The mmapstorage plugin is a high performance storage plugin that supports full Elektra semantics.

The most important constraint for mmapstorage is that any structure (or bytes) that is an allocation unit (e.g. we malloc() the bytes needed for KeySet struct, so this is an unit) needs to have a flag to determine whether those bytes are actually malloc()ed or they are mmap()ed.

The mmapstorage plugin only calls munmap in some error cases, so basically the returned keyset is never invalidated and munmap is never called for its data.

During kdbSet the storage plugins always write to a temp file, due to how the resolver works. We also don't need to mmap the temp file here: when doing kdbSet we already have the KeySet at hand, mmap-ing it is not needed at all, because we have the data. We just want to update the cache file. The mmap/munmap in kdbSet are just so we can write the KeySet to a file in our format. (mmap() is just simpler, but we could also malloc() a region and then fwrite() the stuff)

Therefore the only case where we return a mmap()ed KeySet should be in kdbGet.

When the mmapstorage was designed/implemented, not all structures had refcounters, so there was no way to know when a munmap is safe. This was simply out of scope at that point in time.