t-strvec: improve check_strvec() output

t-strvec: improve check_strvec() output

The macro check_strvec calls the function check_strvec_loc(), which
performs the actual checks.  They report the line number inside that
function on error, which is not very helpful.  Before the previous
patch half of them triggered an assertion that reported the caller's
line number using a custom message, which was more useful, but a bit
awkward.

Improve the output by getting rid of check_strvec_loc() and performing
all checks within check_strvec, as they then report the line number of
the call site, aiding in finding the broken test.  Determine the number
of items and check it up front to avoid having to do them both in the
loop and at the end.

Sanity check the expected items to make sure there are any and that the
last one is NULL, as the compiler no longer does that for us with the
removal of the function attribute LAST_ARG_MUST_BE_NULL.

Use only the actual strvec name passed to the macro, the internal
"expect" array name and an index "i" in the output, for clarity.  While
"expect" does not exist at the call site, it's reasonably easy to infer
that it's referring to the NULL-terminated list of expected strings,
converted to an array.

Here's the output with less items than expected in the strvec before:

 # check "vec->nr > nr" failed at t/unit-tests/t-strvec.c:19
 #    left: 1
 #   right: 1

... and with the patch:

 # check "(&vec)->nr == ARRAY_SIZE(expect) - 1" failed at t/unit-tests/t-strvec.c:53
 #    left: 1
 #   right: 2

With too many items in the strvec we got before:

 # check "vec->nr == nr" failed at t/unit-tests/t-strvec.c:34
 #    left: 1
 #   right: 0
 # check "vec->v[nr] == NULL" failed at t/unit-tests/t-strvec.c:36
 #    left: 0x6000004b8010
 #   right: 0x0

... and with the patch:

 # check "(&vec)->nr == ARRAY_SIZE(expect) - 1" failed at t/unit-tests/t-strvec.c:53
 #    left: 1
 #   right: 0

A broken alloc value was reported like this:

 # check "vec->alloc > nr" failed at t/unit-tests/t-strvec.c:20
 #    left: 0
 #   right: 0

 ... and with the patch:

 # check "(&vec)->nr <= (&vec)->alloc" failed at t/unit-tests/t-strvec.c:56
 #    left: 2
 #   right: 0

An unexpected string value was reported like this:

 # check "!strcmp(vec->v[nr], str)" failed at t/unit-tests/t-strvec.c:24
 #    left: "foo"
 #   right: "bar"
 #      nr: 0

... and with the patch:

 # check "!strcmp((&vec)->v[i], expect[i])" failed at t/unit-tests/t-strvec.c:53
 #    left: "foo"
 #   right: "bar"
 #       i: 0

If the strvec is not NULL terminated, we got:

 # check "vec->v[nr] == NULL" failed at t/unit-tests/t-strvec.c:36
 #    left: 0x102c3abc8
 #   right: 0x0

... and with the patch we get the line number of the caller:

 # check "!strcmp((&vec)->v[i], expect[i])" failed at t/unit-tests/t-strvec.c:53
 #    left: "bar"
 #   right: NULL
 #       i: 1

check_strvec calls without a trailing NULL were detected at compile time
before:

 t/unit-tests/t-strvec.c:71:2: error: missing sentinel in function call [-Werror,-Wsentinel]

... and with the patch it's only found at runtime:

 # check "expect[ARRAY_SIZE(expect) - 1] == NULL" failed at t/unit-tests/t-strvec.c:53
 #    left: 0x100e5a663
 #   right: 0x0

We can let check_strvec add the terminating NULL for us and remove it
from callers, making it impossible to forget.  Leave that conversion for
a future patch, though, since this reimplementation is already intrusive
enough.

Reported-by: Jeff King <peff@peff.net>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>