Make File.Move(..., ..., overwrite: true) more efficient. (#47118)

The current implementation has an inefficiency. When moving a file across volumes, and when the target file doesn't exist, the "rename" syscall will be called twice (unsuccesfully) instead of once. By branching on "overwrite" from the beginning, we avoid calling the first "rename" in vain. Co-authored-by: Konstantin Gukov <kgukov@microsoft.com>

Author: gukoff

src/libraries/System.IO.FileSystem/src/System/IO/FileSystem.Unix.cs

@@ -139,27 +139,6 @@ public static void MoveFile(string sourceFullPath, string destFullPath)
 
  public static void MoveFile(string sourceFullPath, string destFullPath, bool overwrite)
  {
- // 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. 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 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.
- return;
- }
-
  // If overwrite is allowed then just call rename
  if (overwrite)
  {
@@ -179,7 +158,7 @@ public static void MoveFile(string sourceFullPath, string destFullPath, bool ove
  // when the error occurs and our checks, but it's the best we can do, and the
  // worst case in such a race condition (which could occur if the file system is
  // being manipulated concurrently with these checks) is that we throw a
- // FileNotFoundException instead of DirectoryNotFoundexception.
+ // FileNotFoundException instead of DirectoryNotFoundException.
  throw Interop.GetExceptionForIoErrno(errorInfo, destFullPath,
  isDirectory: errorInfo.Error == Interop.Error.ENOENT && !Directory.Exists(Path.GetDirectoryName(destFullPath)) // The parent directory of destFile can't be found
  );
@@ -190,6 +169,28 @@ public static void MoveFile(string sourceFullPath, string destFullPath, bool ove
  return;
  }
 
+ // 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. 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 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.
+ return;
+ }
+
  LinkOrCopyFile(sourceFullPath, destFullPath);
  DeleteFile(sourceFullPath);
  }