strvec: strvec_splice() to a statically initialized vector

We use a singleton empty array to initialize a `struct strvec`,
similar to the empty string singleton we use to initialize a `struct
strbuf`.

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.

    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".

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.

Signed-off-by: Rubén Justo <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: rjusto

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;
 	for (size_t i = 0; i < replacement_len; i++)
 		array->v[idx + i] = xstrdup(replacement[i]);
 }

t/unit-tests/strvec.c

@@ -88,6 +88,16 @@ void test_strvec__pushv(void)
 	strvec_clear(&vec);
 }
 
+void test_strvec__splice_just_initialized_strvec(void)
+{
+	struct strvec vec = STRVEC_INIT;
+	const char *replacement[] = { "foo" };
+
+	strvec_splice(&vec, 0, 0, replacement, ARRAY_SIZE(replacement));
+	check_strvec(&vec, "foo", NULL);
+	strvec_clear(&vec);
+}
+
 void test_strvec__splice_with_same_size_replacement(void)
 {
 	struct strvec vec = STRVEC_INIT;