Use constants in KesterlServerOptionsSystemdExtensions

Author: halter73

src/Microsoft.AspNetCore.Server.Kestrel/Systemd/KesterlServerOptionsSystemdExtensions.cs

@@ -10,6 +10,10 @@ namespace Microsoft.AspNetCore.Hosting
 {
     public static class KesterlServerOptionsSystemdExtensions
     {
+        // SD_LISTEN_FDS_START https://www.freedesktop.org/software/systemd/man/sd_listen_fds.html
+        private const ulong SdListenFdsStart = 3;
+        private const string ListenPidEnvVar = "LISTEN_PID";
+
         /// <summary>
         /// Open file descriptor (SD_LISTEN_FDS_START) initialized by systemd socket-based activation logic if available.
         /// </summary>
@@ -30,10 +34,9 @@ public static KestrelServerOptions UseSystemd(this KestrelServerOptions options)
         /// </returns>
         public static KestrelServerOptions UseSystemd(this KestrelServerOptions options, Action<ListenOptions> configure)
         {
-            if (string.Equals(Process.GetCurrentProcess().Id.ToString(CultureInfo.InvariantCulture), Environment.GetEnvironmentVariable("LISTEN_PID"), StringComparison.Ordinal))
+            if (string.Equals(Process.GetCurrentProcess().Id.ToString(CultureInfo.InvariantCulture), Environment.GetEnvironmentVariable(ListenPidEnvVar), StringComparison.Ordinal))
             {
-                // SD_LISTEN_FDS_START = 3
-                options.ListenHandle(3, configure);
+                options.ListenHandle(SdListenFdsStart, configure);
             }
 
             return options;