[PATCH] packfile: use extra variable to clarify code in use_pack()

[Ramsay Jones <ramsay@ramsayjones.plus.com>]

From: Jeff King <peff@peff.net>

We use the "offset" variable for two purposes. It's the offset into
the packfile that the caller provides us (which is rightly an off_t,
since we might have a packfile much larger than memory). But later we
also use it as the offset within a given mmap'd window, and that
window cannot be larger than a size_t.

For the second use, the fact that we have an off_t leads to some
confusion when we assign it to the "left" variable, which is a size_t.
It is in fact correct (because our earlier "offset -= win->offset" means
we must be within the pack window), but using a separate variable of the
right type makes that much more obvious.

Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
---

Hi Junio,

As promised, I am forwarding a 'saved' patch from Jeff, which was
a by-product of a long-ago discussion regarding commit 5efde212fc
("zlib.c: use size_t for size", 2018-10-14).

I have tested this patch on 'pu' (@6fd68134c8) and directly on top
of commit 5efde212fc. (see branch 'mk/use-size-t-in-zlib').

However, whilst I have been waiting for the tests to finish, I have
been looking at the code and concluded that this does not _have_ to
be applied on top of commit 5efde212fc. (I haven't done it, but just
tweak the context line to read 'unsigned long *left)' rather than
'size_t *left)' and this should apply cleanly to 'master'. Also, it
would have _exactly_ the same effect as the current code! ;-) ).

So, dunno.

ATB,
Ramsay Jones


 packfile.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/packfile.c b/packfile.c
index b0efe8cb3d..0e59f929c5 100644
--- a/packfile.c
+++ b/packfile.c
@@ -622,6 +622,7 @@ unsigned char *use_pack(struct packed_git *p,
 		size_t *left)
 {
 	struct pack_window *win = *w_cursor;
+	size_t offset_in_window;
 
 	/* Since packfiles end in a hash of their content and it's
 	 * pointless to ask for an offset into the middle of that
@@ -683,10 +684,14 @@ unsigned char *use_pack(struct packed_git *p,
 		win->inuse_cnt++;
 		*w_cursor = win;
 	}
-	offset -= win->offset;
+	/*
+	 * We know this difference will fit in a size_t, because our mmap
+	 * window by definition can be no larger than a size_t.
+	 */
+	offset_in_window = xsize_t(offset - win->offset);
 	if (left)
-		*left = win->len - xsize_t(offset);
-	return win->base + offset;
+		*left = win->len - offset_in_window;
+	return win->base + offset_in_window;
 }
 
 void unuse_pack(struct pack_window **w_cursor)
-- 
2.21.0