test: add library, and check example for readiness

example/flake.nix

@@ -57,12 +57,10 @@
           };
 
           testScript = ''
-            machine.wait_for_unit("default.target")
-            machine.succeed("""
-              sleep 10  # HACK
-              journalctl -u process-compose.service
-              curl -v http://localhost:8213/
-            """)
+            process_compose.wait(lambda procs: 
+              procs["sqlite-web"]["is_ready"] == "Ready"
+            )
+            machine.succeed("curl -v http://localhost:8213/")
           '';
         };
       };

nix/process-compose/test.nix

@@ -2,6 +2,41 @@
 
 let
   inherit (lib) types mkOption;
+  testLibrary = ''
+    class ProcessCompose:
+      # GET /processes of process-compose swagger API.
+      def get_processes(self, port=8080):
+        import json
+        url = f"http://localhost:{port}/processes"
+        return json.loads(
+          machine.succeed("${lib.getExe pkgs.xh} get " + url))
+
+      def wait(self, ready, timeout_mins=1):
+        import time
+        import json
+        machine.wait_for_unit("default.target")
+
+        timeout = time.time() + 60*timeout_mins   # 1 minutes from now
+        data = None
+        while True:
+          if time.time() > timeout: 
+            print("Processes not started!")
+            machine.succeed("""
+              journalctl -u process-compose.service
+              cat process-compose*log
+            """)
+            raise Exception("Processes not started!")
+          info = self.get_processes(${if config.port == null then "8080" else builtins.toString config.port})
+          print(json.dumps(info))
+          data = { x["name"]: x for x in info["data"] }
+          if ready(data):
+            break
+          else:
+            time.sleep(1)
+        return data
+    
+    process_compose = ProcessCompose()
+  '';
 in
 {
   options = {
@@ -12,14 +47,19 @@ in
         configuration, followed by the specified testScript.
 
         Useful if you want to test your configuration in CI.
+
+        The testScript will have access to `process_comose.wait` function that
+        it can be used to get the running process information, after waiting for
+        the specified readiness status.
       '';
       default = null;
     };
     outputs.check = mkOption {
       type = types.nullOr types.package;
-      default = if config.testScript == null then null else
+      default =
+        if config.testScript == null then null else
         pkgs.nixosTest {
-          inherit (config) testScript;
+          testScript = testLibrary + "\n" + config.testScript;
           name = "process-compose-${name}-test";
           nodes.machine = {
             systemd.services.process-compose = {