But keep in mind that if your system can't cope with this what you've done there is engineer in unreliability, you've made a system that's deliberately not very robust, unless it's very, very tightly integrated (e.g. two sub-routines inside the same running program) the cost savings had better be _enormous_ or what you're doing is just amplifying a problem and giving it to somebody else, like "solving" a city's waste problem by just dumping all the raw sewage into a neighbouring city's rivers.

Now, the "you can't delete things because then the disk space is unreachable" argument makes plenty of sense for, say, FAT, a filesystem from the 1980s.

But (present year argument) this is 2018. Everybody's main file systems are journalled. Sure enough, both systems _can_ write a record to the journal which will cause the blocks to be freed on replay and then remove that journal entry if the blocks actually get freed up before then. The difference is that Windows doesn't bother doing this.

Only if the libraries that use IPC have changed their wire format between versions, which would be a pretty bad practice, so I wouldn't expect that to happen often (if ever).

If something that's already running has its data files moved around or changed sufficiently, and it later tries to open (that is, the app was running but the data file wasn't open when the upgrade happened) what it thinks is an old data file, but is either new and different or just missing, that could cause problems.

> Because it seems to me that the Windows method might be less flexible but is likely to be more stable, since there's a single coherent global view of the file system at any given time.

In practice I've never had an issue with this (nearly 20 years using various Linux desktop and server distros). Upgrade-in-place is generally the norm, and most people will only reboot if there's a kernel update or an update to the init system.

kgraft for example swaps off each syscall for a process while it is not being used. This lets the OS slowly transfer to the new kernel as it is running.

kpatch does it all in one go but locks up the system for a few milliseconds.

The version that is currently merged into 4.0+ kernels is a hybrid of the two developed by the authors of both systems.

In theory, but Linux systems tend to do very little IPC other than X11, pipelines, and IP-based communication, where the protocols tend to support running with different versions.

In practice you can achieve multi-year uptimes with systems until you get a mandatory kernel security update.

At the first glance this is true, but you can guard against this in several ways. If your process only forks children then it already inherits the loaded libraries from the parent as part of the forked address space. Alternatively you can pass open file descriptors between processes. Another option is to use file-system snapshots, at least if the filesystem supports them.

Yet another option is to not replace individual files but complete directories and swap them out via RENAME_EXCHANGE (an atomic swap, available since kernel 3.15). As long as the process keeps a handle on its original working directory it can keep working with the old version even if it has been replaced with a new one.

Some of those approaches are tricky, but if you want to guard against such inconsistencies at least it is possible. And if your IPC interfaces provide a stable API it shouldn't be necessary.

> And then there's FILE_SHARE_DELETE which allows deletion

That has some issues when the file is mmaped. If I recall correctly you can't replace it as long as a mapping is open.

While this is true, I've never seen this to be a problem. If two programs use IPC, they usually use either stable, or compatible protocol.

To make things even more complicated, you can have two programs, either each in it's own container, or statically linked, or with their private bundles of libraries, doing IPC and then they are free to have different versions of the underlying libraries, while the users still expect them to work fine.

Some versions are known to be incompatible and most Linux distributions do a very good job of recommending and doing a restart of affected services in a way transparent to users. I have been running Linux and home and at work for years, almost never restart those workstations and, as far as I can tell, never had problems from piecemeal upgrades. My 2c.

[1] Note that is not the same as your "must be reachable via some path". It is literally inaccessible by name after delete. Try to access by name and you get STATUS_DELETE_PENDING. This is unrelated to the other misfeature of being able to block deletes by not including FILE_SHARE_DELETE.

Ah. This must be why I can't permanently delete files in use by a program but can sometimes "delete" them and send them to the recycle bin.

Isn't there a $MFT\\{file entry number} virtual directory that gives an alternate access path to each file? Wouldn't that qualify as "a way to access the file?"

Also, you might say that in practice Linux abides by the same rule - the old file can be referenced through some /proc/$pid/fd/$fd entry.

That's why you should restart those programs that were using the library. You can find this out via `lsof`.

Renaming/Deleting files in use is one of those things that us nerds like to complain about, but it makes sense when you think of an accountant that has an open spreadsheet and accidentally deletes a folder with that file. For average non-technical people (on any OS) I would say it makes sense to block that file from being deleted.

Of course there is a theoretical possibility that this will happen; however, in practice, updates (especially security updates) on Linux happen with ABI compatible libraries. E.g. on debian/ubuntu

apt-get update && apt-get upgrade

Will generally only do ABI compatible updates, without installing additional packages (you need 'dist-upgrade' or 'full-upgrade' for that).

Some updates will go as far as to prevent a program restart while updating (by temporarily making the executable unavailable).

Firefox on Ubuntu is an outlier - an update will replace it with one that isn't ABI compatible. It detects this and encourages you to restart it.

All in all, it's not that a reboot is never required for linux theoretically - it is that practically, you MUST reboot only for a kernel update, and may occasionally need to restart other programs that have been updated (but are rarely forced too).