Cleanup config headers

There are two big changes:

1. Public and private config is now separated. Configuration variables
   that are only used internally do not go in a header which is
   installed.

   (Additionally, libutil has a unix-specific private config header,
   which should only be used in unix-specific code. This keeps things a
   bit more organized, in a purely private implementation-internal way.)

2. Secondly, there is no more `-include`. There are very few config
   items that need to be publically exposed, so now it is feasible to
   just make the headers that need them just including the (public)
   configuration header.

And there are also a few more small cleanups on top of those:

- The configuration files have better names.

- The few CPP variables that remain exposed in the public headers are
  now also renamed to always start with `NIX_`. This ensures they should
  not conflict with variables defined elsewhere.

- We now always use `#if` and not `#ifdef`/`#ifndef` for our
  configuration variables, which helps avoid bugs by requiring that
  variables must be defined in all cases.

src/libcmd/meson.build

@@ -44,27 +44,18 @@ if readline_flavor == 'editline'
 elif readline_flavor == 'readline'
   readline = dependency('readline')
   deps_private += readline
-  configdata.set(
-    'USE_READLINE',
-    1,
-    description: 'Use readline instead of editline',
-  )
 else
   error('illegal editline flavor', readline_flavor)
 endif
-
+configdata.set(
-config_h = configure_file(
+  'USE_READLINE',
-  configuration : configdata,
+  (readline_flavor == 'readline').to_int(),
-  output : 'cmd-config-private.hh',
+  description: 'Use readline instead of editline',
 )
 
-add_project_arguments(
+config_priv_h = configure_file(
-  # TODO(Qyriad): Yes this is how the autoconf+Make system did it.
+  configuration : configdata,
-  # It would be nice for our headers to be idempotent instead.
+  output : 'cmd-config-private.hh',
-  '-include', 'nix/config-util.hh',
-  '-include', 'nix/config-store.hh',
-  '-include', 'nix/config-expr.hh',
-  language : 'cpp',
 )
 
 subdir('nix-meson-build-support/common')
@@ -96,7 +87,7 @@ subdir('nix-meson-build-support/windows-version')
 this_library = library(
   'nixcmd',
   sources,
-  config_h,
+  config_priv_h,
   dependencies : deps_public + deps_private + deps_other,
   include_directories : include_dirs,
   link_args: linker_export_flags,

src/libcmd/repl-interacter.cc

@@ -2,7 +2,7 @@
 
 #include <cstdio>
 
-#ifdef USE_READLINE
+#if USE_READLINE
 #include <readline/history.h>
 #include <readline/readline.h>
 #else
@@ -37,7 +37,7 @@ void sigintHandler(int signo)
 
 static detail::ReplCompleterMixin * curRepl; // ugly
 
-#ifndef USE_READLINE
+#if !USE_READLINE
 static char * completionCallback(char * s, int * match)
 {
     auto possible = curRepl->completePrefix(s);
@@ -115,14 +115,14 @@ ReadlineLikeInteracter::Guard ReadlineLikeInteracter::init(detail::ReplCompleter
     } catch (SystemError & e) {
         logWarning(e.info());
     }
-#ifndef USE_READLINE
+#if !USE_READLINE
     el_hist_size = 1000;
 #endif
     read_history(historyFile.c_str());
     auto oldRepl = curRepl;
     curRepl = repl;
     Guard restoreRepl([oldRepl] { curRepl = oldRepl; });
-#ifndef USE_READLINE
+#if !USE_READLINE
     rl_set_complete_func(completionCallback);
     rl_set_list_possib_func(listPossibleCallback);
 #endif
@@ -185,7 +185,7 @@ bool ReadlineLikeInteracter::getLine(std::string & input, ReplPromptType promptT
     // quite useful for reading the test output, so we add it here.
     if (auto e = getEnv("_NIX_TEST_REPL_ECHO"); s && e && *e == "1")
     {
-#ifndef USE_READLINE
+#if !USE_READLINE
         // This is probably not right for multi-line input, but we don't use that
         // in the characterisation tests, so it's fine.
         std::cout << promptForType(promptType) << s << std::endl;

src/libexpr-c/meson.build

@@ -25,18 +25,6 @@ deps_public_maybe_subproject = [
 ]
 subdir('nix-meson-build-support/subprojects')
 
-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', 'nix/config-util.hh',
-  '-include', 'nix/config-store.hh',
-  '-include', 'nix/config-expr.hh',
-
-  language : 'cpp',
-)
-
 subdir('nix-meson-build-support/common')
 
 sources = files(

src/libexpr-c/nix_api_expr.cc

@@ -15,7 +15,7 @@
 #include "nix_api_util.h"
 #include "nix_api_util_internal.h"
 
