Fix CompareFiles task

It's a cheap fix, in that it gives up streaming during the compare, but it's far more likely to be accurate than if I tried to maintain streaming while fixing the original bug of assuming that `Stream.Read` will always fill the buffer.

Fixes #825

Author: AArnott

src/Nerdbank.GitVersioning.Tasks/CompareFiles.cs

@@ -3,6 +3,7 @@
     using System;
     using System.Collections.Generic;
     using System.IO;
+    using System.IO.MemoryMappedFiles;
     using System.Linq;
     using System.Text;
     using Microsoft.Build.Framework;
@@ -64,66 +65,23 @@ private bool IsContentOfFilesTheSame(string file1, string file2)
             if (File.Exists(file1) ^ File.Exists(file2)) return false;
             // If both are missing, that's the same.
             if (!File.Exists(file1)) return true;
-            // If both are present, we need to do a content comparison.
-            using (FileStream fileStream1 = File.OpenRead(file1))
-            {
-                using (FileStream fileStream2 = File.OpenRead(file2))
-                {
-                    if (fileStream1.Length != fileStream2.Length) return false;
-                    byte[] buffer1 = new byte[4096];
-                    byte[] buffer2 = new byte[buffer1.Length];
-                    int bytesRead;
-                    do
-                    {
-                        bytesRead = fileStream1.Read(buffer1, 0, buffer1.Length);
-                        if (fileStream2.Read(buffer2, 0, buffer2.Length) != bytesRead)
-                        {
-                            // We should never get here since we compared file lengths, but
-                            // this is a sanity check.
-                            return false;
-                        }
-                        for (int i = 0; i < bytesRead; i++)
-                        {
-                            if (buffer1[i] != buffer2[i])
-                            {
-                                return false;
-                            }
-                        }
-                    } while (bytesRead == buffer1.Length);
-                }
-            }
 
-            return true;
-        }
+            if (new FileInfo(file1).Length != new FileInfo(file2).Length) return false;
 
-        /// <summary>
-        /// Tests whether a file is up to date with respect to another,
-        /// based on existence, last write time and file size.
-        /// </summary>
-        /// <param name="sourcePath">The source path.</param>
-        /// <param name="destPath">The dest path.</param>
-        /// <returns><c>true</c> if the files are the same; <c>false</c> if the files are different</returns>
-        internal static bool FastFileEqualityCheck(string sourcePath, string destPath)
-        {
-            FileInfo sourceInfo = new FileInfo(sourcePath);
-            FileInfo destInfo = new FileInfo(destPath);
+            // If both are present, we need to do a content comparison.
+            // Keep our comparison simple by loading both in memory.
+            byte[] file1Content = File.ReadAllBytes(file1);
+            byte[] file2Content = File.ReadAllBytes(file2);
 
-            if (sourceInfo.Exists ^ destInfo.Exists)
-            {
-                // Either the source file or the destination file is missing.
-                return false;
-            }
+            // One more sanity check.
+            if (file1Content.Length != file2Content.Length) return false;
 
-            if (!sourceInfo.Exists)
+            for (int i = 0; i < file1Content.Length; i++)
             {
-                // Neither file exists.
-                return true;
+                if (file1Content[i] != file2Content[i]) return false;
             }
 
-            // We'll say the files are the same if their modification date and length are the same.
-            return
-                sourceInfo.LastWriteTimeUtc == destInfo.LastWriteTimeUtc &&
-                sourceInfo.Length == destInfo.Length;
+            return true;
         }
     }
 }