Travis CI fails randomly with "xvfb-run: error: Xvfb failed to start". Example:
https://travis-ci.org/python/cpython/jobs/454776965

whereas the full test suite succeeded.

Extract of .travis.yml:

    $XVFB_RUN make buildbottest TESTOPTS="-j4 -uall,-cpu"
    if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then
      $XVFB_RUN make PYTHON=../python SPHINXOPTS="-q -W -j4" -C Doc/ venv doctest
    fi

I'm not sure what is xvfb, nor why it's needed to run doctests...

xvfb is a utility that allows you to run a command in a virtual X server environment. I assume that is needed for the doctests that use the X server. I assume is related to PR9806

Found a relevant issue but 3 years old : https://github.com/travis-ci/travis-ci/issues/4387#issuecomment-125635027

Doc PR : https://github.com/travis-ci/docs-travis-ci-com/pull/311 . The docs recommend usage of -a to select a free port.

Same error: https://travis-ci.org/python/cpython/jobs/459983088

"make doctest" has been added by commit 859c068e52a31e13e2b9bb6a3f861fa8c290cb0e of bpo-34962.

New changeset a22df4896f6b83c8741203118790ae281716bca5 by Victor Stinner in branch 'master':
bpo-35240: Add "doctest" job to Travis CI (GH-10753)
https://github.com/python/cpython/commit/a22df4896f6b83c8741203118790ae281716bca5

I created a new job to no longer run xvfb-run twice. Let's see if it works around the issue. At least, it should make Travis CI a little bit faster.