Detect systemd when inside a container. (#67288)

* Detect systemd when inside a container.

Modify IsSystemdService to function correctly
for containerised services.

Fix #67203

Adds conditional compilation to fall back
to Process.GetCurrentProcess().Id when
Environment.ProcessId is not available.

* Apply suggestions from code review

The minimum supported .NETCoreApp version in the repository is 6.0, so just using NETCOREAPP.

Co-authored-by: james <judilsteve>
Co-authored-by: NEric Erhardt <eric.erhardt@microsoft.com>

src/libraries/Microsoft.Extensions.Hosting.Systemd/src/SystemdHelpers.cs

@@ -5,6 +5,9 @@
 using System.Globalization;
 using System.IO;
 using System.Text;
+#if !NETCOREAPP
+using System.Diagnostics;
+#endif
 
 namespace Microsoft.Extensions.Hosting.Systemd
 {
@@ -20,9 +23,9 @@ public static partial class SystemdHelpers
         /// </summary>
         /// <returns><c>True</c> if the current process is hosted as a systemd Service, otherwise <c>false</c>.</returns>
         public static bool IsSystemdService()
-            => _isSystemdService ?? (bool)(_isSystemdService = CheckParentIsSystemd());
+            => _isSystemdService ??= GetIsSystemdService();
 
-        private static bool CheckParentIsSystemd()
+        private static bool GetIsSystemdService()
         {
             // No point in testing anything unless it's Unix
             if (Environment.OSVersion.Platform != PlatformID.Unix)
@@ -30,6 +33,21 @@ private static bool CheckParentIsSystemd()
                 return false;
             }
 
+            // To support containerized systemd services, check if we're the main process (PID 1)
+            // and if there are systemd environment variables defined for notifying the service
+            // manager, or passing listen handles.
+#if NETCOREAPP
+            int processId = Environment.ProcessId;
+#else
+            int processId = Process.GetCurrentProcess().Id;
+#endif
+
+            if (processId == 1)
+            {
+                return !string.IsNullOrEmpty(Environment.GetEnvironmentVariable("NOTIFY_SOCKET")) ||
+                       !string.IsNullOrEmpty(Environment.GetEnvironmentVariable("LISTEN_PID"));
+            }
+
             try
             {
                 // Check whether our direct parent is 'systemd'.