Rubén Justo <rjusto@xxxxxxxxx> writes:
Nit: Is the commit subject missing a verb?
> We use a singleton empty array to initialize a `struct strvec`,
> similar to the empty string singleton we use to initialize a `struct
> strbuf`.
>
So a singleton empty array is a statically allocated array element, so
for strvec, this would be `const char *empty_strvec[] = { NULL }`.
> Note that an empty strvec instance (with zero elements) does not
> necessarily need to be an instance initialized with the singleton.
> Let's refer to strvec instances initialized with the singleton as
> "empty-singleton" instances.
>
Right, so when we add elements ideally, we ideally check whether it is a
singleton or not. This is evident in `strvec_push_nodup()`:
void strvec_push_nodup(struct strvec *array, char *value)
{
if (array->v == empty_strvec)
array->v = NULL;
ALLOC_GROW(array->v, array->nr + 2, array->alloc);
array->v[array->nr++] = value;
array->v[array->nr] = NULL;
}
> As a side note, this is the current `strvec_pop()`:
>
> void strvec_pop(struct strvec *array)
> {
> if (!array->nr)
> return;
> free((char *)array->v[array->nr - 1]);
> array->v[array->nr - 1] = NULL;
> array->nr--;
> }
>
> So, with `strvec_pop()` an instance can become empty but it does
> not going to be the an "empty-singleton".
Correct, since we simply set the array element to NULL, but this is
still a dynamically allocated array.
Nit: The sentence reads a bit weirdly.
> This "empty-singleton" circumstance requires us to be careful when
> adding elements to instances. Specifically, when adding the first
> element: we detach the strvec instance from the singleton and set the
> internal pointer in the instance to NULL. After this point we apply
> `realloc()` on the pointer. We do this in `strvec_push_nodup()`, for
> example.
>
> The recently introduced `strvec_splice()` API is expected to be
> normally used with non-empty strvec's. However, it can also end up
> being used with "empty-singleton" strvec's:
>
> struct strvec arr = STRVEC_INIT;
> int a = 0, b = 0;
>
> ... no modification to arr, a or b ...
>
> const char *rep[] = { "foo" };
> strvec_splice(&arr, a, b, rep, ARRAY_SIZE(rep));
>
> So, we'll try to add elements to an "empty-singleton" strvec instance.
>
> Avoid misapplying `realloc()` to the singleton in `strvec_splice()` by
> adding a special case for "empty-singleton" strvec's.
>
So everything said here makes sense, that's a great explanation.
> Signed-off-by: Rubén Justo <rjusto@xxxxxxxxx>
> ---
>
> This iteration adds more detail to the message plus a minor change to
> remove some unnecessary parentheses.
>
> Junio: My message in the previous iteration was aimed at readers like
> Patrick, who is also the author of `strvec_splice()`. I certainly
> assumed too much prior knowledge, which made the review unnecessarily
> laborious.
>
> Rereading what I wrote last night, perhaps the problem now is excess.
> I hope not. In any case, here it is :-)
>
I would say this is very useful over the first iteration, considering I
am someone without prior knowledge here.
> Thanks.
>
> strvec.c | 10 ++++++----
> t/unit-tests/strvec.c | 10 ++++++++++
> 2 files changed, 16 insertions(+), 4 deletions(-)
>
> diff --git a/strvec.c b/strvec.c
> index d1cf4e2496..087c020f5b 100644
> --- a/strvec.c
> +++ b/strvec.c
> @@ -61,16 +61,18 @@ void strvec_splice(struct strvec *array, size_t idx, size_t len,
> {
> if (idx + len > array->nr)
> BUG("range outside of array boundary");
> - if (replacement_len > len)
> + if (replacement_len > len) {
> + if (array->v == empty_strvec)
> + array->v = NULL;
> ALLOC_GROW(array->v, array->nr + (replacement_len - len) + 1,
> array->alloc);
> + }
> for (size_t i = 0; i < len; i++)
> free((char *)array->v[idx + i]);
> - if (replacement_len != len) {
> + if ((replacement_len != len) && array->nr)
> memmove(array->v + idx + replacement_len, array->v + idx + len,
> (array->nr - idx - len + 1) * sizeof(char *));
> - array->nr += (replacement_len - len);
> - }
> + array->nr += replacement_len - len;
Why is this second block of changes needed? Will array-nr ever be 0 when
we reach here?
> for (size_t i = 0; i < replacement_len; i++)
> array->v[idx + i] = xstrdup(replacement[i]);
> }
[snip]
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