object-file: move xmmap() into "wrapper.c"

The `xmmap()` function is provided by "object-file.c" even though its
functionality has nothing to do with the object file subsystem. Move it
into "wrapper.c", whose header already declares those functions.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

wrapper.c

@@ -829,3 +829,51 @@ uint32_t git_rand(unsigned flags)
 
 	return result;
 }
+
+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_gently(void *start, size_t length,
+		  int prot, int flags, int fd, off_t offset)
+{
+	void *ret;
+
+	mmap_limit_check(length);
+	ret = mmap(start, length, prot, flags, fd, offset);
+	if (ret == MAP_FAILED && !length)
+		ret = NULL;
+	return ret;
+}
+
+const char *mmap_os_err(void)
+{
+	static const char blank[] = "";
+#if defined(__linux__)
+	if (errno == ENOMEM) {
+		/* this continues an existing error message: */
+		static const char enomem[] =
+", check sys.vm.max_map_count and/or RLIMIT_DATA";
+		return enomem;
+	}
+#endif /* OS-specific bits */
+	return blank;
+}
+
+void *xmmap(void *start, size_t length,
+	int prot, int flags, int fd, off_t offset)
+{
+	void *ret = xmmap_gently(start, length, prot, flags, fd, offset);
+	if (ret == MAP_FAILED)
+		die_errno(_("mmap failed%s"), mmap_os_err());
+	return ret;
+}