name: "Calling a Module From a Worker Thread (HTTP Server)"
output: cpp-sdk-worker-thread-http.md
release: ""

intro: |
  Logos inter-module calls travel over Qt Remote Objects, whose replicas only
  work on the thread that owns them — the module's main/event-loop thread. So a
  module that wants to call **another** module from a *worker* thread has a
  problem: the call would otherwise run on the worker thread and hang on replica
  acquisition (there's no event loop there to drive it).

  That worker-thread case is not exotic — it's exactly what you hit the moment a
  module embeds a server. The motivating example is a module that serves an HTTP
  `/metrics` endpoint and, on each scrape, calls other modules to gather their
  numbers. The HTTP server runs on its own thread; the inter-module calls happen
  there.

  This doc-test proves that path works on the SDK commit under test, and that the
  module stays **pure C++** — it never touches Qt to make it work. The SDK does
  the marshaling: `LogosAPIClient` transparently runs `getClient` /
  `invokeRemoteMethod` / `requestObject` / `onEvent` on the module's owner thread
  when they're called from another thread.

  It is fully self-contained:

  1. Create `sensor_module`, a tiny **callee** with one method, `readTemperature()`.
  2. Create `http_module`, a **caller** that depends on `sensor_module`, embeds a
     [libmicrohttpd](https://www.gnu.org/software/libmicrohttpd/) server, and on
     every HTTP request calls `sensor_module.readTemperature()` **from the server
     thread** through the generated `modules().sensor_module` wrapper.
  3. Build **both** against the C++ SDK commit under test, run them in
     `logoscore`, start the server, and `curl` it.

  A green run means the `curl` got the sensor's reading back — i.e. the
  worker-thread inter-module call completed instead of hanging. Without the SDK's
  thread marshaling it would deadlock on the server thread and the `curl` would
  time out.

what_you_build: "Two modules — a `sensor_module` callee and an `http_module` caller that embeds an HTTP server — built against this SDK commit and run in `logoscore`, where an HTTP request drives a cross-module call from the server's worker thread."

what_you_learn:
  - Why inter-module calls must run on the module's owner thread (Qt Remote Objects replica affinity)
  - How the SDK lets a module call another module from a worker thread without touching Qt
  - How to embed a third-party C library (libmicrohttpd, via pkg-config) in a universal module
  - How to drive a cross-module call from an HTTP handler and scrape it with `curl`

prerequisites:
  - |
    **Nix** with flakes enabled. Install from [nixos.org](https://nixos.org/download.html), then enable flakes:

    ```bash
    mkdir -p ~/.config/nix
    echo 'experimental-features = nix-command flakes' >> ~/.config/nix/nix.conf
    ```

    Verify: `nix flake --help >/dev/null 2>&1 && echo "Flakes enabled"`
  - "**git** — nix flakes only see files tracked by git."
  - "**curl** — to scrape the endpoint."
  - "A Linux or macOS machine."

sections:
  - title: "Create the callee: sensor_module"
    step: true
    text: |
      `sensor_module` is an ordinary `core` module in the pure-C++ (`interface:
      universal`) style with a single method. `http_module` will call it on every
      HTTP request.
    steps:
      - title: "metadata.json"
        text: "No dependencies; `interface: universal` selects the pure-C++ pattern."
        file:
          path: sensor_module/metadata.json
          language: json
          content: |
            {
              "name": "sensor_module",
              "version": "1.0.0",
              "type": "core",
              "category": "general",
              "description": "A callee module: returns a temperature reading",
              "main": "sensor_module_plugin",
              "interface": "universal",
              "dependencies": [],

              "nix": {
                "packages": {
                  "build": [],
                  "runtime": []
                },
                "external_libraries": [],
                "cmake": {
                  "find_packages": [],
                  "extra_sources": []
                }
              }
            }

      - title: "CMakeLists.txt"
        file:
          path: sensor_module/CMakeLists.txt
          language: cmake
          content: |
            cmake_minimum_required(VERSION 3.14)
            project(SensorModulePlugin LANGUAGES CXX)

            if(DEFINED ENV{LOGOS_MODULE_BUILDER_ROOT})
                include($ENV{LOGOS_MODULE_BUILDER_ROOT}/cmake/LogosModule.cmake)
            elseif(EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/cmake/LogosModule.cmake")
                include(cmake/LogosModule.cmake)
            else()
                message(FATAL_ERROR "LogosModule.cmake not found")
            endif()

            logos_module(
                NAME sensor_module
                SOURCES
                    src/sensor_module_impl.h
                    src/sensor_module_impl.cpp
            )

