Re: [PATCH v2 2/2] Prevent git from rehashing 4GiB files

On Thu, Oct 12, 2023 at 04:09:30PM +0000, brian m. carlson wrote:

+static unsigned int munge_st_size(off_t st_size) {
+	unsigned int sd_size = st_size;
+
+	/*
+	 * If the file is an exact multiple of 4 GiB, modify the value so it
+	 * doesn't get marked as racily clean (zero).
+	 */
+	if (!sd_size && st_size)
+		return 0x80000000;
+	else
+		return sd_size;
+}

Coverity complained that the "true" side of this conditional is
unreachable, since sd_size is assigned from st_size, so the two values
cannot be both true and false. But obviously we are depending here on
the truncation of off_t to "unsigned int". I'm not sure if Coverity is
just dumb, or if it somehow has a different size for off_t.

I don't _think_ this would ever cause confusion in a real compiler, as
assignment from a larger type to a smaller has well-defined truncation,
as far as I know.

But I do wonder if an explicit "& 0xFFFFFFFF" would make it more obvious
what is happening (which would also do the right thing if in some
hypothetical platform "unsigned int" ended up larger than 32 bits).

-Peff