Build pexes that work on all Python patch versions. (#7550)

We want to constrain pants.pex to run on CPython 2.7 or CPython 3.6,
but no further. Fixup CI to only request a specific patch-level in the
Python interpreter used in the two spots that care.

.travis.yml

@@ -596,13 +596,17 @@ py27_osx_build_wheels_ucs4: &py27_osx_build_wheels_ucs4
     - *base_build_wheels_env
     - CACHE_NAME=osxwheelsbuild.ucs4
     - PYTHON_CONFIGURE_OPTS=--enable-unicode=ucs4
-    # We set $PY to ensure the UCS4 interpreter is used when bootstrapping the PEX.
+    # We ensure selection of the the UCS4 interpreter by the pants script and pants.pex with these
+    # env vars.
+    - PYTHON_27_VERSION_UCS4=2.7.13
     - PY=${PYENV_ROOT}/shims/python2.7
+    - PEX_PYTHON_PATH=${PYENV_ROOT}/shims/python2.7
+    - PANTS_PYTHON_SETUP_INTERPRETER_CONSTRAINTS="['CPython==${PYTHON_27_VERSION_UCS4}']"
   before_install:
     - curl -L https://github.com/stedolan/jq/releases/download/jq-1.5/jq-osx-amd64 -o /usr/local/bin/jq
     - chmod 755 /usr/local/bin/jq
     - ./build-support/bin/install_aws_cli_for_ci.sh
-    - ./build-support/bin/install_python_for_ci.sh 2.7.13 ${PYENV_PY36_VERSION}
+    - ./build-support/bin/install_python_for_ci.sh ${PYTHON_27_VERSION_UCS4} ${PYENV_PY36_VERSION}
   script:
     - ./build-support/bin/ci.sh -2b
     - ./build-support/bin/check_pants_pex_abi.py cp27mu

build-support/bin/ci.sh

@@ -122,17 +122,11 @@ else
 fi
 export PY="${PY:-python${py_major_minor}}"
 
-# Also set PANTS_PYTHON_SETUP_INTERPRETER_CONSTRAINTS. We set this to the exact Python version
-# to resolve any potential ambiguity when multiple Python interpreters are discoverable, such as
-# Python 2.7.10 vs. 2.7.13. When running with Python 3, we must also set this constraint to ensure
-# all spawned subprocesses use Python 3 rather than the default of Python 2. This is in part
-# necessary to avoid the _Py_Dealloc error (#6985).
-py_major_minor_patch=$(${PY} -c 'import sys; print(".".join(map(str, sys.version_info[0:3])))')
-export PANTS_PYTHON_SETUP_INTERPRETER_CONSTRAINTS="${PANTS_PYTHON_SETUP_INTERPRETER_CONSTRAINTS:-['CPython==${py_major_minor_patch}']}"
+export PANTS_PYTHON_SETUP_INTERPRETER_CONSTRAINTS="${PANTS_PYTHON_SETUP_INTERPRETER_CONSTRAINTS:-['CPython==${py_major_minor}.*']}"
 banner "Setting interpreter constraints to ${PANTS_PYTHON_SETUP_INTERPRETER_CONSTRAINTS}"
 
 if [[ "${run_bootstrap:-false}" == "true" ]]; then
-  start_travis_section "Bootstrap" "Bootstrapping pants as a Python ${py_major_minor_patch} PEX"
+  start_travis_section "Bootstrap" "Bootstrapping pants as a Python ${py_major_minor} PEX"
   (
     if [[ "${run_bootstrap_clean:-false}" == "true" ]]; then
       ./build-support/python/clean.sh || die "Failed to clean before bootstrapping pants."

build-support/bin/release.sh

@@ -22,12 +22,17 @@ done
 # Reset opt parsing's position to start
 OPTIND=0
 
-# Set the Python interpreter to be used for the virtualenv. Note we allow the user to
-# predefine this value so that they may point to a specific interpreter, e.g. 2.7.13 vs. 2.7.15.
-default_interpreter="python2.7";
+# Setup Python interpreter constraints and pick a suitable interpreter to be used for the
+# virtualenv.
 if [[ "${python_three:-false}" == "true" ]]; then
   default_interpreter="python3.6"
+  interpreter_constraint="CPython==3.6.*"
+else
+  default_interpreter="python2.7"
+  interpreter_constraint="CPython==2.7.*"
 fi
+# Note we allow the user to predefine this value so that they may point to a specific interpreter,
+# e.g. 2.7.13 vs. 2.7.15.
 export PY="${PY:-${default_interpreter}}"
 if ! which "${PY}" >/dev/null; then
   die "Python interpreter ${PY} not discoverable on your PATH."
@@ -37,12 +42,7 @@ if [[ "${py_major_minor}" != "2.7" ]] && [[ "${py_major_minor}" != "3.6" ]]; the
   die "Invalid interpreter. The release script requires Python 2.7 or 3.6 (you are using ${py_major_minor})."
 fi
 
