when we need to ensure that **a newly-created file** is fully
synchronized back to disk, we should call `fsync()` on the parent
directory—that is, the directory containing the newly-created file.
That is to say, In this situation, we should call `fsync()` on
both the newly-created file and its parent directory.
Unfortunately, currently in Doris, in any scenario, directories
are not fsynced.
This patch adds `sync_dir()` interface first, laying the groundwork
for future fixes.
This patch also removes unneeded private method `dir_exists()`.
The upcoming patch will use CREATE_OR_OPEN mode
This patch also remove virtual dtors to cpp file.
* Move the dtors back to env.h
Generally, placing the dtor in an `.h` file(inline) or in a `cpp` file
depends on the trade-off between code expansion and function call overhead.
The code expansion rate is closely related to the number of class members
and the inheritance level.
For the several classes here: `Env`, `ReadableFile`, and `WritableFile`
have no members and are the top level of the inheritance hierarchy, But
for now I have no obvious evidence to prove that make their dtors inline
will cause serious code expansion and more instruction cache-misses,
even if there are thousands of `ReadableFile` objects kept being created
and released during running.
Now Env has unify all environment operation, such as file operation.
However some of our old functions don't leverage it. This change unify
FileUtils::scan_dir to use Env's function.
