[jgit-dev] Should FS become singleton-like?

[Marc Strapetz <marc.strapetz@xxxxxxxxxxx>]

I've just uploaded another FS-related patch [1] which is addressing 
problems with LockFile (which became FS-dependent in version 4.5). I 
understand that most developers didn't fall in love with the 
configurable FS-approach started back in 2010: the idea was to not 
assume a single, auto-detectable Git installation with a single system 
Git config, but to allow using a custom (and possibly portable) Git 
installation and its system config by using a custom FS implementation.

There are still 28 usages of FS.DETECTED in core org.eclipse.jgit 
production code. Some of them are just default values for already 
FS-aware methods, other usages are in central places, like:

FileSnapshot.save(): FS.lastModified()

or

FileUtils: FS.exists(), FS.retryFailedLockFileCommit(), FS.relativize(), ...

I guess similar as for FS.createNewFile() it could happen that their 
implementations will become dependent on the system Git config and then 
new problems will arise. Hence, my question:

Should FS become "singleton-like", defaulting to FS.DETECTED, but with 
an additional setter so it can be (re)configured? I.e. something like:

class FS {
	private FS instance = detect();

	public FS getInstance() {
		return instance;
	}

	public void setInstance(FS instance) {
		this.instance = instance;
	}

        ...
}

This will still allow to use custom Git installations and even change 
them during a program run; just not to use multiple Git installations 
concurrently. It would allow to drop lots of FS parameters and fields 
and help to simplify API somewhat. So something which could be done for 
version 5?

[1] https://git.eclipse.org/r/#/c/114826/

-Marc