run clean-all before switching pants python version runner scripts (#…

…7151)

### Problem

When running `./pants` vs `./pants3`, or changing python interpreter version via `PY=python3.6 ./pants` to `PY=python3.7 ./pants`, it is currently necessary to run `clean-all` first before switching which python interpreter is used to run pants. This is unfortunate, but temporary. Anyone testing with different python interpreter versions has to remember to run `clean-all` in between invocations. I find this difficult to remember.

### Solution

- Make `./pants` write the python version string to a sentinel file `.pants-python-version` at the repo root containing the python version we're running with before doing anything else, and if the version differs from the python version we are currently running pants under, run a `clean-all`.
- Add `ONLY_USING_SINGLE_PYTHON_VERSION` env var to CI to avoid running `clean-all` or needing to generate this sentinel file.

### Result

Many weird errors no longer occur when running `./pants3`.

### Followup Issues
- #7355 -- fix reporting not picking up a run if `clean-all` is one of the goals, which would let us avoid having to make two separate pants invocations here.

.gitignore

@@ -53,3 +53,5 @@ GRTAGS
 GSYMS
 GTAGS
 .mypy_cache/
+# Stores the version of python last used to run pants. See ./pants.
+/.python-interpreter-constraints

.travis.yml

@@ -19,6 +19,10 @@ env:
     - PANTS_CONFIG_FILES="${TRAVIS_BUILD_DIR}/pants.travis-ci.ini"
     - PYTEST_PASSTHRU_ARGS="-v --duration=3"
     - LC_ALL="en_US.UTF-8"
+    # This tells the ./pants runner script to avoid trying to clean the workspace when changing
+    # python versions. CI starts off without the .python-interpreter-constraints file, so it would otherwise
+    # run a clean-all without this env var.
+    - ONLY_USING_SINGLE_PYTHON_VERSION='true'
     - BOOTSTRAPPED_PEX_BUCKET=ci-public.pantsbuild.org
     - BOOTSTRAPPED_PEX_KEY_PREFIX=${TRAVIS_BUILD_NUMBER}/${TRAVIS_BUILD_ID}/pants.pex
     - BOOTSTRAPPED_PEX_URL_PREFIX=s3://${BOOTSTRAPPED_PEX_BUCKET}/${BOOTSTRAPPED_PEX_KEY_PREFIX}

build-support/docker/travis_ci/Dockerfile

@@ -21,4 +21,9 @@ USER ${TRAVIS_USER}:${TRAVIS_GROUP}
 # Our newly created user is unlikely to have a sane environment: set a locale at least.
 ENV LC_ALL="en_US.UTF-8"
 
+# This tells the ./pants runner script to avoid trying to clean the workspace when changing python
+# versions. CI starts off without the .python-interpreter-constraints file, so it would otherwise run a
+# clean-all without this env var.
+ENV ONLY_USING_SINGLE_PYTHON_VERSION 'true'
+
 WORKDIR /travis/workdir

build-support/docker/travis_ci_py27_ucs2/Dockerfile

@@ -46,4 +46,9 @@ USER ${TRAVIS_USER}:${TRAVIS_GROUP}
 # Our newly created user is unlikely to have a sane environment: set a locale at least.
 ENV LC_ALL="en_US.UTF-8"
 
+# This tells the ./pants runner script to avoid trying to clean the workspace when changing python
+# versions. CI starts off without the .python-interpreter-constraints file, so it would otherwise run a
+# clean-all without this env var.
+ENV ONLY_USING_SINGLE_PYTHON_VERSION 'true'
+
 WORKDIR /travis/workdir

build-support/docker/travis_ci_py36/Dockerfile

@@ -43,4 +43,9 @@ USER ${TRAVIS_USER}:${TRAVIS_GROUP}
 # Our newly created user is unlikely to have a sane environment: set a locale at least.
 ENV LC_ALL="en_US.UTF-8"
 
