sed expression in tools/tox-venv is not POSIX compatible

This bug was originally filed in Launchpad as [LP: #1853441](https://bugs.launchpad.net/cloud-init/+bug/1853441)

Launchpad details

affected_projects = []

assignee = None

assignee_name = None

date_closed = None

date_created = 2019-11-21T11:05:46.579756+00:00

date_fix_committed = None

date_fix_released = None

id = 1853441

importance = low

is_complete = False

lp_url = https://bugs.launchpad.net/cloud-init/+bug/1853441

milestone = None

owner = minagalic

owner_name = Mina Galić

private = False

status = triaged

submitter = minagalic

submitter_name = Mina Galić

tags = ['freebsd']

duplicates = []

_Launchpad user **Mina Galić(minagalic)** wrote on 2019-11-21T11:05:46.579756+00:00_

When running tools/tox-venv py3 on FreeBSD, I get the following Error:

sed: 2: ":x; /\\$/ { N; s/\\\n[ ...": unused label 'x; /\\$/ { N; s/\\\n[ ]*//; tx };'

This suggests to me, that the expression

sed -e ':x; /\\$/ { N; s/\\\n[ ]*//; tx };' "${tox_ini}" |

is not POSIX compatible.

Looking at GNU sed's documentation, I see the first issue with making this expression POSIX compliant will be that `N` behaves intentionally different on GNU sed than on POSIX:

https://www.gnu.org/software/sed/manual/html_node/Reporting-Bugs.html#Reporting-Bugs

So the solution here might be simply to require GNU sed, and put it's path/name into an environment variable that can be overriden from the outside.

Start with tools/tox-venv and reproduce the reported `tox-venv py3` failure on FreeBSD. Inspect the sed expression that processes tox_ini, compare its behavior with POSIX sed, and verify the chosen compatibility fix by rerunning the same command without the unused-label error.

Written by the indexing model from the issue text.