mmap_limit: introduce GIT_MMAP_LIMIT to allow testing expected mmap size

In order to test expectations about mmap in a way similar to testing
expectations about malloc with GIT_ALLOC_LIMIT introduced by
d41489a (Add more large blob test cases, 2012-03-07), introduce a
new environment variable GIT_MMAP_LIMIT to limit the largest allowed
mmap length.

xmmap() is modified to check the size of the requested region and
fail it if it is beyond the limit.  Together with GIT_ALLOC_LIMIT
tests can now confirm expectations about memory consumption.

Signed-off-by: Steffen Prohaska <prohaska@zib.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sha1_file.c

@@ -663,10 +663,26 @@ void release_pack_memory(size_t need)
 		; /* nothing */
 }
 
+static void mmap_limit_check(size_t length)
+{
+	static size_t limit = 0;
+	if (!limit) {
+		limit = git_env_ulong("GIT_MMAP_LIMIT", 0);
+		if (!limit)
+			limit = SIZE_MAX;
+	}
+	if (length > limit)
+		die("attempting to mmap %"PRIuMAX" over limit %"PRIuMAX,
+		    (uintmax_t)length, (uintmax_t)limit);
+}
+
 void *xmmap(void *start, size_t length,
 	int prot, int flags, int fd, off_t offset)
 {
-	void *ret = mmap(start, length, prot, flags, fd, offset);
+	void *ret;
+
+	mmap_limit_check(length);
+	ret = mmap(start, length, prot, flags, fd, offset);
 	if (ret == MAP_FAILED) {
 		if (!length)
 			return NULL;