Move plugins infra to libnixmain

They are not actually part of the store layer, but instead part of the
Nix executable infra (libraries don't need plugins, executables do).

This is part of a larger project of moving all of our legacy settings
infra to libmain, and having the underlying libraries just have plain
configuration structs detached from any settings infra / UI layer.

Progress on #5638

meson.build

@@ -26,6 +26,7 @@ subproject('external-api-docs')
 subproject('libutil-c')
 subproject('libstore-c')
 subproject('libexpr-c')
+subproject('libmain-c')
 
 # Language Bindings
 subproject('perl')

packaging/components.nix

@@ -29,6 +29,7 @@ in
   nix-flake-tests = callPackage ../tests/unit/libflake/package.nix { };
 
   nix-main = callPackage ../src/libmain/package.nix { };
+  nix-main-c = callPackage ../src/libmain-c/package.nix { };
 
   nix-cmd = callPackage ../src/libcmd/package.nix { };
 

packaging/hydra.nix

@@ -52,6 +52,7 @@ let
     "nix-flake"
     "nix-flake-tests"
     "nix-main"
+    "nix-main-c"
     "nix-cmd"
     "nix-ng"
   ];

src/build-remote/build-remote.cc

@@ -11,6 +11,7 @@
 
 #include "machines.hh"
 #include "shared.hh"
+#include "plugin.hh"
 #include "pathlocks.hh"
 #include "globals.hh"
 #include "serialise.hh"

src/libmain-c/.version

@@ -0,0 +1 @@
+../../.version

src/libmain-c/build-utils-meson

@@ -0,0 +1 @@
+../../build-utils-meson

src/libmain-c/meson.build

@@ -0,0 +1,84 @@
+project('nix-main-c', 'cpp',
+  version : files('.version'),
+  default_options : [
+    'cpp_std=c++2a',
+    # TODO(Qyriad): increase the warning level
+    'warning_level=1',
+    'debug=true',
+    'optimization=2',
+    'errorlogs=true', # Please print logs for tests that fail
+  ],
+  meson_version : '>= 1.1',
+  license : 'LGPL-2.1-or-later',
+)
+
+cxx = meson.get_compiler('cpp')
+
+subdir('build-utils-meson/deps-lists')
+
+configdata = configuration_data()
+
+deps_private_maybe_subproject = [
+  dependency('nix-util'),
+  dependency('nix-store'),
+  dependency('nix-main'),
+]
+deps_public_maybe_subproject = [
+  dependency('nix-util-c'),
+  dependency('nix-store-c'),
+]
+subdir('build-utils-meson/subprojects')
+
+# TODO rename, because it will conflict with downstream projects
+configdata.set_quoted('PACKAGE_VERSION', meson.project_version())
+
+config_h = configure_file(
+  configuration : configdata,
+  output : 'config-main.h',
+)
+
+add_project_arguments(
+  # TODO(Qyriad): Yes this is how the autoconf+Make system did it.
+  # It would be nice for our headers to be idempotent instead.
+
+  # From C++ libraries, only for internals
+  '-include', 'config-util.hh',
+  '-include', 'config-store.hh',
+  '-include', 'config-main.hh',
+
+  # From C libraries, for our public, installed headers too
+  '-include', 'config-util.h',
+  '-include', 'config-store.h',
+  '-include', 'config-main.h',
+  language : 'cpp',
+)
+
+subdir('build-utils-meson/diagnostics')
+
+sources = files(
+  'nix_api_main.cc',
+)
+
+include_dirs = [include_directories('.')]
+
+headers = [config_h] + files(
+  'nix_api_main.h',
+)
+
+subdir('build-utils-meson/export-all-symbols')
+
+this_library = library(
+  'nixmainc',
+  sources,
+  dependencies : deps_public + deps_private + deps_other,
+  include_directories : include_dirs,
+  link_args: linker_export_flags,
+  prelink : true, # For C++ static initializers
+  install : true,
+)
+
+install_headers(headers, subdir : 'nix', preserve_path : true)
+
+libraries_private = []
+
+subdir('build-utils-meson/export')