-# Also set PANTS_PYTHON_SETUP_INTERPRETER_CONSTRAINTS. We set this to the exact Python version
-# to resolve any potential ambiguity when multiple Python interpreters are discoverable, such as
-# Python 2.7.13 vs. 2.7.15. We must also set this when running with Python 3 to ensure
-# that spawned subprocesses use Python 3.
-py_major_minor_patch=$(${PY} -c 'import sys; print(".".join(map(str, sys.version_info[0:3])))')
-export PANTS_PYTHON_SETUP_INTERPRETER_CONSTRAINTS="${PANTS_PYTHON_SETUP_INTERPRETER_CONSTRAINTS:-['CPython==${py_major_minor_patch}']}"
+export PANTS_PYTHON_SETUP_INTERPRETER_CONSTRAINTS="['${interpreter_constraint}']"
 
 function run_local_pants() {
   ${ROOT}/pants "$@"
@@ -658,11 +658,6 @@ function build_pex() {
     platform_flags=("${platform_flags[@]}" "--platform=${platform}")
   done
 
-  interpreter_constraint="CPython==2.7.*"
-  if [[ "${python_three:-false}" == "true" ]]; then
-    interpreter_constraint="CPython==3.6.*"
-  fi
-
   execute_pex \
     -o "${dest}" \
     --script=pants \

build-support/docker/travis_ci_py27_ucs2/Dockerfile

@@ -27,8 +27,11 @@ RUN /usr/bin/scl enable devtoolset-7 -- ${PYENV_BIN} install ${PYTHON_27_VERSION
 RUN /usr/bin/scl enable devtoolset-7 -- ${PYENV_BIN} global ${PYTHON_27_VERSION_UCS2}
 ENV PATH "${PYENV_ROOT}/shims:${PATH}"
 
+# We ensure selection of the the UCS4 interpreter by the pants script and pants.pex with these
+# env vars.
 ENV PY "${PYENV_ROOT}/shims/python2.7"
 ENV PEX_PYTHON_PATH "${PYENV_ROOT}/shims/python2.7"
+ENV PANTS_PYTHON_SETUP_INTERPRETER_CONSTRAINTS="['CPython==${PYTHON_27_VERSION_UCS2}']"
 
 # Setup mount points for the travis ci user & workdir.
 VOLUME /travis/home

build-support/travis/travis.yml.mustache

@@ -543,11 +543,15 @@ py27_osx_build_wheels_ucs4: &py27_osx_build_wheels_ucs4
     - *base_build_wheels_env
     - CACHE_NAME=osxwheelsbuild.ucs4
     - PYTHON_CONFIGURE_OPTS=--enable-unicode=ucs4
-    # We set $PY to ensure the UCS4 interpreter is used when bootstrapping the PEX.
+    # We ensure selection of the the UCS4 interpreter by the pants script and pants.pex with these
+    # env vars.
+    - PYTHON_27_VERSION_UCS4=2.7.13
     - PY=${PYENV_ROOT}/shims/python2.7
+    - PEX_PYTHON_PATH=${PYENV_ROOT}/shims/python2.7
+    - PANTS_PYTHON_SETUP_INTERPRETER_CONSTRAINTS="['CPython==${PYTHON_27_VERSION_UCS4}']"
   before_install:
     {{>before_install_osx}}
-    - ./build-support/bin/install_python_for_ci.sh 2.7.13 ${PYENV_PY36_VERSION}
+    - ./build-support/bin/install_python_for_ci.sh ${PYTHON_27_VERSION_UCS4} ${PYENV_PY36_VERSION}
   script:
     - ./build-support/bin/ci.sh -2b
     - ./build-support/bin/check_pants_pex_abi.py cp27mu