[Bug 1641832] Re: Bash ignores exit trap on success when part of a command string

[Lee Trager]

I'm having trouble coming up with a simpler test case myself. Here is
how you can reproduce with lp:maas-images on Zesty with
bash-4.4-1ubuntu1

mkdir maas-images
cd maas-images
bzr init
bzr branch lp:maas-images
cd
export PATH=/home/ubuntu/maas-images/trunk/bin:$PATH
wget http://cloud-images.ubuntu.com/daily/server/zesty/20161111/zesty-server-cloudimg-amd64.squashfs
meph2-build --image-format squashfs-image -vv --enable-di amd64 zesty 20161111 zesty-server-cloudimg-amd64.squashfs out.d

If you execute that with 4.4-1ubuntu1 it fails due to the loop back
mount still being mounted(mount | grep maas). If you downgrade to
4.3-15ubuntu1 or use the patch from lp:~ltrager/maas-
images/workaround_lp1641832 umount is called and the script executes
successfully.

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https://bugs.launchpad.net/bugs/1641832

Title:
  Bash ignores exit trap on success when part of a command string

Status in bash package in Ubuntu:
  Triaged

Bug description:
  The MAAS team uses a script, lp:maas-images, which generates the
  images available at images.maas.io. As part of this process we use the
  following to convert a SquashFS image to an ext4 image.

  sudo bash -ec 'src="$1"; img="$2"; trgmp="$3";
      mounts=""
      cleanup() { for m in $mounts; do umount "$m"; done; }
      trap cleanup EXIT
      mount -o loop "$img" "$trgmp"
      mounts="$trgmp"
      unsquashfs -force -xattrs -dest "$trgmp" "$src"' \
      "squashimg-to-image" "$squashimg" "$output" "$trgmp"
  ret=$?
  rm -Rf "$mtmp" || return
  return $ret

  Prior to 4.4-1ubuntu1 the trap would always cause the cleanup function
  to always be called. Its now only called on failure. This causes the
  mount to remain and the following rm to fail. If I add 'false' to the
  end of the command script or downgrade to 4.3-15ubuntu1 the cleanup
  occurs.

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