Re: [PATCH 2/7] get-tar-commit-id: check write_in_full() return against 0

Hi,

Jeff King wrote:

We ask to write 41 bytes and make sure that the return value
is at least 41. This is the same "dangerous" pattern that
was fixed in the prior commit (wherein a negative return
value is promoted to unsigned), though it is not dangerous
here because our "41" is a constant, not an unsigned
variable.

But we should convert it anyway to avoid modeling a
dangerous construct.

Signed-off-by: Jeff King <redacted>
---
 builtin/get-tar-commit-id.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

I kind of disagree with calling this dangerous (and I think that is
what you alluded to above by putting it in quotes), but I like the
postimage more than the preimage.

The variable 'n' could be eliminated to simplify this further.  I
realize that would go against the spirit of this patch, but (1) it's
on-topic for the patch, since it is another ssize_t vs constant
comparison and (2) as mentioned elsewhere in this thread, it's a very
common idiom with read_in_full.  If we want to eliminate it then we
could introduce a separate helper to distinguish between
read_this_much_i_mean_it and read_this_much_or_to_eof.

Anyway, I think the below is an improvement.

With or without this tweak,
Reviewed-by: Jonathan Nieder <redacted>

Thanks.

diff --git i/builtin/get-tar-commit-id.c w/builtin/get-tar-commit-id.c
index 6d9a79f9b3..03ef7c5ba4 100644
--- i/builtin/get-tar-commit-id.c
+++ w/builtin/get-tar-commit-id.c

@@ -20,13 +20,11 @@ int cmd_get_tar_commit_id(int argc, const char **argv, const char *prefix)
 	struct ustar_header *header = (struct ustar_header *)buffer;
 	char *content = buffer + RECORDSIZE;
 	const char *comment;
-	ssize_t n;
 
 	if (argc != 1)
 		usage(builtin_get_tar_commit_id_usage);
 
-	n = read_in_full(0, buffer, HEADERSIZE);
-	if (n < HEADERSIZE)
+	if (read_in_full(0, buffer, HEADERSIZE) < HEADERSIZE)
 		die("git get-tar-commit-id: read error");
 	if (header->typeflag[0] != 'g')
 		return 1;