Warm up MSAL token pipeline at startup

Microsoft.Identity.Web builds its MergedOptions and the underlying MSAL
confidential-client application lazily on the first token request, and
that first initialization is not safe against a second concurrent
caller: a concurrent ISAR call can observe a half-initialized
configuration and fail with "No ClientId was specified.".

Add a MsalWarmupHostedService that acquires a single app token for the
ISAR downstream API before the server accepts requests, so the options
and confidential client are fully built once. A failed warm-up (no
network or invalid secret in local dev) is logged and never blocks
startup.

Author: olaals

backend/api/HostedServices/MsalWarmupHostedService.cs

@@ -0,0 +1,72 @@
+using Microsoft.Identity.Web;
+
+namespace Api.HostedServices
+{
+    /// <summary>
+    /// Warms up the Microsoft.Identity.Web token-acquisition pipeline at startup by
+    /// acquiring a single application (client-credentials) access token for the ISAR
+    /// downstream API.
+    ///
+    /// Microsoft.Identity.Web builds its <c>MergedOptions</c> and the underlying MSAL
+    /// confidential-client application lazily on the first token request. That first
+    /// initialization is not safe against a second concurrent caller: while one thread
+    /// is populating the merged options, another concurrent ISAR call can observe a
+    /// half-initialized configuration and fail with <c>"No ClientId was specified."</c>.
+    ///
+    /// Performing one token acquisition here — before the web server starts accepting
+    /// requests — ensures the options and confidential client are fully built once, so
+    /// the first real mission-scheduling requests never race that initialization.
+    /// </summary>
+    public class MsalWarmupHostedService(
+        ILogger<MsalWarmupHostedService> logger,
+        IServiceScopeFactory scopeFactory,
+        IConfiguration configuration
+    ) : IHostedService
+    {
+        private static readonly TimeSpan WarmupTimeout = TimeSpan.FromSeconds(30);
+
+        public async Task StartAsync(CancellationToken cancellationToken)
+        {
+            var scope = configuration["Isar:Scopes:0"];
+            if (string.IsNullOrWhiteSpace(scope))
+            {
+                logger.LogWarning(
+                    "Skipping MSAL warm-up: no ISAR scope configured under 'Isar:Scopes'."
+                );
+                return;
+            }
+
+            using var timeoutCts = CancellationTokenSource.CreateLinkedTokenSource(
+                cancellationToken
+            );
+            timeoutCts.CancelAfter(WarmupTimeout);
+
+            try
+            {
+                using var serviceScope = scopeFactory.CreateScope();
+                var tokenAcquisition =
+                    serviceScope.ServiceProvider.GetRequiredService<ITokenAcquisition>();
+
+                // Triggers the one-time build of MergedOptions and the MSAL confidential
+                // client used for all subsequent app-token calls to ISAR.
+                await tokenAcquisition.GetAccessTokenForAppAsync(scope);
+
+                logger.LogInformation(
+                    "MSAL token-acquisition pipeline warmed up for ISAR downstream API."
+                );
+            }
+            catch (Exception e)
+            {
+                // A failed warm-up (e.g. no network or invalid secret in local dev) is not
+                // fatal: the merged options are still initialized as a side effect, and the
+                // token will be acquired lazily on first real use. Never block startup.
+                logger.LogWarning(
+                    e,
+                    "MSAL warm-up did not complete successfully. Continuing startup; the token will be acquired on first use."
+                );
+            }
+        }
+
+        public Task StopAsync(CancellationToken cancellationToken) => Task.CompletedTask;
+    }
+}

backend/api/Program.cs

@@ -107,6 +107,7 @@
 
 builder.Services.AddSingleton<EventAggregatorSingletonService>();
 
+builder.Services.AddHostedService<MsalWarmupHostedService>();
 builder.Services.AddHostedService<MqttEventHandler>();
 builder.Services.AddHostedService<MissionEventHandler>();
 builder.Services.AddHostedService<MqttService>();