Support early build cache hints

README.md

@@ -15,6 +15,32 @@ also disables the default shell-oriented `amazon-init` service and enables a
 small generic service that treats EC2 user-data as `/etc/nixos/flake.nix`, then
 runs `nixos-rebuild switch --flake /etc/nixos`.
 
+## First-Boot Build Cache Hints
+
+The first user-data switch is built by the AMI's existing Nix configuration,
+before the target NixOS system has activated its own `nix.settings`. A caller
+flake can optionally expose `quixosEarlyBuildNixConfig` to make that first
+build use known binary caches without baking any project-specific cache into
+the AMI:
+
+```nix
+{
+  quixosEarlyBuildNixConfig = {
+    hosts = {
+      "2600:..." = [ "nix-cache.internal.example.org" ];
+    };
+    substituters = [ "http://nix-cache.internal.example.org/cache" ];
+    trustedPublicKeys = [ "nix-cache.internal.example.org-1:..." ];
+  };
+}
+```
+
+The AMI runner evaluates this output after writing `/etc/nixos/flake.nix`,
+appends the host hints to `/etc/hosts`, and passes the substituters and trusted
+public keys through `NIX_CONFIG` only for the first `nixos-rebuild switch`.
+The target NixOS configuration should still declare the same cache in
+`nix.settings` for all later rebuilds.
+
 ## Publish
 
 The normal flow builds on a temporary EC2 instance, so your local machine does

user-data.sh.tftpl

@@ -75,7 +75,7 @@ write_config() {
             wantedBy = [ "multi-user.target" ];
             after = [ "fetch-ec2-metadata.service" "network-online.target" "nix-daemon.socket" ];
             wants = [ "fetch-ec2-metadata.service" "network-online.target" "nix-daemon.socket" ];
-            path = [ pkgs.coreutils pkgs.git pkgs.systemd ];
+            path = [ pkgs.coreutils pkgs.git pkgs.jq pkgs.nix pkgs.systemd ];
             serviceConfig = {
               Type = "oneshot";
               RemainAfterExit = true;
@@ -102,11 +102,41 @@ write_config() {
 
               install -m 0644 "$user_data" /etc/nixos/flake.nix
 
+              early_build_config=/run/nixos-user-data-flake-early-build-config.json
+              early_build_error=/run/nixos-user-data-flake-early-build-config.err
+              early_nix_config=/run/nixos-user-data-flake-early-nix.conf
+              : > "$early_nix_config"
+
+              if nix --extra-experimental-features 'nix-command flakes' \
+                eval --json /etc/nixos#quixosEarlyBuildNixConfig \
+                > "$early_build_config" 2> "$early_build_error"; then
+                jq -r '(.hosts // {}) | to_entries[] | select(.key != "" and (.value | length > 0)) | [.key, (.value | join(" "))] | @tsv' "$early_build_config" |
+                  while IFS="$(printf '\t')" read -r address names; do
+                    if [ -n "$address" ] && [ -n "$names" ]; then
+                      printf '%s %s # quixos early build nix cache\n' "$address" "$names" >> /etc/hosts
+                    fi
+                  done
+
+                substituters="$(jq -r '(.substituters // []) | join(" ")' "$early_build_config")"
+                trusted_public_keys="$(jq -r '(.trustedPublicKeys // []) | join(" ")' "$early_build_config")"
+                if [ -n "$substituters" ]; then
+                  printf 'extra-substituters = %s\n' "$substituters" >> "$early_nix_config"
+                fi
+                if [ -n "$trusted_public_keys" ]; then
+                  printf 'extra-trusted-public-keys = %s\n' "$trusted_public_keys" >> "$early_nix_config"
+                fi
+              else
+                echo "No quixosEarlyBuildNixConfig flake output found; first switch will use the AMI Nix configuration."
+              fi
+
               runner=/run/nixos-user-data-flake-apply
               cat > "$runner" <<RUNNER_EOF
               #!$${pkgs.runtimeShell}
               set -eu
               export PATH="$${lib.makeBinPath [ pkgs.coreutils pkgs.git pkgs.systemd ]}"
+              if [ -s "$early_nix_config" ]; then
+                export NIX_CONFIG="\$(cat "$early_nix_config")"
+              fi
 
               $${config.system.build.nixos-rebuild}/bin/nixos-rebuild switch --flake /etc/nixos
               printf '%s\n' "$new_hash" > "$applied_hash_file"