Use PYTHON_EXE env var as the python interpreter (#11212)

Setting the PYTHON_EXE environment variable causes new Python virtual environments
to be created using the specified interpreter. Both `make` and `mage` honor the
variable.

For example, `PYTHON_EXE=python2.7 make python-env`.

CHANGELOG-developer.next.asciidoc

@@ -37,3 +37,5 @@ The list below covers the major changes between 7.0.0-beta1 and master only.
 - Support for Logger in the Metricset base instance. {pull}11106[11106]
 - Introduce processing.Support to instance.Setting. This allows Beats to fully modify the event processing. {pull}10801[10801]
 - Filebeat modules can now use ingest pipelines in YAML format. {pull}11209[11209]
+- Added support for using PYTHON_EXE to control what Python interpreter is used
+  by `make` and `mage`. Example: `export PYTHON_EXE=python2.7`. {pull}11212[11212]

dev-tools/mage/pytest.go

@@ -167,10 +167,17 @@ func PythonVirtualenv() (string, error) {
 		return pythonVirtualenvDir, nil
 	}
 
+	// If set use PYTHON_EXE env var as the python interpreter.
+	var args []string
+	if pythonExe := os.Getenv("PYTHON_EXE"); pythonExe != "" {
+		args = append(args, "-p", pythonExe)
+	}
+	args = append(args, ve)
+
 	// Execute virtualenv.
 	if _, err := os.Stat(ve); err != nil {
 		// Run virtualenv if the dir does not exist.
-		if err := sh.Run("virtualenv", ve); err != nil {
+		if err := sh.Run("virtualenv", args...); err != nil {
 			return "", err
 		}
 	}
@@ -181,7 +188,7 @@ func PythonVirtualenv() (string, error) {
 	}
 
 	pip := virtualenvPath(ve, "pip")
-	args := []string{"install"}
+	args = []string{"install"}
 	if !mg.Verbose() {
 		args = append(args, "--quiet")
 	}

libbeat/scripts/Makefile

@@ -250,7 +250,7 @@ load-tests: ## @testing Runs load tests
 # Sets up the virtual python environment
 .PHONY: python-env
 python-env: ${ES_BEATS}/libbeat/tests/system/requirements.txt
-	@test -d ${PYTHON_ENV} || virtualenv ${VIRTUALENV_PARAMS} ${PYTHON_ENV}
+	@test -d ${PYTHON_ENV} || virtualenv $(if ${PYTHON_EXE},-p ${PYTHON_EXE}) ${VIRTUALENV_PARAMS} ${PYTHON_ENV}
 	@. ${PYTHON_ENV}/bin/activate && pip install ${PIP_INSTALL_PARAMS} -q --upgrade pip ; \
 	if [ -a ./tests/system/requirements.txt ] && [ ! ${ES_BEATS}/libbeat/tests/system/requirements.txt -ef ./tests/system/requirements.txt ] ; then \
 		. ${PYTHON_ENV}/bin/activate && pip install ${PIP_INSTALL_PARAMS} -qUr ${ES_BEATS}/libbeat/tests/system/requirements.txt -Ur ./tests/system/requirements.txt ; \