Fix Process StartTime offset on Linux due to device sleeping (#25312)

* Fix Process StartTime offset on Linux due to device sleeping

* PR feedback

* uptimeFile -> UptimeFile

* remove Debug.Assert

* pal_io.c: revert unrelated fix

Author: tmds

src/System.Diagnostics.Process/src/System.Diagnostics.Process.csproj

@@ -435,6 +435,7 @@
     <Reference Include="System.Diagnostics.FileVersionInfo" />
     <Reference Include="System.Diagnostics.Tools" />
     <Reference Include="System.IO.FileSystem" />
+    <Reference Include="System.Memory" />
     <Reference Include="System.Resources.ResourceManager" />
     <Reference Include="System.Runtime" />
     <Reference Include="System.Runtime.Extensions" />

src/System.Diagnostics.Process/src/System/Diagnostics/Process.Linux.cs

@@ -2,8 +2,10 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System;
 using System.Collections.Generic;
 using System.ComponentModel;
+using System.Globalization;
 using System.IO;
 using System.Text;
 
@@ -57,6 +59,42 @@ internal DateTime StartTimeCore
             }
         }
 
+        /// <summary>Computes a time based on a number of ticks since boot.</summary>
+        /// <param name="timespanAfterBoot">The timespan since boot.</param>
+        /// <returns>The converted time.</returns>
+        internal static DateTime BootTimeToDateTime(TimeSpan timespanAfterBoot)
+        {
+            // Use the uptime and the current time to determine the absolute boot time.
+            DateTime bootTime = DateTime.UtcNow - Uptime;
+
+            // And use that to determine the absolute time for timespan.
+            DateTime dt = bootTime + timespanAfterBoot;
+
+            // The return value is expected to be in the local time zone.
+            // It is converted here (rather than starting with DateTime.Now) to avoid DST issues.
+            return dt.ToLocalTime();
+        }
+
+        /// <summary>Gets the elapsed time since the system was booted.</summary>
+        private static TimeSpan Uptime
+        {
+            get
+            {
+                // '/proc/uptime' accounts time a device spends in sleep mode.
+                const string UptimeFile = Interop.procfs.ProcUptimeFilePath;
+                string text = File.ReadAllText(UptimeFile);
+
+                double uptimeSeconds = 0;
+                int length = text.IndexOf(' ');
+                if (length != -1)
+                {
+                    Double.TryParse(text.AsReadOnlySpan().Slice(0, length), NumberStyles.AllowDecimalPoint, NumberFormatInfo.InvariantInfo, out uptimeSeconds);
+                }
+
+                return TimeSpan.FromSeconds(uptimeSeconds);
+            }
+        }
+
         /// <summary>Gets execution path</summary>
         private string GetPathToOpenFile()
         {

src/System.Diagnostics.Process/src/System/Diagnostics/Process.Unix.cs

@@ -473,23 +473,6 @@ internal static TimeSpan TicksToTimeSpan(double ticks)
             return TimeSpan.FromSeconds(ticks / (double)ticksPerSecond);
         }
 
-        /// <summary>Computes a time based on a number of ticks since boot.</summary>
-        /// <param name="timespanAfterBoot">The timespan since boot.</param>
-        /// <returns>The converted time.</returns>
-        internal static DateTime BootTimeToDateTime(TimeSpan timespanAfterBoot)
-        {
-            // Use the uptime and the current time to determine the absolute boot time. This implementation is relying on the 
-            // implementation detail that Stopwatch.GetTimestamp() uses a value based on time since boot.
-            DateTime bootTime = DateTime.UtcNow - TimeSpan.FromSeconds(Stopwatch.GetTimestamp() / (double)Stopwatch.Frequency);
-
-            // And use that to determine the absolute time for timespan.
-            DateTime dt = bootTime + timespanAfterBoot;
-
-            // The return value is expected to be in the local time zone.
-            // It is converted here (rather than starting with DateTime.Now) to avoid DST issues.
-            return dt.ToLocalTime();
-        }
-
         /// <summary>Opens a stream around the specified file descriptor and with the specified access.</summary>
         /// <param name="fd">The file descriptor.</param>
         /// <param name="access">The access mode.</param>