Implement process CPU usage measurement

src/Tgstation.Server.Host/Components/Session/SessionController.cs

@@ -519,6 +519,10 @@ namespace Tgstation.Server.Host.Components.Session
 			return process.CreateDump(outputFile, minidump, cancellationToken);
 		}
 
+		/// <inheritdoc />
+		public double MeasureProcessorTimeDelta()
+			=> process.MeasureProcessorTimeDelta();
+
 		/// <summary>
 		/// The <see cref="Task{TResult}"/> for <see cref="LaunchResult"/>.
 		/// </summary>

src/Tgstation.Server.Host/System/IProcessBase.cs

@@ -24,6 +24,12 @@ namespace Tgstation.Server.Host.System
 		/// </summary>
 		long? MemoryUsage { get; }
 
+		/// <summary>
+		/// Gets the estimated CPU usage fraction of the process based on the last time this was called.
+		/// </summary>
+		/// <returns>The CPU's estimated usage as a value between 0 and 1.</returns>
+		double MeasureProcessorTimeDelta();
+
 		/// <summary>
 		/// Set's the owned <see cref="global::System.Diagnostics.Process.PriorityClass"/> to a non-normal value.
 		/// </summary>

src/Tgstation.Server.Host/System/Process.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Diagnostics;
 using System.Threading;
 using System.Threading.Tasks;
@@ -87,6 +87,26 @@ namespace Tgstation.Server.Host.System
 		/// </summary>
 		readonly Task<string?>? readTask;
 
+		/// <summary>
+		/// <see langword="lock"/> object for measuring processor time usage.
+		/// </summary>
+		readonly object processTimeMeasureLock;
+
+		/// <summary>
+		/// The last time <see cref="MeasureProcessorTimeDelta"/> was called.
+		/// </summary>
+		DateTimeOffset lastProcessorMeasureTime;
+
+		/// <summary>
+		/// The last value of <see cref="global::System.Diagnostics.Process.TotalProcessorTime"/>.
+		/// </summary>
+		TimeSpan lastProcessorUsageTime;
+
+		/// <summary>
+		/// The last valid return value of <see cref="MeasureProcessorTimeDelta"/>.
+		/// </summary>
+		double lastProcessorUsageEstimation;
+
 		/// <summary>
 		/// If the <see cref="Process"/> was disposed.
 		/// </summary>
@@ -123,6 +143,8 @@ namespace Tgstation.Server.Host.System
 
 			this.logger = logger ?? throw new ArgumentNullException(nameof(logger));
 
+			processTimeMeasureLock = new object();
+
 			Lifetime = WrapLifetimeTask();
 
 			if (preExisting)
@@ -147,6 +169,8 @@ namespace Tgstation.Server.Host.System
 				DefaultIOManager.BlockingTaskCreationOptions,
 				TaskScheduler.Current);
 
+			MeasureProcessorTimeDelta();
+
 			logger.LogTrace("Created process ID: {pid}", Id);
 		}
 
@@ -267,6 +291,34 @@ namespace Tgstation.Server.Host.System
 			return processFeatures.CreateDump(handle, outputFile, minidump, cancellationToken);
 		}
 
+		/// <inheritdoc />
+		public double MeasureProcessorTimeDelta()
+		{
+			lock (processTimeMeasureLock)
+			{
+				try
+				{
+					var now = DateTimeOffset.UtcNow;
+					var newTime = handle.TotalProcessorTime;
+
+					var timeDelta = now - lastProcessorMeasureTime;
+					var newUsage = newTime - lastProcessorUsageTime;
+
+					lastProcessorMeasureTime = now;
+					lastProcessorUsageTime = newTime;
+
+					if (timeDelta != TimeSpan.Zero)
+						lastProcessorUsageEstimation = newUsage / (Environment.ProcessorCount * timeDelta);
+				}
+				catch (Exception ex)
+				{
+					logger.LogWarning(ex, "Error measuring processor time delta!");
+				}
+
+				return lastProcessorUsageEstimation;
+			}
+		}
+
 		/// <summary>
 		/// Attaches a log message to the process' exit event.
 		/// </summary>