I'm writing a file that takes minutes to write. External software monitors for this file to appear, but unfortunately doesn't monitor for inotify IN_CLOSE_WRITE events, but rather checks periodically "the file is there" and then starts to process it, which will fail if the file is incomplete. I cannot fix the external software. A workaround I've been using so far is to write a temporary file and then rename it when it's finished, but this workaround complicates my workflow for reasons beyond the scope of this question¹.
Files are not directory entries. Using hardlinks, there can be multiple pointers to the same file. When I open a file for writing, both the inode and the directory entry are created immediately. Can I prevent this? Can I postpone the creation of the directory entry until the file is closed, rather than when the file is opened for writing?
Example Python-code, but the question is not specific to Python:
fp = open(dest, 'w') # currently both inode and directory entry are created here
fp.write(...)
fp.write(...)
fp.write(...)
fp.close() # I would like to create the directory entry only here
Reading everything into memory and then writing it all in one go is not a good solution, because writing will still take time and the file might not fit into memory.
I found the related question Is it possible to create an unlinked file on a selected filesystem?, but I would want to first create an anonymous/unnamed file, then naming it when I'm done writing (I agree with the answer there that creating an inode is unavoidable, but that's fine; I just want to postpone naming it).
Tagging this as linux, because I suspect the answer might be different between Linux and Windows and I only need a solution on Linux.
¹Many files are produced in parallel within dask graphs, and injecting a "move as soon as finished" task in our system would be complicated, so we're really renaming 50 files when 50 files have been written, which causes delays.
