feat: cache decrypted repository "secrets"

nix/extra-builtins.nix

@@ -22,8 +22,11 @@
   in
     lenContent >= lenSuffix && builtins.substring (lenContent - lenSuffix) lenContent content == suffix;
 in {
+  # Instead of calling rage directly here, we call a wrapper script that will cache the output
+  # in a predictable path in /tmp, which allows us to only require the password for each encrypted
+  # file once.
   rageImportEncrypted = identities: nixFile:
     assert assertMsg (builtins.isPath nixFile) "The file to decrypt must be given as a path to prevent impurity.";
     assert assertMsg (hasSuffix ".nix.age" nixFile) "The content of the decrypted file must be a nix expression and should therefore end in .nix.age";
-      exec (["rage" "-d"] ++ (builtins.concatMap (x: ["-i" x]) identities) ++ [nixFile]);
+      exec ([./rage-decrypt.sh nixFile] ++ identities);
 }

nix/rage-decrypt.sh

@@ -0,0 +1,33 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+file="$1"
+[[ "$file" == "/nix/store/"* ]] || { echo "Input must be a store path!"; exit 1; }
+shift
+identities=("$@")
+
+# Strip .age suffix and store path prefix
+basename="${file%".age"}"
+basename="${basename#*"-"}"
+
+# Calculate a unique content-based identifier (relocations of
+# the source file in the nix store should not affect caching)
+new_name="$(sha512sum "$file")"
+new_name="${new_name:0:32}-${basename//"/"/"%"}"
+
+# Derive the path where the decrypted file will be stored
+out="/tmp/nix-import-encrypted/$new_name"
+mkdir -p "$(dirname "$out")"
+
+# Decrypt only if necessary
+if [[ ! -e "$out" ]]; then
+	args=()
+	for i in "${identities[@]}"; do
+		args+=("-i" "$i")
+	done
+	rage -d "${args[@]}" -o "$out" "$file"
+fi
+
+# Print decrypted content
+cat "$out"