+# This tells the ./pants runner script to avoid trying to clean the workspace when changing python
+# versions. CI starts off without the .python-interpreter-constraints file, so it would otherwise run a
+# clean-all without this env var.
+ENV ONLY_USING_SINGLE_PYTHON_VERSION 'true'
+
 WORKDIR /travis/workdir

build-support/travis/travis.yml.mustache

@@ -12,6 +12,10 @@ env:
     - PANTS_CONFIG_FILES="${TRAVIS_BUILD_DIR}/pants.travis-ci.ini"
     - PYTEST_PASSTHRU_ARGS="-v --duration=3"
     - LC_ALL="en_US.UTF-8"
+    # This tells the ./pants runner script to avoid trying to clean the workspace when changing
+    # python versions. CI starts off without the .python-interpreter-constraints file, so it would otherwise
+    # run a clean-all without this env var.
+    - ONLY_USING_SINGLE_PYTHON_VERSION='true'
     - BOOTSTRAPPED_PEX_BUCKET=ci-public.pantsbuild.org
     - BOOTSTRAPPED_PEX_KEY_PREFIX=${TRAVIS_BUILD_NUMBER}/${TRAVIS_BUILD_ID}/pants.pex
     - BOOTSTRAPPED_PEX_URL_PREFIX=s3://${BOOTSTRAPPED_PEX_BUCKET}/${BOOTSTRAPPED_PEX_KEY_PREFIX}

pants

@@ -2,6 +2,42 @@
 # Copyright 2014 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+REPO_ROOT=$(cd $(dirname "${BASH_SOURCE[0]}") && pwd -P)
+
+interpreter_constraints_sentinel_file_path='.python-interpreter-constraints'
+
+function clean_if_interpreter_constraints_changed() {
+  new_interpreter_constraints="$1"
+
+  if [[ -n "${recursive_pants_shell_script_invocation:-}" ]]; then
+    return 0
+  fi
+
+  interpreter_constraints_file="${REPO_ROOT}/${interpreter_constraints_sentinel_file_path}"
+
+  if [[ -f "${interpreter_constraints_file}" ]]; then
+    previous_interpreter_constraints="$(cat "${interpreter_constraints_file}")"
+    if [[ "${new_interpreter_constraints}" == "${previous_interpreter_constraints}" ]]; then
+      return 0
+    fi
+  else
+    cat >&2 <<EOF
+${interpreter_constraints_sentinel_file_path} not found in the buildroot.
+Forcing an initial clean-all.
+EOF
+  fi
+
+  export recursive_pants_shell_script_invocation='y'
+  cat >&2 <<EOF
+Different interpreter constraints were set than were used in the previous Pants run.
+Clearing the cache and preparing a Python environment with these interpreter constraints:
+${new_interpreter_constraints}
+EOF
+  ./pants clean-all
+  echo "${new_interpreter_constraints}" > "${interpreter_constraints_file}"
+  unset recursive_pants_shell_script_invocation
+}
+
 # This bootstrap script runs pants from the live sources in this repo.
 #
 # Further support is added for projects wrapping pants with custom external extensions.  In the
@@ -67,6 +103,11 @@ export PY="${PY:-python3}"
 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}']}"
 
+# We can currently assume the CI environment does not switch python versions within a shard.
+if [[ "${ONLY_USING_SINGLE_PYTHON_VERSION:-false}" != 'true' ]]; then
+  clean_if_interpreter_constraints_changed "${PANTS_PYTHON_SETUP_INTERPRETER_CONSTRAINTS}"
+fi
+
 PANTS_EXE="${HERE}/src/python/pants/bin/pants_loader.py"
 
 if [[ ! -z "${WRAPPER_REQUIREMENTS}" ]]; then

tests/python/pants_test/pants_run_integration_test.py

@@ -183,6 +183,10 @@ def hermetic_env_whitelist(cls):
         # Ensure that the underlying ./pants invocation doesn't run from sources
         # (and therefore bootstrap) if we don't want it to.
         'RUN_PANTS_FROM_PEX',
+        # This tells the ./pants runner script to avoid trying to clean the workspace when changing
+        # python versions. CI starts off without the .python-interpreter-constraints file, so it
+        # would otherwise run a clean-all without this env var.
+        'ONLY_USING_SINGLE_PYTHON_VERSION',
       ]
 
   def setUp(self):