address some posix-shell issues in runtime env interpreter script

Author: rickeylev

python/private/runtime_env_toolchain_interpreter.sh

@@ -57,20 +57,24 @@ fi
 # detecting virtualenvs like normal (i.e. using the venv symlink to find the
 # real interpreter). To work around this, we have to manually detect the venv,
 # then trick the interpreter into understanding we're in a virtual env.
-self_dir=$(dirname $0)
-if [[ -e "$self_dir/pyvenv.cfg" || -e "$self_dir/../pyvenv.cfg" ]]; then
-  if [[ "$0" == /* ]]; then
-    venv_bin="$0"
-  else
-    venv_bin="$PWD/$0"
-  fi
+self_dir=$(dirname "$0")
+if [ -e "$self_dir/pyvenv.cfg" ] || [ -e "$self_dir/../pyvenv.cfg" ]; then
+  case "$0" in
+    /*)
+      venv_bin="$0"
+      ;;
+    *)
+      venv_bin="$PWD/$0"
+      ;;
+  esac
 
   # PYTHONEXECUTABLE is also used because `exec -a` doesn't fully trick the
   # pyenv wrappers.
   # NOTE: The PYTHONEXECUTABLE envvar only works for non-Mac starting in Python 3.11
   export PYTHONEXECUTABLE="$venv_bin"
   # Python looks at argv[0] to determine sys.executable, so use exec -a
   # to make it think it's the venv's binary, not the actual one invoked.
+  # NOTE: exec -a isn't strictly posix-compatible, but very widespread
   exec -a "$venv_bin" "$PYTHON_BIN" "$@"
 else
   exec "$PYTHON_BIN" "$@"