FileSystem.Unix.File.Move uses "rename" in more cases (#40611) (#395)

* FileSystem.Unix.File.Move uses "rename" in more cases (#40611)

* FileSystem.Unix.File.Move use rename in more cases, avoid link/copy when possible, improve performance on file systems that do not support hard links, such as FAT

* Adapt Unit Tests for accounting FileSystemWatcher events fired by FileSystem.Unix.File.Move implementation that use rename in more cases, avoiding link/copy when possible

* [FileSystem] ReAdd check for same device

Co-authored-by: Sylvain <sf@ellisys.com>
Co-authored-by: Mitchell Hwang <mihw@microsoft.com>

2 files changed, 15 insertions, 14 deletions

diff --git a/src/System.IO.FileSystem.Watcher/tests/FileSystemWatcher.File.Move.cs b/src/System.IO.FileSystem.Watcher/tests/FileSystemWatcher.File.Move.cs
index be9d25490f..e772c85dae 100644
--- a/src/System.IO.FileSystem.Watcher/tests/FileSystemWatcher.File.Move.cs
+++ b/src/System.IO.FileSystem.Watcher/tests/FileSystemWatcher.File.Move.cs
@@ -22,7 +22,7 @@ namespace System.IO.Tests
         [PlatformSpecific(TestPlatforms.AnyUnix)]  // Expected WatcherChangeTypes are different based on OS
         public void Unix_File_Move_To_Same_Directory()
         {
-            FileMove_SameDirectory(WatcherChangeTypes.Created | WatcherChangeTypes.Deleted);
+            FileMove_SameDirectory(WatcherChangeTypes.Renamed);
         }
 
         [Fact]
@@ -73,7 +73,7 @@ namespace System.IO.Tests
         [PlatformSpecific(TestPlatforms.OSX)]  // Expected WatcherChangeTypes are different based on OS
         public void OSX_File_Move_To_Different_Watched_Directory()
         {
-            FileMove_DifferentWatchedDirectory(WatcherChangeTypes.Changed);
+            FileMove_DifferentWatchedDirectory(0);
         }
 
         [Fact]
@@ -104,7 +104,7 @@ namespace System.IO.Tests
         [PlatformSpecific(TestPlatforms.AnyUnix)]  // Expected WatcherChangeTypes are different based on OS
         public void Unix_File_Move_In_Nested_Directory(bool includeSubdirectories)
         {
-            FileMove_NestedDirectory(includeSubdirectories ? WatcherChangeTypes.Created | WatcherChangeTypes.Deleted : 0, includeSubdirectories);
+            FileMove_NestedDirectory(includeSubdirectories ? WatcherChangeTypes.Renamed : 0, includeSubdirectories);
         }
 
         [Fact]
@@ -118,7 +118,7 @@ namespace System.IO.Tests
         [PlatformSpecific(TestPlatforms.AnyUnix)]  // Expected WatcherChangeTypes are different based on OS
         public void Unix_File_Move_With_Set_NotifyFilter()
         {
-            FileMove_WithNotifyFilter(WatcherChangeTypes.Deleted);
+            FileMove_WithNotifyFilter(WatcherChangeTypes.Renamed);
         }
 
         #region Test Helpers
diff --git a/src/System.IO.FileSystem/src/System/IO/FileSystem.Unix.cs b/src/System.IO.FileSystem/src/System/IO/FileSystem.Unix.cs
index b2007ca32d..94b4557643 100644
--- a/src/System.IO.FileSystem/src/System/IO/FileSystem.Unix.cs
+++ b/src/System.IO.FileSystem/src/System/IO/FileSystem.Unix.cs
@@ -160,18 +160,19 @@ namespace System.IO
         {
             // The desired behavior for Move(source, dest) is to not overwrite the destination file
             // if it exists. Since rename(source, dest) will replace the file at 'dest' if it exists,
-            // link/unlink are used instead. However, if the source path and the dest path refer to
-            // the same file, then do a rename rather than a link and an unlink.  This is important
-            // for case-insensitive file systems (e.g. renaming a file in a way that just changes casing),
-            // so that we support changing the casing in the naming of the file. If this fails in any
-            // way (e.g. source file doesn't exist, dest file doesn't exist, rename fails, etc.), we
-            // just fall back to trying the link/unlink approach and generating any exceptional messages
-            // from there as necessary.
+            // link/unlink are used instead. Rename is more efficient than link/unlink on file systems
+            // where hard links are not supported (such as FAT). Therefore, given that source file exists,
+            // rename is used in 2 cases: when dest file does not exist or when source path and dest
+            // path refer to the same file (on the same device). This is important for case-insensitive
+            // file systems (e.g. renaming a file in a way that just changes casing), so that we support
+            // changing the casing in the naming of the file. If this fails in any way (e.g. source file
+            // doesn't exist, dest file doesn't exist, rename fails, etc.), we just fall back to trying the
+            // link/unlink approach and generating any exceptional messages from there as necessary.
             Interop.Sys.FileStatus sourceStat, destStat;
             if (Interop.Sys.LStat(sourceFullPath, out sourceStat) == 0 && // source file exists
-                Interop.Sys.LStat(destFullPath, out destStat) == 0 && // dest file exists
-                sourceStat.Dev == destStat.Dev && // source and dest are on the same device
-                sourceStat.Ino == destStat.Ino && // and source and dest are the same file on that device
+                (Interop.Sys.LStat(destFullPath, out destStat) != 0 || // dest file does not exist
+                (sourceStat.Dev == destStat.Dev && // source and dest are on the same device
+                sourceStat.Ino == destStat.Ino)) && // source and dest are the same file on that device
                 Interop.Sys.Rename(sourceFullPath, destFullPath) == 0) // try the rename
             {
                 // Renamed successfully.