1 files changed, 47 insertions, 0 deletions
diff --git a/ops/terraform/deploy-nixos/nix-eval.sh b/ops/terraform/deploy-nixos/nix-eval.sh
new file mode 100755
index 0000000000..65f534180b
--- /dev/null
+++ b/ops/terraform/deploy-nixos/nix-eval.sh
@@ -0,0 +1,47 @@
+#!/usr/bin/env bash
+
+# SPDX-FileCopyrightText: 2023 The TVL Authors
+#
+# SPDX-License-Identifier: MIT
+set -ueo pipefail
+
+# Evaluates a Nix expression.
+#
+# Receives input parameters as JSON from stdin.
+# It expects a dict with the following keys:
+#
+#  - `attrpath`: the attribute.path pointing to the expression to instantiate.
+#    Required.
+#  - `entrypoint`: the path to the Nix file to invoke.
+#    Optional. If omitted, will shell out to git to determine the repo root,
+#    and Nix will use `default.nix` in there.
+#  - `argstr_json`: A string JSON-encoding a map containing string keys and
+#    values which should be passed to Nix as `--argstr $key $value`.
+#    command line args. Optional.
+#  - `build`: A boolean (or string being "true" or "false") stating whether the
+#    expression should also be built/substituted on the machine executing this script.
+#
+# jq's @sh format takes care of escaping.
+eval "$(jq -r '@sh "attrpath=\(.attrpath) && entrypoint=\(.entrypoint) && argstr=\((.argstr_json // "{}"|fromjson) | to_entries | map ("--argstr", .key, .value) | join(" ")) build=\(.build)"')"
+
+# Evaluate the expression.
+[[ -z "$entrypoint" ]] && entrypoint=$(git rev-parse --show-toplevel)
+# shellcheck disable=SC2086,SC2154
+drv=$(nix-instantiate -A "${attrpath}" "${entrypoint}" ${argstr})
+
+# If `build` is set to true, invoke nix-build on the .drv.
+# We need to swallow all stdout, to not garble the JSON printed later.
+# shellcheck disable=SC2154
+if [ "${build}" == "true" ]; then
+  nix-build --no-out-link "${drv}" > /dev/null
+fi
+
+# Determine the output path.
+outPath=$(nix show-derivation "${drv}" | jq -r ".\"${drv}\".outputs.out.path")
+
+# Return a JSON back to stdout.
+# It contains the following keys:
+#
+# - `drv`: the store path of the Derivation that has been instantiated.
+# - `outPath`: the output store path.
+jq -n --arg drv "$drv" --arg outPath "$outPath" '{"drv":$drv, "outPath":$outPath}'