      - title: "flake.nix"
        file:
          path: sensor_module/flake.nix
          language: nix
          content: |
            {
              description = "Sensor core module - a callee for the worker-thread doc-test";

              inputs = {
                logos-module-builder.url = "github:logos-co/logos-module-builder{release}";
              };

              outputs = inputs@{ logos-module-builder, ... }:
                logos-module-builder.lib.mkLogosModule {
                  src = ./.;
                  configFile = ./metadata.json;
                  flakeInputs = inputs;
                };
            }

      - title: "src/sensor_module_impl.h — the class"
        text: "A plain C++ class — no base, no Qt. Its one public method becomes callable over IPC."
        file:
          path: sensor_module/src/sensor_module_impl.h
          language: cpp
          content: |
            #pragma once

            #include <cstdint>

            // A trivial sensor. http_module calls readTemperature() on every HTTP
            // request, from its server thread.
            class SensorModuleImpl {
            public:
                /// Returns the current temperature reading (degrees Celsius).
                int64_t readTemperature();
            };

      - title: "src/sensor_module_impl.cpp — the implementation"
        file:
          path: sensor_module/src/sensor_module_impl.cpp
          language: cpp
          content: |
            #include "sensor_module_impl.h"

            int64_t SensorModuleImpl::readTemperature()
            {
                return 42;
            }

  - title: "Create the caller: http_module"
    step: true
    text: |
      `http_module` declares `sensor_module` as a dependency (so the builder
      generates a typed `modules().sensor_module` wrapper) and embeds an HTTP
      server using **libmicrohttpd**. The server runs on its own thread; its
      request handler calls `sensor_module.readTemperature()` from there. The
      module code never mentions Qt — the SDK marshals the cross-module call onto
      the module's owner thread.
    steps:
      - title: "metadata.json — declare the dependency and the C library"
        text: |
          `dependencies` lists `sensor_module`. `nix.packages.runtime` adds
          `libmicrohttpd` (a build input, so the plugin can link it) and
          `nix.packages.build` adds `pkg-config` so CMake can find it.
        file:
          path: http_module/metadata.json
          language: json
          content: |
            {
              "name": "http_module",
              "version": "1.0.0",
              "type": "core",
              "category": "general",
              "description": "A caller module: serves HTTP and reads sensor_module from the server thread",
              "main": "http_module_plugin",
              "interface": "universal",
              "dependencies": ["sensor_module"],

              "nix": {
                "packages": {
                  "build": ["pkg-config"],
                  "runtime": ["libmicrohttpd"]
                },
                "external_libraries": [],
                "cmake": {
                  "find_packages": [],
                  "extra_sources": []
                }
              }
            }

      - title: "CMakeLists.txt — link libmicrohttpd"
        text: "After `logos_module(...)`, find libmicrohttpd via pkg-config and link it into the generated plugin target (`<name>_module_plugin`)."
        file:
          path: http_module/CMakeLists.txt
          language: cmake
          content: |
            cmake_minimum_required(VERSION 3.14)
            project(HttpModulePlugin LANGUAGES CXX)

            if(DEFINED ENV{LOGOS_MODULE_BUILDER_ROOT})
                include($ENV{LOGOS_MODULE_BUILDER_ROOT}/cmake/LogosModule.cmake)
            elseif(EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/cmake/LogosModule.cmake")
                include(cmake/LogosModule.cmake)
            else()
                message(FATAL_ERROR "LogosModule.cmake not found")
            endif()

            logos_module(
                NAME http_module
                SOURCES
                    src/http_module_impl.h
                    src/http_module_impl.cpp
            )

            find_package(PkgConfig REQUIRED)
            pkg_check_modules(MHD REQUIRED IMPORTED_TARGET libmicrohttpd)
            target_link_libraries(http_module_module_plugin PRIVATE PkgConfig::MHD)

      - title: "flake.nix — add the dependency input"
        text: |
          Declare `sensor_module` as a flake input (the input name **must match**
          the dependency name). The `path:` value is a placeholder — we lock it to
          the real sensor checkout in the build step.
        file:
          path: http_module/flake.nix
          language: nix
          content: |
            {
              description = "HTTP core module - reads sensor_module from its server thread";

              inputs = {
                logos-module-builder.url = "github:logos-co/logos-module-builder{release}";

