[PATCH v3] strvec: `strvec_splice()` to a statically initialized vector

["Rubén Justo" <rjusto@gmail.com>]

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:  when 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 strvec's initialized with the singleton.

Signed-off-by: Rubén Justo <rjusto@gmail.com>
---

This iteration fixes a problem we saw when running with SANITIZE=leak.
Although it wasn't a leak.

We need to end the array because `realloc(NULL)` is not going to give
us that { NULL }.  I know it's something I considered at some point
because I thought about a change like `CALLOC_GROW()`.  Perhaps
another time.

 strvec.c              | 11 +++++++----
 t/unit-tests/strvec.c | 10 ++++++++++
 2 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/strvec.c b/strvec.c
index d1cf4e2496..62283fcef2 100644
--- a/strvec.c
+++ b/strvec.c
@@ -61,16 +61,19 @@ 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);
+		array->v[array->nr + (replacement_len - len) + 1] = NULL;
+	}
 	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]);
 }
diff --git a/t/unit-tests/strvec.c b/t/unit-tests/strvec.c
index 855b602337..e66b7bbfae 100644
--- a/t/unit-tests/strvec.c
+++ b/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;

Interdiff against v2:
  diff --git a/strvec.c b/strvec.c
  index 087c020f5b..62283fcef2 100644
  --- a/strvec.c
  +++ b/strvec.c
  @@ -66,6 +66,7 @@ void strvec_splice(struct strvec *array, size_t idx, size_t len,
   			array->v = NULL;
   		ALLOC_GROW(array->v, array->nr + (replacement_len - len) + 1,
   			   array->alloc);
  +		array->v[array->nr + (replacement_len - len) + 1] = NULL;
   	}
   	for (size_t i = 0; i < len; i++)
   		free((char *)array->v[idx + i]);
-- 
2.47.0.281.g735430a4cf