Re: [PATCH v3 1/3] ref-filter: add worktreepath atom

On Sun, Dec 16, 2018 at 01:57:57PM -0800, nbelakovski@gmail.com wrote:

From: Nickolai Belakovski <redacted>

Add an atom proving the path of the linked worktree where this ref is
checked out, if it is checked out in any linked worktrees, and empty
string otherwise.

I stumbled over the word "proving" here. Maybe "showing" would be more
clear?

Normally single-quotes are used in asciidoc to emphasize text, and the
quotes aren't passed through. Asciidoc (and asciidoctor) do seem to
render the literal quotes here, which is good. I wonder if it would be
more clear to just write it out, though, like:

  ...any linked worktree. Otherwise, replaced with a single space.

Also, why are we replacing it with a single space? Wouldn't the empty
string be more customary (and work with the other "if empty, then do
this" formatting options)?

Minor style nit: we put the "*" in a pointer declaration next to the
variable name, without intervening whitespace. Like:

  static struct worktree **worktrees;

A few style nits:

  - the "*" space thing from above (it's in other places below, too, but
    I won't point out each)

  - we prefer "/* */" comments, even for single-liners

  - since we do all-lowercase identifiers, use more underscores to break
    things up. E.g., ref_to_worktree_entry.

Here we store the refname as a separate variable, but then point to the
worktree itself to access wt->path. Why do we treat these differently?
I.e., I'd expect to see either:

  1. Each entry holding a single worktree object, and using its head_ref
     and path fields, like:

       struct ref_to_worktree_entry {
               struct hashmap_entry ent; /* must be first */
	       struct worktree *wt;
       };
       ....
       entry = xmalloc(sizeof(*entry));
       entry->wt = wt;
       hashmap_entry_init(entry, strhash(wt->head_ref));
       ...
       strbuf_addstr(&out, result->wt->path);


  2. Each entry containing just the bits it needs, like:

       struct ref_to_worktree_entry {
               struct hashmap_entry ent; /* must be first */
	       char *ref;
	       char *path;
       };
       ...
       /*
        * We could use FLEXPTR_ALLOC_STR() here, but it doesn't actually
	* support holding _two_ strings. Separate allocations probably
	* aren't a huge deal here, since there are only a handful of
	* worktrees.
	*/
       entry = xmalloc(sizeof(*entry));
       entry->ref = wt->head_ref;
       entry->path = wt->path;
       hashmap_entry_init(entry, strhash(entry->ref));
       ...
       strbuf_addstr(&out, result->path);


I think the first one is strictly preferable unless we're worried about
the lifetime of the "struct worktree" going away. I don't think that's
an issue, though; they are ours until we call free_worktrees().

This uses one map for each %(worktree) we use. But won't they all be the
same? It would ideally be associated with the ref-filter.  There's no
ref-filter context struct to hold this kind of data, just static globals
in ref-filter.c (including this used_atom struct!). That's something
we'll probably need to fix in the long run, but I think it would be
reasonable to just have:

  static struct hashmap ref_to_worktree_map;

next to the declaration of used_atom_cnt, need_symref, etc. And then
those can all eventually get moved into a struct together.

Marking as SOURCE_NONE makes sense.

+static const char * get_worktree_info(const struct used_atom *atom, const struct ref_array_item *ref)
+{
+	struct strbuf val = STRBUF_INIT;
+	struct reftoworktreeinfo_entry * entry;
+	struct reftoworktreeinfo_entry * lookup_result;
+
+	FLEXPTR_ALLOC_STR(entry, ref, ref->refname);
+	hashmap_entry_init(entry, strhash(entry->ref));
+	lookup_result = hashmap_get(&(atom->u.reftoworktreeinfo_map), entry, NULL);
+	free(entry);

We shouldn't need to do an allocation just for a lookup. That's what the
extra "keydata" parameter is for in the comparison function. And I guess
this is what led you to have "char *ref" in the struct, rather than
reusing wt->head_ref (because you don't have a "struct worktree" here).

You should be able to do it like this:

  struct hashmap_entry entry;
  struct ref_to_worktree_entry *result;

  hashmap_entry_init(entry, strhash(ref->refname));
  result = hashmap_get(&ref_to_worktree_map, &entry, ref->refname));
  ...

and then your comparison function would look like this:

  int ref_to_worktree_hashcmp(const void *data,
                              const void *entry,
			      const void *entry_or_key,
			      const void *keydata)
  {
          const struct ref_to_worktree_entry *a = entry;
          const struct ref_to_worktree_entry *b = entry;
	  if (keydata)
	          return strcmp(a->wt->head_ref, keydata);
	  else
	          return strcmp(a->wt->head_ref, b->wt->head_ref);
  }

If you're thinking that this API is totally confusing and hard to figure
out, I agree. It's optimized to avoid extra allocations. I wish we had a
better one for simple cases (especially string->string mappings like
this).

Speaking of comparison functions, I didn't see one in your patch. Don't
you need to pass one to hashmap_init?

+	if (lookup_result)
+	{
+		if (!strncmp(atom->name, "worktreepath", strlen(atom->name)))
+			strbuf_addstr(&val, lookup_result->wt->path);
+	}
+	else
+		strbuf_addstr(&val, " ");

What's this extra strncmp about? If we're _not_ a worktreepath atom,
we'd still do the lookup only to put nothing in the string?

I think we'd only call this function when populate_value() sees a
worktreepath atom, though:

So it would be OK to drop the check of atom->name again inside
get_worktree_info().

And if we move the mapping out to a static global, then this only has to
be done once, not once per atom. In fact, I think this could double-free
"worktrees" with your current patch if you have two "%(worktree)"
placeholders, since "worktrees" already is a global.

It's probably worth testing that the path we get is actually sane, too.
I.e., expect something more like:

   cat >expect <<-\EOF
   master: $PWD
   master: $PWD/worktree
   side: not checked out
   EOF
   git for-each-ref \
     --format="%(refname:short): %(if)%(worktreepath)%(then)%(worktreepath)%(else)not checked %out%(end)

(I wish there was a way to avoid that really long line, but I don't
think there is).

-Peff