Improved "rename" support

[aktau]

As mentioned here: howeyc/fsnotify#104

Currently, it's not straightforward to detect a rename correctly. In most cases, one will see this:

RENAME file1
CREATE file2

Yet this doesn't guarantee that file1 and file2 are the same file (there could be a file creation in between, or file1 could be moved outside a watched folder and file2 could be moved in). To work around this, one could store the os.FileInfo for every watched file and use os.SameFile to compare it with the new file. This is a bit cumbersome though, as it requires keeping a lot of os.FileInfo structs in memory + bookkeeping them.

However, some platforms provide a way to atomically see what was the old name and what's the new name of a file. To quote @nathany:

On Linux there is a "cookie" used to tie the RENAME FROM and TO events together, but this isn't currently exposed by fsnotify. I still need to do more research across each platform.

This is a tracker issue to see if there is sufficient cross-platform support to enable this in the default API.

[jmhodges]

I think watchdog (https://github.com/gorakhargosh/watchdog) has figured out a cross-platform API, by the way.

[nathany]

@jmhodges Do you have any more details on "Weird Things" vim does? Is this when saving or is there a rename file function people are using in Vim?

[nathany]

ref: rjeczalik/notify#78

[omeid]

Vim uses swap and backup files, involves rename and moving.

[F21]

Do you think keeping a list of os.FileInfos in a concurrently safe map like map[string]*os.FileInfo as mentioned by @aktau is probably the best way to get this implemented?

[F21]

Having done some experimentation on Windows, the os.Samefile() method doesn't work on Windows, because it compares by using the file path. So, if you had test.txt, the fileInfo stores the path to test.txt. After renaming it to test2.txt, using os.Lstat() on the new file stores the path as test2.txt. When you do a comparison with os.Samefile(), it sees that the file paths for the 2 FileInfos are different and returns false.