src/libmain-c/nix_api_main.cc

@@ -0,0 +1,16 @@
+#include "nix_api_store.h"
+#include "nix_api_store_internal.h"
+#include "nix_api_util.h"
+#include "nix_api_util_internal.h"
+
+#include "plugin.hh"
+
+nix_err nix_init_plugins(nix_c_context * context)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        nix::initPlugins();
+    }
+    NIXC_CATCH_ERRS
+}

src/libmain-c/nix_api_main.h

@@ -0,0 +1,40 @@
+#ifndef NIX_API_MAIN_H
+#define NIX_API_MAIN_H
+/**
+ * @defgroup libmain libmain
+ * @brief C bindings for nix libmain
+ *
+ * libmain has misc utilities for CLI commands
+ * @{
+ */
+/** @file
+ * @brief Main entry for the libmain C bindings
+ */
+
+#include "nix_api_util.h"
+#include <stdbool.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+// cffi start
+
+/**
+ * @brief Loads the plugins specified in Nix's plugin-files setting.
+ *
+ * Call this once, after calling your desired init functions and setting
+ * relevant settings.
+ *
+ * @param[out] context Optional, stores error information
+ * @return NIX_OK if the initialization was successful, an error code otherwise.
+ */
+nix_err nix_init_plugins(nix_c_context * context);
+
+// cffi end
+#ifdef __cplusplus
+}
+#endif
+/**
+ * @}
+ */
+#endif // NIX_API_MAIN_H

src/libmain-c/package.nix

@@ -0,0 +1,83 @@
+{ lib
+, stdenv
+, mkMesonDerivation
+, releaseTools
+
+, meson
+, ninja
+, pkg-config
+
+, nix-util-c
+, nix-store
+, nix-store-c
+, nix-main
+
+# Configuration Options
+
+, version
+}:
+
+let
+  inherit (lib) fileset;
+in
+
+mkMesonDerivation (finalAttrs: {
+  pname = "nix-main-c";
+  inherit version;
+
+  workDir = ./.;
+  fileset = fileset.unions [
+    ../../build-utils-meson
+    ./build-utils-meson
+    ../../.version
+    ./.version
+    ./meson.build
+    # ./meson.options
+    (fileset.fileFilter (file: file.hasExt "cc") ./.)
+    (fileset.fileFilter (file: file.hasExt "hh") ./.)
+    (fileset.fileFilter (file: file.hasExt "h") ./.)
+  ];
+
+  outputs = [ "out" "dev" ];
+
+  nativeBuildInputs = [
+    meson
+    ninja
+    pkg-config
+  ];
+
+  propagatedBuildInputs = [
+    nix-util-c
+    nix-store
+    nix-store-c
+    nix-main
+  ];
+
+  preConfigure =
+    # "Inline" .version so it's not a symlink, and includes the suffix.
+    # Do the meson utils, without modification.
+    ''
+      chmod u+w ./.version
+      echo ${version} > ../../.version
+    '';
+
+  mesonFlags = [
+  ];
+
+  env = lib.optionalAttrs (stdenv.isLinux && !(stdenv.hostPlatform.isStatic && stdenv.system == "aarch64-linux")) {
+    LDFLAGS = "-fuse-ld=gold";
+  };
+
+  enableParallelBuilding = true;
+
+  separateDebugInfo = !stdenv.hostPlatform.isStatic;
+
+  strictDeps = true;
+
+  hardeningDisable = lib.optional stdenv.hostPlatform.isStatic "pie";
+
+  meta = {
+    platforms = lib.platforms.unix ++ lib.platforms.windows;
+  };
+
+})

src/libmain/common-args.cc

@@ -5,6 +5,7 @@
 #include "logging.hh"
 #include "loggers.hh"
 #include "util.hh"
+#include "plugin.hh"
 
