[Libguestfs] [libnbd PATCH v4 0/2] lib/utils: introduce async-signal-safe execvpe()

Wed Mar 22 10:24:37 UTC 2023

On Tue, Mar 21, 2023 at 03:56:22PM +0100, Laszlo Ersek wrote:
> On 3/21/23 15:05, Eric Blake wrote:
> > On Tue, Mar 21, 2023 at 07:04:59AM +0100, Laszlo Ersek wrote:
> >> On 3/20/23 20:41, Eric Blake wrote:
> >>> On Sun, Mar 19, 2023 at 10:41:37AM +0100, Laszlo Ersek wrote:
> >>>> This is version 4 of the following sub-series:
> >>>>
> >>>> [libnbd PATCH v3 09/29] lib/utils: introduce async-signal-safe execvpe()
> >>>> [libnbd PATCH v3 10/29] lib/utils: add unit tests for async-signal-safe execvpe()
> >>>>
> > ...
> >>>
> >>
> >> Series merged as commit range 742cbd8c7adc..0b7172b3cffa.
> >
> > I see you already fixed one pipeline failure due to some gcc versions
> > being more picky about function __attribute__ placement than others.
> > The remaining failures are with alpine Linux, where /bin/expr comes
> > from busybox instead of coreutils, and has the unfortunate quality of
> > having its behavior dependent on argv[0].
> 
> :)
> 
> I just wanted to announce on-list that I disabled
> "lib/test-fork-safe-execvpe.sh" on Alpine Linux because of this :)
> 
> >
> > Starting from a clean clone, I reproduced it locally with:
> >
> > $ git diff
> > diff --git i/lib/test-fork-safe-execvpe.sh w/lib/test-fork-safe-execvpe.sh
> > index 838bac9..4b3700c 100755
> > --- i/lib/test-fork-safe-execvpe.sh
> > +++ w/lib/test-fork-safe-execvpe.sh
> > @@ -18,7 +18,7 @@
> >
> >  . ../tests/functions.sh
> >
> > -set -e
> > +set -ex
> >
> >  # Determine the absolute pathname of the execvpe helper binary. The "realpath"
> >  # utility is not in POSIX, but Linux, FreeBSD and OpenBSD all have it.
> > @@ -155,7 +155,7 @@ success()
> >
> >  # Create a temporary directory and change the working directory to it.
> >  tmpd=$(mktemp -d)
> > -cleanup_fn rm -r -- "$tmpd"
> > +#cleanup_fn rm -r -- "$tmpd"
> >  cd "$tmpd"
> >
> >  # If the "file" parameter of execvpe() is an empty string, then we must fail --
> >
> > $ podman build -f ci/containers/alpine-edge.Dockerfile -t libnbd-alpine-edge
> > $ podman run -it --rm --userns=keep-id -v .:/repo:z -w /repo libnbd-alpine-edge bash
> > $ ./configure
> > $ make check
> > $ grep tmpd= lib/test-suite.log
> > + tmpd=/tmp/tmp.EMgKeF
> > $ /tmp/tmp.EMgKeF/bin/f 1 + 1
> > f: applet not found
> > 0b748c9fe495:~$
> >
> > So it looks like we need some way to work around busybox' insistance
> > that argv[0] determines which applet to run.
> 
> I couldn't come up with a reproducer like yours. I couldn't figure out
> how to *quickly* get an interactive Alpine Linux environment with the
> test failing, and I also couldn't figure out how to trigger "verbose"
> test runs on gitlab, without polluting the master branch with debug
> patches. (I tried forking the project in my own space on gitlab, and
> pushed a debug patch with just "set -x" onto a non-master branch *there*
> -- but CI didn't start in response to that.)
> 
> So, I only installed a new Alpine Linux virtual machine -- what a pain
> *that* was --, and investigated what /usr/bin/expr was. When I found it
> was a symlink to /bin/busybox, I started looking for Alpine Linux
> specific tweaks that could replace busybox (in this role) with a real
> binary executable "expr" utility.
> 
> I was relieved to find the following wiki article:
> 
>   https://wiki.alpinelinux.org/wiki/How_to_get_regular_stuff_working
> 
> which promised -- I thought anyways -- a real coreutils package.
> 
> Imagine my dismay when I found that, after installing coreutils with apk
> in the Alpine Linux VM, the symlink stayed in place, only its target
> binary changed from "/bin/busybox" to "coreutils". Well done, Alpine
> Linux, well done.
> 
> So, no, this mess (= Alpine Linux) is not salvageable. The
> "lib/test-fork-safe-execvpe.sh" script depends on "expr" being
> functional under the name "f". And yes I want "f" to be a
> single-character filename; otherwise the nice tabular formatting of the
> script falls apart (or produces overlong lines).
> 
> As last step, I learned about ci/skipped_tests, and used it to disable
> the test on alpine linux.
> 
> The latest pipeline passed:
> <https://gitlab.com/nbdkit/libnbd/-/pipelines/813280321>.
> 
> Either way, I wanted to highlight the following commits on the list:
> 
> 1  b29ff42e5d00 lib: account for realpath deficiency on some platforms
> 2  65631e5dfff5 lib/utils: try to placate attribute placement restriction from gcc
> 3  4cae20ccefaf Revert "lib: account for realpath deficiency on some platforms"
> 4  f5a065aa3a9c ci: skip "lib/test-fork-safe-execvpe.sh" on Alpine Linux

IIUC, that skips the test in CI, but if a developer or downstream user
runs 'make check' won't they still execute test-fork-safe-execvpe.sh ?
IOW, will the next release of libnbd have a broken test when it gets
imported into the Alpine distro ?

FWIW, in terms of CI I'd suggest that skipping Alpine is something that
is an especially bad idea. Alpine is the only distro in CI that uses
Musl instead of GLibc, and Alpine also does other relatively unusual
things compared to other Linux distros, such as the use of busybox that
impacted us here. Thus despite the pain, testing on Alpine has a pretty
significant value-add for test scenario coverage, compared to testing
on lots of other Linux distros which are effectively just different
versions of the same regular software setup.

With regards,
Daniel
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