Implement SynchronousIOManager.WriteFileChecked

src/Tgstation.Server.Host/Core/SynchronousIOManager.cs

@@ -1,6 +1,9 @@
 using System;
 using System.Collections.Generic;
+using System.Globalization;
 using System.IO;
+using System.Linq;
+using System.Security.Cryptography;
 using System.Threading;
 
 namespace Tgstation.Server.Host.Core
@@ -36,7 +39,37 @@ namespace Tgstation.Server.Host.Core
 		/// <inheritdoc />
 		public bool WriteFileChecked(string path, byte[] data, string previousSha1, CancellationToken cancellationToken)
 		{
-			throw new NotImplementedException();
+			cancellationToken.ThrowIfCancellationRequested();
+			using (var file = File.Open(path, FileMode.OpenOrCreate, FileAccess.ReadWrite, FileShare.None))
+			{
+				using (var readMs = new MemoryStream())
+				{
+					cancellationToken.ThrowIfCancellationRequested();
+					file.CopyTo(readMs);
+					if (readMs.Length != 0 && previousSha1 == null)
+						return false;   //no sha1? no write
+					//suppressed due to only using for consistency checks
+#pragma warning disable CA5350 // Do not use insecure cryptographic algorithm SHA1.
+					using (var sha1 = new SHA1Managed())
+#pragma warning restore CA5350 // Do not use insecure cryptographic algorithm SHA1.
+					{
+						var sha1String = String.Join("", sha1.ComputeHash(readMs).Select(b => b.ToString("x2", CultureInfo.InvariantCulture)));
+						if (sha1String != previousSha1)
+							return false;
+					}
+				}
+
+				cancellationToken.ThrowIfCancellationRequested();
+				file.Seek(0, SeekOrigin.Begin);
+
+				cancellationToken.ThrowIfCancellationRequested();
+				file.SetLength(data.Length);
+
+				cancellationToken.ThrowIfCancellationRequested();
+				file.Write(data, 0, data.Length);
+
+				return true;
+			}
 		}
 	}
 }