 namespace nix {
 

src/libmain/plugin.cc

@@ -0,0 +1,117 @@
+#ifndef _WIN32
+#  include <dlfcn.h>
+#endif
+
+#include "config-global.hh"
+#include "signals.hh"
+
+namespace nix {
+
+struct PluginFilesSetting : public BaseSetting<Paths>
+{
+    bool pluginsLoaded = false;
+
+    PluginFilesSetting(
+        Config * options,
+        const Paths & def,
+        const std::string & name,
+        const std::string & description,
+        const std::set<std::string> & aliases = {})
+        : BaseSetting<Paths>(def, true, name, description, aliases)
+    {
+        options->addSetting(this);
+    }
+
+    Paths parse(const std::string & str) const override;
+};
+
+Paths PluginFilesSetting::parse(const std::string & str) const
+{
+    if (pluginsLoaded)
+        throw UsageError(
+            "plugin-files set after plugins were loaded, you may need to move the flag before the subcommand");
+    return BaseSetting<Paths>::parse(str);
+}
+
+struct PluginSettings : Config
+{
+    PluginFilesSetting pluginFiles{
+        this,
+        {},
+        "plugin-files",
+        R"(
+          A list of plugin files to be loaded by Nix. Each of these files will
+          be dlopened by Nix. If they contain the symbol `nix_plugin_entry()`,
+          this symbol will be called. Alternatively, they can affect execution
+          through static initialization. In particular, these plugins may construct
+          static instances of RegisterPrimOp to add new primops or constants to the
+          expression language, RegisterStoreImplementation to add new store
+          implementations, RegisterCommand to add new subcommands to the `nix`
+          command, and RegisterSetting to add new nix config settings. See the
+          constructors for those types for more details.
+
+          Warning! These APIs are inherently unstable and may change from
+          release to release.
+
+          Since these files are loaded into the same address space as Nix
+          itself, they must be DSOs compatible with the instance of Nix
+          running at the time (i.e. compiled against the same headers, not
+          linked to any incompatible libraries). They should not be linked to
+          any Nix libs directly, as those will be available already at load
+          time.
+
+          If an entry in the list is a directory, all files in the directory
+          are loaded as plugins (non-recursively).
+        )"};
+};
+
+static PluginSettings pluginSettings;
+
+static GlobalConfig::Register rPluginSettings(&pluginSettings);
+
+void initPlugins()
+{
+    assert(!pluginSettings.pluginFiles.pluginsLoaded);
+    for (const auto & pluginFile : pluginSettings.pluginFiles.get()) {
+        std::vector<std::filesystem::path> pluginFiles;
+        try {
+            auto ents = std::filesystem::directory_iterator{pluginFile};
+            for (const auto & ent : ents) {
+                checkInterrupt();
+                pluginFiles.emplace_back(ent.path());
+            }
+        } catch (std::filesystem::filesystem_error & e) {
+            if (e.code() != std::errc::not_a_directory)
+                throw;
+            pluginFiles.emplace_back(pluginFile);
+        }
+        for (const auto & file : pluginFiles) {
+            checkInterrupt();
+            /* handle is purposefully leaked as there may be state in the
+               DSO needed by the action of the plugin. */
+#ifndef _WIN32 // TODO implement via DLL loading on Windows
+            void * handle = dlopen(file.c_str(), RTLD_LAZY | RTLD_LOCAL);
+            if (!handle)
+                throw Error("could not dynamically open plugin file '%s': %s", file, dlerror());
+
+            /* Older plugins use a statically initialized object to run their code.
+               Newer plugins can also export nix_plugin_entry() */
+            void (*nix_plugin_entry)() = (void (*)()) dlsym(handle, "nix_plugin_entry");
+            if (nix_plugin_entry)
+                nix_plugin_entry();
+#else
+            throw Error("could not dynamically open plugin file '%s'", file);
+#endif
+        }
+    }
+
+    /* Since plugins can add settings, try to re-apply previously
+       unknown settings. */
+    globalConfig.reapplyUnknownSettings();
+    globalConfig.warnUnknownSettings();
+
+    /* Tell the user if they try to set plugin-files after we've already loaded */
+    pluginSettings.pluginFiles.pluginsLoaded = true;
+}
+
+}