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About linux : Just-in-time-read-only-filesystem-using-mkfifo-and-inotifywait

Question Detail

I am writing some gross middleware – basically, I have some old code that needs to open 100,000 files for reading only, expecting them all to be in one folder. It never writes. It is multiprocess so it can try to open ~30 files at the same time. The old way, I would have to actually copy the files into that folder (or use links, NFS, etc.). Worth noting I have no ability to change this old code – its just a binary.

I have some new, fancy code that can retrieve a file almost instantly. I want to tie these things together, so when the old code tries to open the file, it is actually, in real time, running the new code.

So I thought of mkfifo and inotifywait. Instead of a folder of 100,000 files, I can make a folder of 100,000 named pipes. So far so good. The legacy code goes to open the files, not knowing that they are indeed named pipes. The problem is, I don’t know what order the legacy code is going to open the files (nice, right?). So I would like to TRIGGER the named pipe WRITE (from my fancy new code) when the legacy code goes in for the read. I can’t spawn 100,000 writes and have them all block. So I thought hey – inotifywait makes sense. Every time the legacy goes to open the pipe, it triggers a read event, which can then be used to spawn the pipe writer in the background. The problem is.. inotifywait doesn’t trigger the read event until AFTER the writer has been spawned!

Any ideas of how to solve this? Basically – I want to intercept a file open, block for a couple hundred ms while I retrieve the contents of the file, then return that contents. Ideally I don’t have to create a custom FUSE filesystem to do this.. its just a read-only file open. The problem is this needs to run fast and in parallel.. and I don’t know which files are going to be opened in what order. Gotta be a quick and dirty way!

Thanks in advance for everyone’s time.

Question Answer

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