On Wed, Feb 20, 2019 at 03:55:58PM +0100, Ævar Arnfjörð Bjarmason wrote:
> > @@ -376,11 +377,15 @@ corrupt_graph_and_verify() {
> > data="${2:-\0}"
> > grepstr=$3
> > cd "$TRASH_DIRECTORY/full" &&
> > + orig_size=$(wc -c < $objdir/info/commit-graph) &&
> > + zero_pos=${4:-${orig_size}} &&
> > test_when_finished mv commit-graph-backup $objdir/info/commit-graph &&
> > cp $objdir/info/commit-graph commit-graph-backup &&
> > printf "$data" | dd of="$objdir/info/commit-graph" bs=1 seek="$pos"
> > conv=notrunc &&
> > + dd of="$objdir/info/commit-graph" bs=1 seek="$zero_pos" count=0 &&
> > + dd if=/dev/zero of="$objdir/info/commit-graph" bs=1 seek="$zero_pos"
> > count=$(($orig_size - $zero_pos)) &&
>
> In the limited time I had to dig it starts failing at test 46, when
> count=0 is given. dd on NetBSD exits with 127 when given count=0 it
> seems.
So the first 'dd' is supposed to truncate the commit-graph file at
$zero_pos. I don't think we need 'count=0' for that: in the absence
of the 'if=...' operand, 'dd' reads from standard input, which is
redirected from /dev/null in our test scripts, i.e. there is nothing
to read, and, consequently, there is nothing to write, either.
Though not strictly necessary, I would feel more comfortable if
'if=/dev/null' would be explicitly specified, and even more so with a
"# truncate at $zero_pos" comment above that command.
As to the second 'dd', I think we should not run it at all when count
would be zero, i.e. when $orig_size = $zero_pos, because in
combination with 'if=/dev/zero' it's asking for trouble. According to
POSIX [1]:
count=n
Copy only n input blocks. If n is zero, it is unspecified
whether no blocks or all blocks are copied.
Imagine a 'dd' that implements the second option: there are infinite
blocks in /dev/zero to copy! OTOH, if an implementation chooses the
first option (e.g. the usual Linux 'dd' from coreutils), then both of
these 'dd' invocations will leave the commit-graph file as-is, so it
doesn't matter whether we run them or not.
[1] http://pubs.opengroup.org/onlinepubs/9699919799/utilities/dd.html
