[PATCH v4 2/4] Introduce GIT_MMAP_LIMIT to allow testing expected mmap size

[Steffen Prohaska <prohaska@zib.de>]

Similar to testing expectations about malloc with GIT_ALLOC_LIMIT (see
commit d41489), it can be useful to test expectations about mmap.

This introduces a new environment variable GIT_MMAP_LIMIT to limit the
largest allowed mmap length (in KB).  xmmap() is modified to check the
limit.  Together with GIT_ALLOC_LIMIT tests can now easily confirm
expectations about memory consumption.

GIT_MMAP_LIMIT will be used in the next commit to test that data will be
streamed to an external filter without mmaping the entire file.

[commit d41489]: d41489a6424308dc9a0409bc2f6845aa08bd4f7d Add more large
    blob test cases

Signed-off-by: Steffen Prohaska <prohaska@zib.de>
---
 sha1_file.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/sha1_file.c b/sha1_file.c
index 00c07f2..603673b 100644
--- a/sha1_file.c
+++ b/sha1_file.c
@@ -663,10 +663,25 @@ void release_pack_memory(size_t need)
 		; /* nothing */
 }
 
+static void mmap_limit_check(size_t length)
+{
+	static ssize_t limit = -1;
+	if (limit == -1) {
+		const char *env = getenv("GIT_MMAP_LIMIT");
+		limit = env ? atol(env) * 1024 : 0;
+	}
+	if (limit && 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;
-- 
2.1.0.6.gb452461