Re: [PATCH v3] [PR gdb/27614] gdb-add-index fails on symlinks.

[Lancelot SIX <lsix@lancelotsix.com>]

Le Sun, Mar 28, 2021 at 12:11:22PM -0400, Simon Marchi a écrit :
> 
> 
> On 2021-03-27 1:27 p.m., Lancelot SIX via Gdb-patches wrote:
> > Since V2:
> >         - Use GDB to follow symlink instead of readlink. Unlike
> >           readlink, GDB is guaranteed to be available.
> > 
> > Since V1:
> >         - Replace '&>/dev/null' with '>/dev/null 2>&1'
> > 
> > --
> > 
> > PR 27614 shows that gdb-add-index fails to generate the index when its
> > argument is a symlink.
> > 
> > The following one liner illustrates the reported problem:
> > 
> >         $ echo 'int main(){}'|gcc -g -x c -;ln -s a.out symlink;gdb-add-index symlink
> >         gdb-add-index: No index was created for symlink
> >         gdb-add-index: [Was there no debuginfo? Was there already an index?]
> >         $ ls -l
> >         -rwxr-xr-x 1 25712 Mar 19 23:05 a.out*
> >         -rw------- 1  8277 Mar 19 23:05 a.out.gdb-index
> >         lrwxrwxrwx 1     5 Mar 19 23:05 symlink -> a.out*
> > 
> > GDB generates the .gdb-index file with a name that matches the name of
> > the actual program (a.out.gdb-index here), not the symlink that
> > references it.  The remaining of the script is looking for a file named
> > after the provided argument (would be 'symlink.gdb-index' in our
> > example).
> > 
> > The common option to solve such issue would be to use readlink to follow
> > the symlink.  Unfortunately, this command is not available in the POSIX
> > standard.  This commit therefore proposes to use GDB itself to identify
> > where the symlink points to.  This requires some parsing of GDB output.
> > The added test should be enough to detect regression if GDB where to
> > change the way it formats its output.
> 
> I preferred your previous approach, compared to relying on a maintenance
> command.  Relying on a maintenance command is fine in tests, for example,
> but here somebody could use gdb-add-index from a given GDB version with
> a GDB of a different version.
> 
>   GDB=/my/newer/gdb gdb-add-index a.out

Yes, agreed. I was reluctant to rely on maint command in the first
place, I should have kept it that way!

> 
> In the previous review, you said:
> 
> >> Would it work with just `readlink <file>`?
> 
> > This would fail if $file is a symlink to a symlink.  This is what
> > ldconfig usually does (libfoo.so -> libfoo.so.x -> libfoo.so.x.y).
> 
> Can't you just call it in a loop then?
> 
>   while file is a symlink:
>     file=readlink $file
> 

Yes. It would look somethink like:


	if test -L "$1"; then
		if command -v readlink >/dev/null 2>&1; then
			file="$1"
			while test -L "$file"; do
				target=$(readlink "$file")
				case "$target" in
					/*)
						file="$target"
						;;
					*)
						file="$(dirname "$file")/$target"
						;;
				esac
			done
		else
			echo "$myname: 'readlink' missing.  Failed to follow symlink $1." 1>&2
			exit 1
		fi
	else
		file="$1"
	fi

I’ll test this and prepare a V4 shortly.

Thanks for the comments.
Lancelot.

> Simon