[GSoC] Git Blog 5

My fifth week blog finished: The web version is here: https://adlternative.github.io/GSOC-Git-Blog-5/

## Week5: Tempting apple

This week, I spent a lot of time working on the digital circuit course design of the school. So this week's patches for git was completed in a hurry. This week `Ævar Arnfjörð Bjarmason` gave a lot of useful suggestions for the patch I wrote earlier. Some are related to code style improvements, and some are better design ideas.

Before, I wanted to use a `<s, s_size>` style string in `atom_value` to help copy and compare data containing '\0'.

Like this:

```c struct atom_value { const char *s; size_t s_size; ... }; ```

But `Ævar Arnfjörð Bjarmason` thinks it is more reasonable to use `strbuf` instead of `<s, s_size>`.

Like this:

```c struct atom_value { struct strbuf s; ... }; ```

Since the `strbuf` API has a natural `<s, s_size>`, we can add data
that may contain '\0' through `strbuf_add()`, `strbuf_addbuf()`, and
`strbuf_addf()` can also be used to fill strbuf with format string. Use
 `strbuf_addstr()` to replace `xstrdup()`, `strbuf_add()` to replace
 `xmemdupz()`, this is indeed a very tempting choice.

But in my actual refactoring process, this is not very easy to achieve.

For example, the original interface is like this,

```c v->s = copy_email(wholine, &used_atom[i]); ```

`v->s` will be filled with the data dynamically allocated by `copy_email()`.

```c static const char *copy_email(const char *buf, struct used_atom *atom) { ... if (!eoemail) return xstrdup(""); return xmemdupz(email, eoemail - email); } ```

Then if we want use `strbuf` type `v->s`, we should change the
 `copy_email()` interface parameters and return value.

```c static void copy_email(struct strbuf *str, const char *buf, struct used_atom *atom) { ... if (!eoemail) return; return strbuf_add(str, email, eoemail - email); } ```

Then the caller can do:

```c copy_email(&v->s, wholine, &used_atom[i]); ```

This is in line with our expectations.

But something like `fill_remote_ref_details()`, things gradually become complicated and difficult. `fill_remote_ref_details()`. Just consider the `show_ref()` called in `fill_remote_ref_details()`, `show_ref()` may call `shorten_unambiguous_ref()` internally, and then another function is called internally in `shorten_unambiguous_ref()`... This makes us fail the method of passing `v->s` in parameters like `copy_email()` does. Another way of thinking: what if I can "attach" the data directly? Using `strbuf_attach()` may be a viable option, but...

```c size_t len; void *data = shorten_unambiguous_ref(refname, warn_ambiguous_refs); len = strlen(data); strbuf_attach(&str, data, len, len); ```

...we need to get the length of the data, but this is not easy to do, `strlen()` can only be used on data that does not contain '\0', and we are not sure whether a function like `shorten_unambiguous_ref()` will return a `NULL`.

Well, this is one of the reasons why I cannot move on.

On the other hand, look at the following piece of code, it appears in `populate_value()`.

```c for (i = 0; i < used_atom_cnt; i++) { struct atom_value *v = &ref->value[i]; if (v->s == NULL && used_atom[i].source == SOURCE_NONE) return strbuf_addf_ret(err, -1, _("missing object %s for %s"), oid_to_hex(&ref->objectname), ref->refname); } ```

We need to determine whether `v->s` equals to `NULL`. We can use c-style strings to easily distinguish between empty strings(`xstrdup("")`) and `NULL`, but if we use strbuf, it is not easy to distinguish, because an empty strbuf has the following characteristics: `s.buf == strbuf_slopbuf` or `sb->buf[0] == '\0'`. It can be said that we shouldn’t even use `NULL` to assign to `s.buf`.

So in the end, I rejected this seemingly very attractive solution, and use the previous strategy: `<s, s_size>`, unless someone can think of a better solution later `;-)`.

* FootNote:
1. Why is there no `strbuf_empty()` in `strbuf` API? I think this may be
a very important thing.

```c #define strbuf_empty(sb) \ (sb->buf == strbuf_slopbuf) ? \ (!strbuf_slopbuf[0]) : \ (sb->buf[0] == '\0') ```

2. Another thing worth mentioning is: I will have the school final exam
between July 7th and July 14th, I may be busy during this period.

Thanks for Git community, reviewers and mentors. -- ZheNing Hu