                # The module this one depends on. Placeholder path — locked to the
                # real checkout in the build step via --override-input.
                sensor_module.url = "path:/path/to/your/sensor_module";
              };

              outputs = inputs@{ logos-module-builder, sensor_module, ... }:
                logos-module-builder.lib.mkLogosModule {
                  src = ./.;
                  configFile = ./metadata.json;
                  flakeInputs = inputs;
                };
            }

      - title: "src/http_module_impl.h — the class"
        text: |
          Pure C++: `LogosModuleContext` (for `modules()`), a `std::mutex`, and an
          opaque `void* m_daemon` (the libmicrohttpd handle stays out of the header
          the generator parses). `start`/`stop` control the server; `readSensor`
          does the cross-module call and is what the HTTP handler invokes.
        file:
          path: http_module/src/http_module_impl.h
          language: cpp
          content: |
            #pragma once

            #include <cstdint>
            #include <mutex>

            #include <logos_module_context.h>  // LogosModuleContext base + modules()

            // Serves HTTP via libmicrohttpd. On each request the server thread calls
            // sensor_module through modules().sensor_module — the SDK marshals that
            // call onto this module's owner thread. No Qt here.
            class HttpModuleImpl : public LogosModuleContext {
            public:
                HttpModuleImpl() = default;
                ~HttpModuleImpl();

                /// Start the HTTP server on `port`. Returns 1 on success, 0 on
                /// failure (already running / bad port / bind error).
                int64_t start(int64_t port);

                /// Stop the HTTP server. Returns 1 if it was running, 0 otherwise.
                int64_t stop();

                /// Read sensor_module.readTemperature(). The HTTP handler calls this
                /// from the server (worker) thread; exposed as a method so it can
                /// also be driven directly for comparison.
                int64_t readSensor();

            private:
                std::mutex m_mutex;
                void* m_daemon = nullptr;  // struct MHD_Daemon*
            };

      - title: "src/http_module_impl.cpp — the implementation"
        text: |
          The handler runs on a libmicrohttpd worker thread and calls
          `self->readSensor()`, which goes through `modules().sensor_module`. That
          cross-module call is what the SDK marshals onto the owner thread — the
          whole point of this doc-test.
        file:
          path: http_module/src/http_module_impl.cpp
          language: cpp
          content: |
            #include "http_module_impl.h"

            #include <cstdint>
            #include <string>

            #include <microhttpd.h>

            // Generated at build time: defines LogosModules with the typed
            // modules().sensor_module accessor. Included only in the .cpp so the impl
            // header the generator parses stays free of Qt / codegen types.
            #include "logos_sdk.h"

            namespace {

            // libmicrohttpd access handler. `cls` is the HttpModuleImpl*. Runs on an
            // MHD worker thread; the cross-module call inside is marshaled onto the
            // module's owner thread by the SDK.
            MHD_Result onRequest(void* cls, struct MHD_Connection* connection,
                                 const char* /*url*/, const char* /*method*/,
                                 const char* /*version*/, const char* /*upload_data*/,
                                 size_t* /*upload_data_size*/, void** /*req_cls*/)
            {
                auto* self = static_cast<HttpModuleImpl*>(cls);
                const std::string body =
                    "temperature " + std::to_string(self->readSensor()) + "\n";

                MHD_Response* response = MHD_create_response_from_buffer(
                    body.size(), const_cast<char*>(body.data()), MHD_RESPMEM_MUST_COPY);
                MHD_add_response_header(response, "Content-Type", "text/plain; charset=utf-8");
                MHD_Result ret = MHD_queue_response(connection, MHD_HTTP_OK, response);
                MHD_destroy_response(response);
                return ret;
            }

            }  // namespace

            HttpModuleImpl::~HttpModuleImpl()
            {
                stop();
            }

            int64_t HttpModuleImpl::readSensor()
            {
                // Cross-module call. From the HTTP handler this runs on the server's
                // worker thread; the SDK marshals it onto this module's owner thread.
                return modules().sensor_module.readTemperature();
            }

            int64_t HttpModuleImpl::start(int64_t port)
            {
                std::lock_guard<std::mutex> lock(m_mutex);
                if (m_daemon) return 0;
                if (port <= 0 || port > 65535) return 0;

                MHD_Daemon* daemon = MHD_start_daemon(
                    MHD_USE_INTERNAL_POLLING_THREAD, static_cast<uint16_t>(port),
                    nullptr, nullptr, &onRequest, this, MHD_OPTION_END);
                if (!daemon) return 0;

                m_daemon = daemon;
                return 1;
            }

            int64_t HttpModuleImpl::stop()
            {
                std::lock_guard<std::mutex> lock(m_mutex);
                if (!m_daemon) return 0;
                MHD_stop_daemon(static_cast<MHD_Daemon*>(m_daemon));
                m_daemon = nullptr;
                return 1;
            }

  - title: "Build both modules against this SDK"
    step: true
    text: |
      Nix flakes only see git-tracked files, so initialise a repo in each module
      first, then build each `.lgx`, overriding `logos-cpp-sdk` to the commit
      under test.

