Add Nix deployment support

nix-bitcoin is archived and no longer receives security fixes. Provide
a maintained upstream path for reproducible builds and declarative
NixOS deployment.

The flake pins Nixpkgs and exports a package and NixOS module. The
package builds the server and CLI, takes its version from Cargo, and
installs shell completions. The module creates a service, user,
configuration, state directory, and optional firewall ports for each
named instance. The documentation covers local builds, deployment,
secrets, and multiple instances.

Generated with OpenAI Codex.

Author: benthecarman

README.md

@@ -64,6 +64,7 @@ See [Getting Started](docs/getting-started.md) for a full walkthrough.
 | [API Guide](docs/api-guide.md) | gRPC transport, authentication, and endpoint reference |
 | [Tor](docs/tor.md) | Connecting to and receiving connections over Tor |
 | [Operations](docs/operations.md) | Production deployment, backups, and monitoring |
+| [Nix deployment](docs/nix.md) | Reproducible builds and a NixOS service module |
 
 ### API
 

docs/nix.md

@@ -0,0 +1,105 @@
+# Nix deployment
+
+The flake builds `ldk-server` and `ldk-server-cli`. It also provides a NixOS
+module for the daemon.
+
+## Run without installing
+
+Build and run the server from this repository:
+
+```bash
+nix run . -- /path/to/config.toml
+```
+
+Run the command-line client:
+
+```bash
+nix shell .#ldk-server -c ldk-server-cli --help
+```
+
+## Deploy on NixOS
+
+Add the flake to your system inputs:
+
+```nix
+{
+  inputs.ldk-server.url = "github:lightningdevkit/ldk-server";
+
+  outputs = { nixpkgs, ldk-server, ... }: {
+    nixosConfigurations.my-host = nixpkgs.lib.nixosSystem {
+      system = "x86_64-linux";
+      modules = [
+        ldk-server.nixosModules.default
+        {
+          services.ldk-server.instances = {
+            mainnet = {
+              enable = true;
+              openFirewall = true;
+              lightningPort = 9735;
+              grpcPort = 3536;
+              settings = {
+                node = {
+                  network = "bitcoin";
+                  listening_addresses = [ "0.0.0.0:9735" ];
+                  grpc_service_address = "0.0.0.0:3536";
+                };
+                esplora.server_url = "https://mempool.space/api";
+                log = {
+                  level = "Info";
+                  log_to_file = false;
+                };
+              };
+            };
+
+            signet = {
+              enable = true;
+              lightningPort = 19735;
+              grpcPort = 13536;
+              settings = {
+                node = {
+                  network = "signet";
+                  listening_addresses = [ "127.0.0.1:19735" ];
+                  grpc_service_address = "127.0.0.1:13536";
+                };
+                esplora.server_url = "https://mutinynet.com/api";
+              };
+            };
+          };
+        }
+      ];
+    };
+  };
+}
+```
+
+By default, each instance gets a separate service, user, configuration, and
+data directory. For example, `mainnet` uses `ldk-server-mainnet.service` and
+stores data in `/var/lib/ldk-server/mainnet`.
+
+Use different Lightning and gRPC addresses for each instance. The module sets
+each data path even if the TOML file contains a different storage path.
+
+The `settings` option writes values to the Nix store. Do not put passwords or
+other secrets in this option. Use `environmentFiles` for secrets:
+
+```nix
+services.ldk-server.instances.mainnet.environmentFiles = [
+  "/run/secrets/ldk-server-mainnet"
+];
+```
+
+The file can override supported settings with environment variables:
+
+```text
+LDK_SERVER_BITCOIND_RPC_USER=rpc-user
+LDK_SERVER_BITCOIND_RPC_PASSWORD=rpc-password
+```
+
+You can also set an instance's `configFile` to a complete TOML file. You
+cannot use `configFile` and `settings` on the same instance.
+
+After deployment, inspect the service with this command:
+
+```bash
+systemctl status ldk-server-mainnet
+```

flake.lock

@@ -0,0 +1,41 @@
+{
+  description = "LDK Server";
+
+  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+
+  outputs =
+    { self, nixpkgs }:
+    let
+      supportedSystems = [
+        "aarch64-darwin"
+        "aarch64-linux"
+        "x86_64-linux"
+      ];
+      forAllSystems = nixpkgs.lib.genAttrs supportedSystems;
+    in
+    {
+      packages = forAllSystems (
+        system:
+        let
+          pkgs = nixpkgs.legacyPackages.${system};
+        in
+        {
+          default = self.packages.${system}.ldk-server;
+          ldk-server = pkgs.callPackage ./nix/package.nix {
+            gitHash = self.rev or self.dirtyRev or "unknown";
+          };
+        }
+      );
+
+      checks = forAllSystems (system: {
+        inherit (self.packages.${system}) ldk-server;
+      });
+
+      nixosModules = {
+        default = self.nixosModules.ldk-server;
+        ldk-server = import ./nix/module.nix;
+      };
+
+      formatter = forAllSystems (system: nixpkgs.legacyPackages.${system}.nixfmt-tree);
+    };
+}