBeTrue();
+        (await store.IsAvailableAsync(Token)).ShouldBeTrue();
+        await store.TryLoadAsync(Token);
+
+        probes.ShouldBe(1);
+        probedOn.ShouldNotBe(asking);
+
+        // Availability is the choice, not a second probe through the store it chose.
+        await inner.DidNotReceive().IsAvailableAsync(Arg.Any<CancellationToken>());
+        await inner.Received(1).TryLoadAsync(Arg.Any<CancellationToken>());
+    }
+
+    [Fact]
+    public async Task AMachineWithNoStore_IsUnavailable_AndForgettingIsHarmless()
+    {
+        var store = new ProbedDeviceKeyStore(static () => new UnavailableDeviceKeyStore());
+
+        (await store.IsAvailableAsync(Token)).ShouldBeFalse();
+        (await store.TryLoadAsync(Token)).ShouldBeNull();
+
+        await store.ForgetAsync(Token);
+    }
+}