      > The override URLs carry a `{release}` placeholder the runner expands to a
      > concrete ref — locally this checkout's `HEAD`, in CI the commit being
      > tested.
    steps:
      - title: "Initialise git repos"
        run: |
          (cd sensor_module && git init -q && git add -A)
          (cd http_module && git init -q && git add -A)
        check_file: "sensor_module/.git/HEAD"

      - title: "Build the sensor's .lgx against this SDK"
        run: |
          nix build 'path:./sensor_module#lgx' \
            --override-input logos-module-builder 'github:logos-co/logos-module-builder{release}' \
            --override-input logos-module-builder/logos-cpp-sdk 'github:logos-co/logos-cpp-sdk{release}' \
            -o sensor-lgx
        code_block: |
          nix build 'path:./sensor_module#lgx' \
            --override-input logos-module-builder/logos-cpp-sdk 'github:logos-co/logos-cpp-sdk' \
            -o sensor-lgx
        post_text: "The sensor package is under `./sensor-lgx/`:"
        extra_run:
          run: "ls sensor-lgx/*.lgx"

      - title: "Build the http module's .lgx against this SDK"
        text: |
          Lock `sensor_module` to the local checkout and override `logos-cpp-sdk`
          in both builders, so the dependency wrapper and both plugins are built
          against one consistent SDK.
        run: |
          nix build 'path:./http_module#lgx' \
            --override-input sensor_module 'path:./sensor_module' \
            --override-input logos-module-builder 'github:logos-co/logos-module-builder{release}' \
            --override-input logos-module-builder/logos-cpp-sdk 'github:logos-co/logos-cpp-sdk{release}' \
            --override-input sensor_module/logos-module-builder 'github:logos-co/logos-module-builder{release}' \
            --override-input sensor_module/logos-module-builder/logos-cpp-sdk 'github:logos-co/logos-cpp-sdk{release}' \
            -o http-lgx
        code_block: |
          nix build 'path:./http_module#lgx' \
            --override-input sensor_module 'path:./sensor_module' \
            --override-input logos-module-builder/logos-cpp-sdk 'github:logos-co/logos-cpp-sdk' \
            --override-input sensor_module/logos-module-builder/logos-cpp-sdk 'github:logos-co/logos-cpp-sdk' \
            -o http-lgx
        post_text: "The http package is under `./http-lgx/`:"
        extra_run:
          run: "ls http-lgx/*.lgx"

  - title: "Build the runtime and install both modules"
    step: true
    text: |
      Build `logoscore` and `lgpm` (against this SDK), seed the modules directory
      with the capability module, and install both modules.
    steps:
      - title: "Build logoscore against this SDK"
        run: |
          nix build 'github:logos-co/logos-logoscore-cli{release}' \
            --override-input logos-cpp-sdk 'github:logos-co/logos-cpp-sdk{release}' \
            --override-input logos-liblogos/logos-cpp-sdk 'github:logos-co/logos-cpp-sdk{release}' \
            --override-input logos-module-client/logos-cpp-sdk 'github:logos-co/logos-cpp-sdk{release}' \
            --override-input logos-capability-module/logos-module-builder 'github:logos-co/logos-module-builder{release}' \
            --override-input logos-capability-module/logos-module-builder/logos-cpp-sdk 'github:logos-co/logos-cpp-sdk{release}' \
            --out-link ./logos
        code_block: |
          nix build 'github:logos-co/logos-logoscore-cli' \
            --override-input logos-cpp-sdk 'github:logos-co/logos-cpp-sdk' \
            --override-input logos-liblogos/logos-cpp-sdk 'github:logos-co/logos-cpp-sdk' \
            --override-input logos-module-client/logos-cpp-sdk 'github:logos-co/logos-cpp-sdk' \
            --override-input logos-capability-module/logos-module-builder/logos-cpp-sdk 'github:logos-co/logos-cpp-sdk' \
            --out-link ./logos
        check_file: "logos/bin/logoscore"

