test-delta: read input into a heap buffer

We currently read the input to test-delta by mmap()-ing it.
However, memory-checking tools like valgrind and ASan are
less able to detect reads/writes past the end of an mmap'd
buffer, because the OS is likely to give us extra bytes to
pad out the final page size. So instead, let's read into a
heap buffer.

As a bonus, this also makes it possible to write tests with
empty bases, as mmap() will complain about a zero-length
map.

This is based on a patch by Jann Horn <jannh@google.com>
which actually aligned the data at the end of a page, and
followed it with another page marked with mprotect(). That
would detect problems even without a tool like ASan, but it
was significantly more complex and may have introduced
portability problems. By comparison, this approach pushes
the complexity onto existing memory-checking tools.

Note that this could be done even more simply by using
strbuf_read_file(), but that would defeat the purpose:
strbufs generally overallocate (and at the very least
include a trailing NUL which we do not care about), which
would defeat most memory checkers.

Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Nicolas Pitre <nico@fluxnic.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/helper/test-delta.c

@@ -34,8 +34,8 @@ int cmd__delta(int argc, const char **argv)
 		return 1;
 	}
 	from_size = st.st_size;
-	from_buf = mmap(NULL, from_size, PROT_READ, MAP_PRIVATE, fd, 0);
-	if (from_buf == MAP_FAILED) {
+	from_buf = xmalloc(from_size);
+	if (read_in_full(fd, from_buf, from_size) < 0) {
 		perror(argv[2]);
 		close(fd);
 		return 1;
@@ -48,8 +48,8 @@ int cmd__delta(int argc, const char **argv)
 		return 1;
 	}
 	data_size = st.st_size;
-	data_buf = mmap(NULL, data_size, PROT_READ, MAP_PRIVATE, fd, 0);
-	if (data_buf == MAP_FAILED) {
+	data_buf = xmalloc(data_size);
+	if (read_in_full(fd, data_buf, data_size) < 0) {
 		perror(argv[3]);
 		close(fd);
 		return 1;