Add PANTS_DEBUG-ability (#136)

Adds the ability to debug Pants itself through DAP leveraging `debugpy`.

Now you can use `PANTS_DEBUG=1 ./pants --no-pantsd ...` and attach VS Code to the server and debug rule code.

pants

@@ -362,17 +362,27 @@ function bootstrap_pants {
   local pants_version="$1"
   local python="$2"
   local pants_sha="${3:-}"
+  local pants_debug="${4:-}"
 
-  local pants_requirement="pantsbuild.pants==${pants_version}"
+  local pants_requirements=(pantsbuild.pants==${pants_version})
   local maybe_find_links
   if [[ -z "${pants_sha}" ]]; then
     maybe_find_links=""
   else
     maybe_find_links="--find-links=$(find_links_url "${pants_version}" "${pants_sha}")"
-   fi
+  fi
+
+  local debug_suffix
+  if [[ -z "${pants_debug}" ]]; then
+    debug_suffix=""
+  else
+    debug_suffix="-debug"
+    pants_requirements+=(debugpy==1.6.0)
+  fi
+
   local python_major_minor_version
   python_major_minor_version="$(get_python_major_minor_version "${python}")"
-  local target_folder_name="${pants_version}_py${python_major_minor_version}"
+  local target_folder_name="${pants_version}_py${python_major_minor_version}${debug_suffix}"
   local bootstrapped="${PANTS_BOOTSTRAP}/${target_folder_name}"
 
   if [[ ! -d "${bootstrapped}" ]]; then
@@ -382,15 +392,15 @@ function bootstrap_pants {
       staging_dir=$(tempdir "${PANTS_BOOTSTRAP}")
       local virtualenv_path
       virtualenv_path="$(bootstrap_virtualenv "${python}")" || exit 1
-      green "Installing ${pants_requirement} into a virtual environment at ${bootstrapped}"
+      green "Installing ${pants_requirements[@]} into a virtual environment at ${bootstrapped}"
       (
         scrub_env_vars
         # shellcheck disable=SC2086
         "${python}" "${virtualenv_path}" --quiet --no-download "${staging_dir}/install" && \
         # Grab the latest pip, but don't advance setuptools past 58 which drops support for the
         # `setup` kwarg `use_2to3` which Pants 1.x sdist dependencies (pystache) use.
         "${staging_dir}/install/bin/pip" install --quiet -U pip "setuptools<58" && \
-        "${staging_dir}/install/bin/pip" install ${maybe_find_links} --quiet --progress-bar off "${pants_requirement}"
+        "${staging_dir}/install/bin/pip" install ${maybe_find_links} --quiet --progress-bar off "${pants_requirements[@]}"
       ) && \
       ln -s "${staging_dir}/install" "${staging_dir}/${target_folder_name}" && \
       mv "${staging_dir}/${target_folder_name}" "${bootstrapped}" && \
@@ -474,15 +484,27 @@ fi
 cd "$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd -P)"
 pants_version="$(determine_pants_version)"
 python="$(determine_python_exe "${pants_version}")"
-pants_dir="$(bootstrap_pants "${pants_version}" "${python}" "${PANTS_SHA:-}")" || exit 1
+pants_dir="$(bootstrap_pants "${pants_version}" "${python}" "${PANTS_SHA:-}" "${PANTS_DEBUG:-}")" || exit 1
 
 pants_python="${pants_dir}/bin/python"
-pants_binary="${pants_dir}/bin/pants"
+pants_binary=(${pants_dir}/bin/pants)
 pants_extra_args=""
 if [[ -n "${PANTS_SHA:-}" ]]; then
   pants_extra_args="${pants_extra_args} --python-repos-repos=$(find_links_url "$pants_version" "$PANTS_SHA")"
 fi
 
+pants_debug_args=()
+if [ -n "${PANTS_DEBUG:-}" ]; then
+  if [[ "$*" != *"--no-pantsd"* ]]; then
+    echo "Error! Must pass '--no-pantsd' when using PANTS_DEBUG"
+    exit 1
+  fi
+  # NB: We can't invoke `-m debugpy` as that'll prepend CWD to sys.path which might have unintended side-effects.
+  # `-c` also prepends, but we strip that ourselves.
+  pants_binary=(-c "__import__(\"sys\").path.pop(0);__import__(\"debugpy.server.cli\").server.cli.main()" --listen 127.0.0.1:5678 --wait-for-client "${pants_binary[@]}")
+  echo "Will launch debugpy server at '127.0.0.1:5678' waiting for client connection."
+fi
+
 # shellcheck disable=SC2086
-exec "${pants_python}" "${pants_binary}" ${pants_extra_args} \
+exec "${pants_python}" "${pants_binary[@]}" ${pants_extra_args} \
   --pants-bin-name="${PANTS_BIN_NAME}" --pants-version=${pants_version} "$@"