Add DB::open_cf_with_ttl method.

[fdeantoni]

Added the ability to open a database with column families and TTL. It uses method ffi::rocksdb_open_column_families_with_ttl.

[aleksuss]

Overall looks good 👍🏻

[aleksuss]

Couple nitpicks.

1. As I can see, you provide default options when create ColumnFamilyDescriptor. I guess it should be added to the function description for notifying user about this.
2. Sometimes a necessity occurs to create a ColumnFamilyDescriptor with custom options. What do you think about adding extra function with custom options for ColumnFamilyDescriptors ?

[fdeantoni]

Thanks for reviewing this PR @aleksuss! I have pushed a new commit that contains the fixes you suggested. Let me know if this addresses what you were suggesting.

[aleksuss]

I mean to create a method like this:

pub fn open_cf_with_ttl_opt<P, I, N>(
         opts: &Options,
         path: P,
         cfs: I,
         ttl: Duration,
         cfs_opts: Options,
     ) -> Result<DB, Error>
     where
         P: AsRef<Path>,
         I: IntoIterator<Item = N>,
         N: AsRef<str>,
     {
         let cfs = cfs
             .into_iter()
             .map(|name| ColumnFamilyDescriptor::new(name.as_ref(), cfs_opts.clone()));

         DB::open_cf_descriptors_internal(opts, path, cfs, &AccessType::WithTTL { ttl })
}

[aleksuss]

It seems this method is redundant.

[aleksuss]

Suggested change

	DB::open_cf_descriptors_with_ttl(opts, path, cfs, ttl)
	DB::open_cf_descriptors_internal(opts, path, cfs, &AccessType::WithTTL { ttl })

[aleksuss]

I mean to create a method like this:

pub fn open_cf_with_ttl_opt<P, I, N>(
         opts: &Options,
         path: P,
         cfs: I,
         ttl: Duration,
         cfs_opts: Options,
     ) -> Result<DB, Error>
     where
         P: AsRef<Path>,
         I: IntoIterator<Item = N>,
         N: AsRef<str>,
     {
         let cfs = cfs
             .into_iter()
             .map(|name| ColumnFamilyDescriptor::new(name.as_ref(), cfs_opts.clone()));

         DB::open_cf_descriptors_internal(opts, path, cfs, &AccessType::WithTTL { ttl })
}

[fdeantoni]

Thanks for the feedback @aleksuss. Just a question:

I actually patterned DB::open_cf_descriptors_with_ttl after the existing DB::open_cf_descriptors method. I made DB::open_cf_with_ttl() use DB::open_cf_descriptors_with_ttl() so that the existing test would exercise both methods.

If DB::open_cf_descriptors_with_ttl is redundant, perhaps DB::open_cf_descriptors should be marked deprecated with a new method like the one you describe above (but without the TTL)? Something like:

    /// Opens a database with the given database options and column family descriptors.
    pub fn open_cf_opt<P, I>(opts: &Options, path: P, cfs: I, cfs_opts: Options) -> Result<DB, Error>
    where
        P: AsRef<Path>,
        I: IntoIterator<Item = ColumnFamilyDescriptor>,
    {
         let cfs = cfs
             .into_iter()
             .map(|name| ColumnFamilyDescriptor::new(name.as_ref(), cfs_opts.clone()));

        DB::open_cf_descriptors_internal(opts, path, cfs, &AccessType::ReadWrite)
    }

Follow up question: Implementing things this way does take away the ability to create / re-open a DB with ColumnFamilies having different configurations. I'm assuming there may be use cases where a database may want to have different configurations per column family?

[aleksuss]

I'm assuming there may be use cases where a database may want to have different configurations per column family?

It is exactly what I've meant. But I rethought and I think the better way to do it in another PR.