Allow users to opt-out creating root dir when building Fs backend

[knight42]

https://github.com/apache/incubator-opendal/blob/a04a724059d7631318cb1543382b7462190f43eb/core/src/services/fs/backend.rs#L141-L163

Currently when creating Fs backend, OpenDAL will always create the root dir even if users intend to read data from the Fs backend, which might suprise users.

I propose we could create the necessary dirs in the implemetation of Accessor::write and Accessor::create rather than when we are building Fs backend. Or at least provide users with a option to opt-out this behavior.

[Xuanwo]

I don't understand the purpose of creating a file system backend on a non-existent root. For instance, if a user creates a backend with /not_exist and tries to read hello.txt, they will receive a NotFound error. This can cause confusion for the user who then needs to figure out what went wrong.

I would prefer to return an error instead, but this change will break the fs backend. Let's keep this issue open and see if others share the same concern.

[knight42]

I don't understand the purpose of creating a file system backend on a non-existent root.

Users might not create a fs backend with a non-existent root on purpose.

For instance I once tested oli cat by reading a local file and I mistyped the path, like oli cat /tmp/not-exist/foo, and then a dir named non-exist was created under /tmp.

Another case is that if I run the test test_parse_fs_location locally, a dir named foo/bar will be created under PWD, which supprises me as no read/write operation is actually performed.

[Xuanwo]

Users might not create a fs backend with a non-existent root on purpose.

That's true.

For instance I once tested oli cat by reading a local file and I mistyped the path, like oli cat /tmp/not-exist/foo, and then a dir named non-exist was created under /tmp.

Actually, oli must avoid using /tmp/not-exist as the root directory. Instead, all serivces should stick to / as root to make the internal logic cleaner.

Another case is that if I run the test test_parse_fs_location locally, a dir named foo/bar will be created under PWD, which supprises me as no read/write operation is actually performed.

This is the same issue as I statemented in last comment.

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Thank you for the proposal, but I don't think it's a significant enough issue to warrant a change.

[knight42]

all serivces should stick to / as root to make the internal logic cleaner.

Got it, this is a convention that I am not aware of. To follow this convention, I think oli should turn a relative path into an absolute path when parseing location. I could do that in a follow-up PR.