In Linux what happens if 1000 files in a directory are moved to another location while another 300 files were added to the source directory?
This depends on which tools you use: Let's check a few cases:
If you run something along the lines of mv /path/to/source/* /path/to/dest/ int a shell, you will end up with the original 1000 files being moved, the new 300 being untouched. This comes from the fact, that the shell will expand the * before starting the move operation, so when the move is in progress, the list is already fixed.
If you use Nautilus (and other GUI friends), you will end up the same way: It will run the move operation based on which files were selected - this doesn't change when new files show up.
If you use your own program using syscalls along the line of loop over glob and only one mv until glob stays empty, you will end up with all 1300 files in the new directory. This is because every new glob will pick up the new files, that have showed up in the meantime.
When you tell the system to move all the files from a directory, it lists all the files and then starts moving them. If new files appear in the directory, they aren't added to the list of files to move, so they'll remain in the original location.
You can, of course, program a way of moving files different to mv which will periodically check for new files in the source directory.
The kernel itself can't be "in the middle" of a "move 1000 files" operation. You need to be much more specific about what operation you're proposing.
One thread can only move one file at a time with the rename(*oldpath, const char *newpath) or renameat system calls (and only within the same filesystem1). Or Linux renameat2 which has flags like RENAME_EXCHANGE to atomically exchange two pathnames, or RENAME_NOREPLACE to not replace the destination if it exists. (e.g. allowing a mv -i implementation that avoids the race condition of stat and then rename, which would still overwrite a file created after stat.
link + unlink could also solve that, because link fails if the new name exists.)
But each of these system calls only renames a single directory entry per system call. Using POSIX renameat with olddirfd and newdirfd (opened with open(O_DIRECTORY)) would allow you to keep looping over files in a directory even if the source or destination directory itself had been renamed. (Using relative paths could also allow that with regular rename().)
Anyway, as the other answers say, most programs that use the rename system call will figure out a list of filenames before doing the first rename. (Usually using the readdir(3) POSIX library function as a wrapper for platform-specific system calls like Linux getdents).
But if you're talking about find -exec ... {} \; to run one command per file, or the more efficient -exec {} + with so many files that they don't fit on one command line, then you can certainly have renames happening while still scanning. e.g.
find . -name '*.txt' -exec mv -t ../txtfiles {} \; # Intentionally inefficient
If you created some new .txt files while this was running, you might see some of them in ../txtfiles. But internally find(1) will have used open(O_DIRECTORY) and getdents on ..
If one system call was enough to return all the directory entries in . (which find will loop over one at a time, only making further system calls if needed for -type or to recurse, or fork+exec on a match), then the list is a snapshot of the directory entries at one point in time. Further changes to the directory can't affect what find does, because it already has a copy of the directory listing what it will loop over. (Probably it internally uses readdir(3), which returns one entry at a time, but inside glibc we know from using strace find . that it makes a getdents64 system call with a buffer size of count=32768 entries.)
But if the directory is huge and/or the kernel doesn't fill find's buffer, it will have to make a 2nd getdents system call after looping over what it got the first time. So it could maybe see new entries after doing some renames.
But see discussion in comments under other answers: the kernel might have snapshotted for us, because (I think) getdents isn't allowed to return the same filename twice. Different filesystems use different sorting / indexing mechanisms for making access to an entry in a huge directory more efficient than a linear search. So adding or removing a directory might possibly have other effects on the order of the remaining entries. Hmm, probably it's more likely that filesystems keep a stable order, and just update an actual index (like the EXT4 dir_index feature), so a directory FD's position can just be a directory entry to resume from? I really don't know how the telldir(3) library interface maps onto lseek, or if that's purely a user-space thing for looping over the buffer obtained by user-space. But multiple getdents can be needed to get all the entries from a huge directory, so even if seeking isn't supported, the kernel needs to be able to record a current position.
Footnote 1:
To "move" between filesystems, it's up to user-space to copy and unlink. (e.g. with open and either read+write, mmap+write or sendfile(2) or copy_file_range(2), the latter two totally avoiding bouncing the file data through user-space.)