[Build] Allow Python version to be set by ENV VAR

Author: AutomatedTester

.bazelrc

@@ -4,33 +4,42 @@ build --flag_alias=pin_browsers=//common:pin_browsers
 build --flag_alias=headless=//common:headless
 
 # Set the default java toolchain
+
 build --java_runtime_version=remotejdk_11
 build --tool_java_language_version=11
 
 # We target java 8 by default
+
 build --javacopt="--release 8"
 
 # Require java dependencies to be used and first-order
+
 build --experimental_strict_java_deps=strict
 build --explicit_java_test_deps
 
 # Ensure builds are unpolluted by the user env
+
 build --incompatible_strict_action_env
 
 # Except for the PATH environment variable
+
 build --action_env=PATH
 
 # For build stamping
+
 build --workspace_status_command=scripts/build-info.py
 
 # Make sure we get something helpful when tests fail
+
 build --verbose_failures
 build --test_output=errors
 
 # Tests need to be able to open sockets on localhost
+
 build --noexperimental_sandbox_default_allow_network
 
 # pass environment variables to the test environment
+
 build --test_env=CI
 build --test_env=DASHBOARD_URL
 build --test_env=DISPLAY
@@ -41,6 +50,7 @@ build --test_env=MOZ_HEADLESS
 build --test_env=PATH  # Remove once browser pinning works
 build --test_env=SELENIUM_BROWSER
 build --test_env=TRAVIS
+build --test_env=PYTHON_VERSION
 
 test --test_timeout=1800
 

WORKSPACE

@@ -6,6 +6,14 @@ workspace(
     },
 )
 
+load("//common/private:env.bzl", "env")
+env(
+    name = "python_version",
+    env_var=["PYTHON_VERSION"]
+)
+load("@python_version//:defs.bzl", "PYTHON_VERSION")
+
+
 register_toolchains(":py_toolchain")
 
 load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
@@ -61,7 +69,7 @@ load("@rules_python//python:repositories.bzl", "python_register_toolchains")
 
 python_register_toolchains(
     name = "python_toolchain",
-    python_version = "3.9",
+    python_version = PYTHON_VERSION,
 )
 
 load("@python_toolchain//:defs.bzl", "interpreter")

common/private/env.bzl

@@ -0,0 +1,11 @@
+def _env_impl(rctx):
+  rctx.file("BUILD.bazel") # So we can refer to the defs.bzl file we're about to create
+  defs = ["%s = %s" % (k, repr(rctx.os.environ.get(k, "3.8"))) for k in rctx.attr.env_var]
+  rctx.file("defs.bzl", "\n".join(defs))
+
+env = repository_rule(
+    implementation = _env_impl,
+    attrs = {
+        "env_var": attr.string_list(),
+    },
+)