Add nix-flake-c, nix_flake_init_global, nix_flake_settings_new

src/libflake-c/.version

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

src/libflake-c/build-utils-meson

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+../../build-utils-meson/

src/libflake-c/meson.build

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+project('nix-flake-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-expr'),
+  dependency('nix-flake'),
+]
+deps_public_maybe_subproject = [
+  dependency('nix-util-c'),
+  dependency('nix-store-c'),
+  dependency('nix-expr-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-flake.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-expr.hh',
+  # not generated (yet?)
+  # '-include', 'config-flake.hh',
+
+  # From C libraries, for our public, installed headers too
+  '-include', 'config-util.h',
+  '-include', 'config-store.h',
+  '-include', 'config-expr.h',
+  '-include', 'config-flake.h',
+  language : 'cpp',
+)
+
+subdir('build-utils-meson/common')
+
+sources = files(
+  'nix_api_flake.cc',
+)
+
+include_dirs = [include_directories('.')]
+
+headers = [config_h] + files(
+  'nix_api_flake.h',
+)
+
+# TODO move this header to libexpr, maybe don't use it in tests?
+headers += files('nix_api_flake.h')
+
+subdir('build-utils-meson/export-all-symbols')
+subdir('build-utils-meson/windows-version')
+
+this_library = library(
+  'nixflakec',
+  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/libflake-c/nix_api_flake.cc

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+#include "nix_api_flake.h"
+#include "nix_api_flake_internal.hh"
+#include "nix_api_util_internal.h"
+
+#include "flake/flake.hh"
+
+nix_flake_settings * nix_flake_settings_new(nix_c_context * context)
+{
+    try {
+        auto settings = nix::make_ref<nix::flake::Settings>();
+        return new nix_flake_settings{settings};
+    }
+    NIXC_CATCH_ERRS_NULL
+}
+
+void nix_flake_settings_free(nix_flake_settings * settings)
+{
+    delete settings;
+}
+
+nix_err nix_flake_init_global(nix_c_context * context, nix_flake_settings * settings)
+{
+    static std::shared_ptr<nix::flake::Settings> registeredSettings;
+    try {
+        if (registeredSettings)
+            throw nix::Error("nix_flake_init_global already initialized");
+
+        registeredSettings = settings->settings;
+        nix::flake::initLib(*registeredSettings);
+    }
+    NIXC_CATCH_ERRS
+}

src/libflake-c/nix_api_flake.h

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+#ifndef NIX_API_FLAKE_H
+#define NIX_API_FLAKE_H
+/** @defgroup libflake libflake
+ * @brief Bindings to the Nix Flakes library
+ *
+ * @{
+ */
+/** @file
+ * @brief Main entry for the libflake C bindings
+ */
+
+#include "nix_api_store.h"
+#include "nix_api_util.h"
+#include "nix_api_expr.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+// cffi start
+
+typedef struct nix_flake_settings nix_flake_settings;
+
+// Function prototypes
+/**
+ * Create a nix_flake_settings initialized with default values.
+ * @param[out] context Optional, stores error information
+ * @return A new nix_flake_settings or NULL on failure.
+ * @see nix_flake_settings_free
+ */
+nix_flake_settings * nix_flake_settings_new(nix_c_context * context);
+
+/**
+ * @brief Release the resources associated with a nix_flake_settings.
+ */
+void nix_flake_settings_free(nix_flake_settings * settings);
+
+/**
+ * @brief Register Flakes support process-wide.
+ */
+nix_err nix_flake_init_global(nix_c_context * context, nix_flake_settings * settings);
+
+#ifdef __cplusplus
+} // extern "C"
+#endif
+
+#endif

src/libflake-c/nix_api_flake_internal.hh

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+#pragma once
+
+#include "ref.hh"
+#include "flake/settings.hh"
+
+struct nix_flake_settings
+{
+    nix::ref<nix::flake::Settings> settings;
+};

src/libflake-c/package.nix

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+{ lib
+, stdenv
+, mkMesonLibrary
+
+, nix-store-c
+, nix-expr-c
+, nix-flake
+
+# Configuration Options
+
+, version
+}:
+
+let
+  inherit (lib) fileset;
+in
+
+mkMesonLibrary (finalAttrs: {
+  pname = "nix-flake-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") ./.)
+  ];
+
+  propagatedBuildInputs = [
+    nix-expr-c
+    nix-store-c
+    nix-flake
+  ];
+
+  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";
+  };
+
+  meta = {
+    platforms = lib.platforms.unix ++ lib.platforms.windows;
+  };
+
+})