[PATCH v5 3/4] 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 and previous commit), 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.  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 | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/sha1_file.c b/sha1_file.c
index 00c07f2..3204f66 100644
--- a/sha1_file.c
+++ b/sha1_file.c
@@ -663,10 +663,29 @@ void release_pack_memory(size_t need)
 		; /* nothing */
 }
 
+static void mmap_limit_check(size_t length)
+{
+	static size_t limit = SIZE_MAX;
+	if (limit == SIZE_MAX) {
+		const char *var = "GIT_MMAP_LIMIT";
+		unsigned long val = 0;
+		const char *env = getenv(var);
+		if (env && !git_parse_ulong(env, &val))
+			die("Failed to parse %s", var);
+		limit = val;
+	}
+	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;
-- 
2.1.0.8.gd3b6067