devproxy logs: say which instance is shown when multiple are running

DevProxy.Tests/LogsCommandTests.cs

@@ -0,0 +1,59 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using DevProxy.Commands;
+using DevProxy.State;
+
+using Xunit;
+
+namespace DevProxy.Tests;
+
+public class LogsCommandTests
+{
+    [Fact]
+    public void GetMultipleInstancesNotice_SingleInstance_ReturnsNull()
+    {
+        var state = new ProxyInstanceState { Pid = 100 };
+
+        Assert.Null(LogsCommand.GetMultipleInstancesNotice(state, [state]));
+    }
+
+    [Fact]
+    public void GetMultipleInstancesNotice_MultipleInstances_ListsSelectedAndOtherPids()
+    {
+        var now = DateTimeOffset.UtcNow;
+        var systemProxy = new ProxyInstanceState { Pid = 100, AsSystemProxy = true, StartedAt = now.AddMinutes(-10) };
+        var other1 = new ProxyInstanceState { Pid = 200, AsSystemProxy = false, StartedAt = now };
+        var other2 = new ProxyInstanceState { Pid = 300, AsSystemProxy = false, StartedAt = now.AddMinutes(-5) };
+        List<ProxyInstanceState> states = [other1, systemProxy, other2];
+
+        var selected = StateManager.SelectPrimaryState(states);
+        Assert.Same(systemProxy, selected);
+
+        var notice = LogsCommand.GetMultipleInstancesNotice(selected!, states);
+
+        Assert.NotNull(notice);
+        Assert.Contains("PID 100 (system proxy instance)", notice, StringComparison.Ordinal);
+        Assert.Contains("Other instances: 200, 300", notice, StringComparison.Ordinal);
+        Assert.Contains("--pid", notice, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void GetMultipleInstancesNotice_NoSystemProxy_SelectsMostRecentlyStarted()
+    {
+        var now = DateTimeOffset.UtcNow;
+        var older = new ProxyInstanceState { Pid = 100, AsSystemProxy = false, StartedAt = now.AddMinutes(-10) };
+        var newer = new ProxyInstanceState { Pid = 200, AsSystemProxy = false, StartedAt = now };
+        List<ProxyInstanceState> states = [older, newer];
+
+        var selected = StateManager.SelectPrimaryState(states);
+        Assert.Same(newer, selected);
+
+        var notice = LogsCommand.GetMultipleInstancesNotice(selected!, states);
+
+        Assert.NotNull(notice);
+        Assert.Contains("PID 200 (most recently started instance)", notice, StringComparison.Ordinal);
+        Assert.Contains("Other instances: 100.", notice, StringComparison.Ordinal);
+    }
+}

DevProxy/Commands/LogsCommand.cs

@@ -62,12 +62,19 @@ private async Task<int> RunAsync(ParseResult parseResult, CancellationToken canc
         }
         else
         {
-            state = await StateManager.LoadStateAsync(cancellationToken);
+            var states = await StateManager.LoadAllStatesAsync(cancellationToken);
+            state = StateManager.SelectPrimaryState(states);
             if (state is null)
             {
                 Console.WriteLine("Dev Proxy is not running.");
                 return 1;
             }
+
+            var notice = GetMultipleInstancesNotice(state, states);
+            if (notice is not null)
+            {
+                await Console.Error.WriteLineAsync(notice);
+            }
         }
 
         var logFile = state.LogFile;
@@ -92,6 +99,22 @@ private async Task<int> RunAsync(ParseResult parseResult, CancellationToken canc
         return 0;
     }
 
+    internal static string? GetMultipleInstancesNotice(ProxyInstanceState selected, IReadOnlyCollection<ProxyInstanceState> states)
+    {
+        var otherPids = states
+            .Where(s => s.Pid != selected.Pid)
+            .Select(s => s.Pid.ToString(CultureInfo.InvariantCulture))
+            .ToList();
+        if (otherPids.Count == 0)
+        {
+            return null;
+        }
+
+        var selectedDescription = selected.AsSystemProxy ? "system proxy instance" : "most recently started instance";
+        return $"Multiple Dev Proxy instances are running. Showing logs from PID {selected.Pid} ({selectedDescription}). " +
+            $"Other instances: {string.Join(", ", otherPids)}. Use --pid to show logs from a specific instance.";
+    }
+
     private static DateTime? ParseSinceOption(string? since)
     {
         if (string.IsNullOrEmpty(since))

DevProxy/State/StateManager.cs

@@ -104,6 +104,15 @@ public static Task SaveStateAsync(ProxyInstanceState state, CancellationToken ca
     public static async Task<ProxyInstanceState?> LoadStateAsync(CancellationToken cancellationToken = default)
     {
         var states = await LoadAllStatesAsync(cancellationToken);
+        return SelectPrimaryState(states);
+    }
+
+    /// <summary>
+    /// Selects the primary instance from the given states.
+    /// Prefers the system-proxy instance, then the most recently started instance.
+    /// </summary>
+    public static ProxyInstanceState? SelectPrimaryState(IEnumerable<ProxyInstanceState> states)
+    {
         return states
             .OrderByDescending(s => s.AsSystemProxy ? 1 : 0)
             .ThenByDescending(s => s.StartedAt)