python: add a wrapper to indirect 2/3

bazelbuild/bazel#3517

.bazelrc

@@ -1,3 +1,4 @@
+common --python_top=//python
 build --show_loading_progress
 build --show_progress
 build --show_progress_rate_limit=60.0
@@ -16,5 +17,4 @@ build:ci --nouse_action_cache
 build:ci --sandbox_debug
 build:ci --spawn_strategy=standalone
 common:ci --color=no
-common:ci --python_top=//python:py2
 test:ci --test_strategy=standalone

python/BUILD.bazel

@@ -1,13 +1,13 @@
-py_runtime(
-    name = "py2",
-    files = [],
-    interpreter_path = "/usr/bin/python2",
-    visibility = ["//visibility:public"],
+load("@io_bazel_rules_go//go:def.bzl", "go_binary")
+
+go_binary(
+    name = "wrapper",
+    srcs = ["wrapper.go"],
 )
 
 py_runtime(
-    name = "py3",
+    name = "python",
     files = [],
-    interpreter_path = "/usr/bin/python3",
+    interpreter = ":wrapper",
     visibility = ["//visibility:public"],
 )

python/wrapper.go

@@ -0,0 +1,61 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"log"
+	"os"
+	"strings"
+	"syscall"
+)
+
+func main() {
+	args := os.Args
+	env := os.Environ()
+	if err := wrap(args, env); err != nil {
+		log.Fatal(err)
+	}
+}
+
+func wrap(args []string, env []string) error {
+	python, _ := guess(args)
+
+	args = append([]string{python}, args[1:]...)
+	return syscall.Exec(python, args, env)
+}
+
+func guess(args []string) (string, error) {
+	python := python2()
+
+	if len(args) < 2 {
+		return python, fmt.Errorf("missing file argument")
+	}
+	file, err := os.Open(os.Args[1])
+	if err != nil {
+		return python, err
+	}
+	defer file.Close()
+
+	scanner := bufio.NewScanner(file)
+	scanner.Scan()
+	shebang := scanner.Text()
+	if err := scanner.Err(); err != nil {
+		return python, err
+	}
+
+	switch {
+	case strings.Contains(shebang, "python3"):
+		python = python3()
+	default:
+		python = python2()
+	}
+	return python, nil
+}
+
+func python2() string {
+	return "/usr/bin/python2"
+}
+
+func python3() string {
+	return "/usr/bin/python3"
+}