      - title: "Build lgpm"
        run: "nix build 'github:logos-co/logos-package-manager#cli' -o lgpm"
        check_file: "lgpm/bin/lgpm"

      - title: "Seed the modules directory with the capability module"
        run: |
          mkdir -p modules
          cp -RL ./logos/modules/. ./modules/
        check_file: "modules/capability_module/manifest.json"

      - title: "Install the sensor"
        run: "./lgpm/bin/lgpm --modules-dir ./modules --allow-unsigned install --file sensor-lgx/*.lgx"
        expect_contains:
          - "Installed to:"

      - title: "Install the http module"
        run: "./lgpm/bin/lgpm --modules-dir ./modules --allow-unsigned install --file http-lgx/*.lgx"
        expect_contains:
          - "Installed to:"

      - title: "Confirm both modules are installed"
        run: "./lgpm/bin/lgpm --modules-dir ./modules list"
        expect_contains:
          - "sensor_module"
          - "http_module"
        check_file: "modules/http_module/manifest.json"

  - title: "Serve over HTTP and scrape from the worker thread"
    step: true
    text: |
      Start the daemon, load both modules, then start the HTTP server and `curl`
      it. The `curl` triggers a request whose handler — on the server's worker
      thread — calls `sensor_module.readTemperature()`. Getting `temperature 42`
      back is the proof that the worker-thread cross-module call completed.
    steps:
      - title: "Start the daemon"
        run: "sh -c './logos/bin/logoscore -D -m ./modules > logs.txt 2>&1 &'"
        code_block: "logoscore -D -m ./modules > logs.txt &"

      - run: "sleep 3"

      - title: "Load the sensor (the dependency first)"
        run: "./logos/bin/logoscore load-module sensor_module"
        code_block: "logoscore load-module sensor_module"
        expect_contains:
          - "sensor_module"

      - title: "Load the http module"
        run: "./logos/bin/logoscore load-module http_module"
        code_block: "logoscore load-module http_module"
        expect_contains:
          - "http_module"

      - title: "Read the sensor directly (main thread)"
        text: |
          Called via `logoscore`, `readSensor()` runs on the module's own event-loop
          thread — the easy case. It returns the sensor's reading:
        run: "./logos/bin/logoscore call http_module readSensor"
        code_block: "logoscore call http_module readSensor"
        expect_contains:
          - '"result":42'

      - title: "Start the HTTP server"
        run: "./logos/bin/logoscore call http_module start 8080"
        code_block: "logoscore call http_module start 8080"
        expect_contains:
          - '"result":1'

      - run: "sleep 1"

      - title: "Scrape it — the cross-module call now happens on the server thread"
        text: |
          The HTTP handler runs on a libmicrohttpd worker thread and calls
          `sensor_module.readTemperature()` from there. The SDK marshals that call
          onto the module's owner thread, so it completes and the response carries
          the sensor's reading. Without the marshaling this request would hang.
        run: "curl -s --max-time 15 http://127.0.0.1:8080/"
        code_block: "curl http://127.0.0.1:8080/"
        expect_contains:
          - "temperature 42"

      - title: "Stop the HTTP server"
        run: "./logos/bin/logoscore call http_module stop"
        code_block: "logoscore call http_module stop"
        expect_contains:
          - '"result":1'

      - title: "Stop the daemon"
        run: "./logos/bin/logoscore stop"
        code_block: "logoscore stop"

      - run: "sleep 2"

      - title: "Confirm the daemon has stopped"
        run: "./logos/bin/logoscore status || true"
        code_block: "logoscore status"
        expect_contains:
          - '"status":"not_running"'

  - title: "Recap"
    text: |
      | Call site | Thread | Result |
      | --------- | ------ | ------ |
      | `logoscore call http_module readSensor` | module event-loop thread | `42` |
      | `curl http://127.0.0.1:8080/` → HTTP handler → `readSensor()` | libmicrohttpd worker thread | `temperature 42` |

      Both reach `sensor_module.readTemperature()` through the generated
      `modules().sensor_module` wrapper. The second does it from a worker thread —
      and it works because the SDK marshals the call onto the module's owner
      thread, where Qt Remote Objects replicas live. The module itself stays pure
      C++. A green run is evidence that worker-thread inter-module calls work on
      this SDK commit.