On 2/10/24 10:26 AM, Andrew Burgess wrote:
GDB makes use of the libiberty function buildargv for splitting the
inferior (program being debugged) argument string in the case where
the inferior is not being started under a shell.
I have recently been working to improve this area of GDB, and noticed
some unexpected behaviour to the libiberty function buildargv, when
the input is a string consisting only of white space.
What I observe is that if the input to buildargv is a string
containing only white space, then buildargv will return an argv list
containing a single empty argument, e.g.:
char **argv = buildargv (" ");
assert (*argv[0] == '\0');
assert (argv[1] == NULL);
We get the same output from buildargv if the input is a single space,
or multiple spaces. Other white space characters give the same
results.
This doesn't seem right to me, and in fact, there appears to be a work
around for this issue in expandargv where we have this code:
/* If the file is empty or contains only whitespace, buildargv would
return a single empty argument. In this context we want no arguments,
instead. */
if (only_whitespace (buffer))
{
file_argv = (char **) xmalloc (sizeof (char *));
file_argv[0] = NULL;
}
else
/* Parse the string. */
file_argv = buildargv (buffer);
I think that the correct behaviour in this situation is to return an
empty argv array, e.g.:
char **argv = buildargv (" ");
assert (argv[0] == NULL);
And it turns out that this is a trivial change to buildargv. The diff
does look big, but this is because I've re-indented a block. Check
with 'git diff -b' to see the minimal changes. I've also removed the
work around from expandargv.
When testing this sort of thing I normally write the tests first, and
then fix the code. In this case test-expandargv.c has sort-of been
used as a mechanism for testing the buildargv function (expandargv
does call buildargv most of the time), however, for this particular
issue the work around in expandargv (mentioned above) masked the
buildargv bug.
I did consider adding a new test-buildargv.c file, however, this would
have basically been a copy & paste of test-expandargv.c (with some
minor changes to call buildargv). This would be fine now, but feels
like we would eventually end up with one file not being updated as
much as the other, and so test coverage would suffer.
Instead, I have added some explicit buildargv testing to the
test-expandargv.c file, this reuses the test input that is already
defined for expandargv.
Of course, once I removed the work around from expandargv then we now
do always call buildargv from expandargv, and so the bug I'm fixing
would impact both expandargv and buildargv, so maybe the new testing
is redundant? I tend to think more testing is always better, so I've
left it in for now.
So just an FYI. Sometimes folks include the -b diffs as well for these
scenarios. THe problem with doing so (as I recently stumbled over
myself) is the bots which monitor the list and apply patches get quite
confused by that practice. Anyway, just something to be aware of.
As for testing, I tend to agree, more is better unless we're highly
confident its redundant. So I'll go with your judgment on
redundant-ness of the test.
As with the prior patch, you'll need to run it through the usual
bootstrap/regression cycle and cobble together a ChangeLog.
OK once those things are taken care of.
jeff
