Reduce the number of syscalls in FileCache::loadMetadata

[Algunenano]

Changelog category (leave one):

• Performance Improvement

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

• Reduce the number of syscalls in FileCache::loadMetadata

Documentation entry for user-facing changes

--

References #52037

A couple changes focused only on the iteration over the folders to check what's empty:

• Using path = it->path(); fs::is_directory(path) requires a call to newfstatat/stat. We avoid this syscall by using the information already present in the iterator.
• fs::is_empty(path) adds another syscall in order to check the type of path. As we know per the previous condition that it is a directory we can also avoid that syscall to get the type. To do that, we simply build a directory iterator with the path and see if it's empty.

cc @kssenii

The impact is hard to measure since it depends on what's in cache and what isn't and the rest of the operations that I haven't measured.

In a synthetic benchmark, with 1000 folders and each one of them 1000 subfolders with 1 file in each, and everything in kernel/disk cache, the iteration time and checks for emptiness go from 4039 ms to 3150 ms in my machine.

for i in {0..1000}; do for j in {0..1000}; do echo "$i $j"; mkdir -p $i/$j; touch $i/$j/a.txt; done; done

Before (4039 ms):

size_t empties_fs(const std::string &path)
{
    size_t empties = 0;

    for (auto level_1_it = directory_iterator{path};
        level_1_it != directory_iterator();
        level_1_it++)
    {
        const auto level_1_path = level_1_it->path();

        if (!is_directory(level_1_path))
            continue;
        if (is_empty(level_1_path))
            empties++;
        else
        {
            for (auto level_2_it = directory_iterator{level_1_path};
                level_2_it != directory_iterator();
                level_2_it++)
            {
                const auto level_2_path = level_2_it->path();
                if (!is_directory(level_2_path))
                    continue;
                if (is_empty(level_2_path))
                    empties++;
            }
        }
    }

    return empties;
}

After (3150 ms):

size_t empties_fs(const std::string &path)
{
    size_t empties = 0;
    for (auto level_1_it = directory_iterator{path};
        level_1_it != directory_iterator();
        level_1_it++)
    {
        if (!level_1_it->is_directory())
            continue;
        const auto level_1_path = level_1_it->path();

        auto level_2_it = directory_iterator{level_1_path};
        if (level_2_it == directory_iterator{})
            empties++;
        else
        {
            for (;
                level_2_it != directory_iterator();
                level_2_it++)
            {
                if (!level_2_it->is_directory())
                    continue;

                const auto level_2_path = level_2_it->path();
                if (directory_iterator{level_2_path} == directory_iterator{})
                    empties++;
            }
        }
    }


    return empties;
}

cc @kssenii

[robot-ch-test-poll3]

This is an automated comment for commit 3acb600 with description of existing statuses. It's updated for the latest CI running
The full report is available here
The overall status of the commit is 🟡 pending

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