Re: [PATCH] clear_pattern_list(): clear embedded hashmaps

[Jeff King <peff@peff.net>]

On Mon, Aug 17, 2020 at 10:22:27AM -0700, Elijah Newren wrote:

> > > +       hashmap_free_entries(&pl->recursive_hashmap, struct pattern_entry, ent);
> > > +       hashmap_free_entries(&pl->parent_hashmap, struct pattern_entry, ent);
> >
> > This clears up the hash entries, but continues to leak the hash table.
> > Since you submitted first, can you fix this to use hashmap_free_()
> > instead, as per
> > https://lore.kernel.org/git/932741d7598ca2934dbca40f715ba2d3819fcc51.1597561152.git.gitgitgadget@gmail.com/?
> >  Then I'll rebase my series on yours and drop my first patch (since
> > it'll then be identical).
> 
> Nevermind, I got confused once again by the name.
> hashmap_free_entries() doesn't mean just free the entries, it means
> free what hashmap_free() would plus all the entries, i.e. do what
> hashmap_free() *should* *have* *been* defined to do.  Such a confusing
> API.  And hashmap_free() really perplexes me -- it seems like a
> function that can't possibly be useful; it's sole purpose seems to be
> a trap for the unwary.

There used to be an "also free entries" flag, but that got complicated
by the loosening of the "hashmap_entry must be at the front of the
struct to be freed" rule.

With this kind of embedded-entry data structure (and list.h is in the
same boat) it _is_ sometimes useful to be part of a data structure
without giving up ownership of the memory. But I agree that the more
normal case is to free items when the hashmap is destroyed.

Likewise, the whole "you have to define a struct that contains the map
entry" thing is flexible and efficient, but a pain to use.

I generally find khash's "map this type to that type, the hash owns the
memory" much more natural. And it doesn't lose efficiency (and indeed
sometimes even gains it) because it uses macros to store concrete types.
But of course macros create their own headaches. :)

Anyway, I'm definitely open to renaming to something more sensible. I
already mentioned the free/clear thing earlier, but
hashmap_clear_entries() ends up _very_ confusing. Because it's clearing
the hashmap but freeing the entries. hashmap_clear_and_free_entries() is
kind of long, but a lot more descriptive.

-Peff