Cast 64 bit off_t to 32 bit size_t

Some systems have sizeof(off_t) == 8 while sizeof(size_t) == 4.
This implies that we are able to access and work on files whose
maximum length is around 2^63-1 bytes, but we can only malloc or
mmap somewhat less than 2^32-1 bytes of memory.

On such a system an implicit conversion of off_t to size_t can cause
the size_t to wrap, resulting in unexpected and exciting behavior.
Right now we are working around all gcc warnings generated by the
-Wshorten-64-to-32 option by passing the off_t through xsize_t().

In the future we should make xsize_t on such problematic platforms
detect the wrapping and die if such a file is accessed.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

git-compat-util.h

@@ -258,6 +258,11 @@ static inline ssize_t xwrite(int fd, const void *buf, size_t len)
 	}
 }
 
+static inline size_t xsize_t(off_t len)
+{
+	return (size_t)len;
+}
+
 static inline int has_extension(const char *filename, const char *ext)
 {
 	size_t len = strlen(filename);