-#if HAVE_BOEHMGC
+#if NIX_USE_BOEHMGC
 #  include <mutex>
 #endif
 
@@ -209,7 +209,7 @@ void nix_state_free(EvalState * state)
     delete state;
 }
 
-#if HAVE_BOEHMGC
+#if NIX_USE_BOEHMGC
 std::unordered_map<
     const void *,
     unsigned int,
@@ -285,7 +285,7 @@ nix_err nix_value_decref(nix_c_context * context, nix_value *x)
 
 void nix_gc_register_finalizer(void * obj, void * cd, void (*finalizer)(void * obj, void * cd))
 {
-#if HAVE_BOEHMGC
+#if NIX_USE_BOEHMGC
     GC_REGISTER_FINALIZER(obj, finalizer, cd, 0, 0);
 #endif
 }

src/libexpr-c/nix_api_external.cc

@@ -168,7 +168,7 @@ ExternalValue * nix_create_external_value(nix_c_context * context, NixCExternalV
         context->last_err_code = NIX_OK;
     try {
         auto ret = new
-#if HAVE_BOEHMGC
+#if NIX_USE_BOEHMGC
             (GC)
 #endif
                 NixCExternalValue(*desc, v);

src/libexpr-c/nix_api_value.cc

@@ -125,7 +125,7 @@ PrimOp * nix_alloc_primop(
     try {
         using namespace std::placeholders;
         auto p = new
-#if HAVE_BOEHMGC
+#if NIX_USE_BOEHMGC
             (GC)
 #endif
                 nix::PrimOp{
@@ -497,7 +497,7 @@ ListBuilder * nix_make_list_builder(nix_c_context * context, EvalState * state,
     try {
         auto builder = state->state.buildList(capacity);
         return new
-#if HAVE_BOEHMGC
+#if NIX_USE_BOEHMGC
             (NoGC)
 #endif
                 ListBuilder{std::move(builder)};
@@ -519,7 +519,7 @@ nix_list_builder_insert(nix_c_context * context, ListBuilder * list_builder, uns
 
 void nix_list_builder_free(ListBuilder * list_builder)
 {
-#if HAVE_BOEHMGC
+#if NIX_USE_BOEHMGC
     GC_FREE(list_builder);
 #else
     delete list_builder;
@@ -578,7 +578,7 @@ BindingsBuilder * nix_make_bindings_builder(nix_c_context * context, EvalState *
     try {
         auto bb = state->state.buildBindings(capacity);
         return new
-#if HAVE_BOEHMGC
+#if NIX_USE_BOEHMGC
             (NoGC)
 #endif
                 BindingsBuilder{std::move(bb)};
@@ -600,7 +600,7 @@ nix_err nix_bindings_builder_insert(nix_c_context * context, BindingsBuilder * b
 
 void nix_bindings_builder_free(BindingsBuilder * bb)
 {
-#if HAVE_BOEHMGC
+#if NIX_USE_BOEHMGC
     GC_FREE((nix::BindingsBuilder *) bb);
 #else
     delete (nix::BindingsBuilder *) bb;

src/libexpr-test-support/meson.build

@@ -29,15 +29,6 @@ subdir('nix-meson-build-support/subprojects')
 rapidcheck = dependency('rapidcheck')
 deps_public += rapidcheck
 
-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.
-  '-include', 'nix/config-util.hh',
-  '-include', 'nix/config-store.hh',
-  '-include', 'nix/config-expr.hh',
-  language : 'cpp',
-)
-
 subdir('nix-meson-build-support/common')
 
 sources = files(

src/libexpr-tests/meson.build

@@ -35,13 +35,12 @@ deps_private += gtest
 gtest = dependency('gmock')
 deps_private += gtest
 
-add_project_arguments(
+configdata = configuration_data()
-  # TODO(Qyriad): Yes this is how the autoconf+Make system did it.
+configdata.set_quoted('PACKAGE_VERSION', meson.project_version())
-  # It would be nice for our headers to be idempotent instead.
+
-  '-include', 'nix/config-util.hh',
+config_priv_h = configure_file(
-  '-include', 'nix/config-store.hh',
+  configuration : configdata,
-  '-include', 'nix/config-expr.hh',
+  output : 'expr-tests-config.hh',
-  language : 'cpp',
 )
 
 subdir('nix-meson-build-support/common')
@@ -69,6 +68,7 @@ include_dirs = [include_directories('.')]
 this_exe = executable(
   meson.project_name(),
   sources,
+  config_priv_h,
   dependencies : deps_private_subproject + deps_private + deps_other,
   include_directories : include_dirs,
   # TODO: -lrapidcheck, see ../libutil-support/build.meson

src/libexpr-tests/nix_api_expr.cc

@@ -12,6 +12,8 @@
 #include <gmock/gmock.h>
 #include <gtest/gtest.h>
 
+#include "expr-tests-config.hh"
+
 namespace nixC {
 
 TEST_F(nix_api_store_test, nix_eval_state_lookup_path)

src/libexpr/eval-gc.cc

@@ -5,7 +5,9 @@
 #include "nix/serialise.hh"
 #include "nix/eval-gc.hh"
 
-#if HAVE_BOEHMGC
+#include "expr-config-private.hh"
+
+#if NIX_USE_BOEHMGC
 
 #  include <pthread.h>
 #  if __FreeBSD__
@@ -24,7 +26,7 @@
 
 namespace nix {
 
-#if HAVE_BOEHMGC
+#if NIX_USE_BOEHMGC
 /* Called when the Boehm GC runs out of memory. */
 static void * oomHandler(size_t requested)
 {
@@ -94,7 +96,7 @@ void initGC()
     if (gcInitialised)
         return;
 
-#if HAVE_BOEHMGC
+#if NIX_USE_BOEHMGC
     initGCReal();
 
     gcCyclesAfterInit = GC_get_gc_no();

src/libexpr/eval.cc

@@ -295,7 +295,7 @@ EvalState::EvalState(
     , debugStop(false)
     , trylevel(0)
     , regexCache(makeRegexCache())
-#if HAVE_BOEHMGC
+#if NIX_USE_BOEHMGC
     , valueAllocCache(std::allocate_shared<void *>(traceable_allocator<void *>(), nullptr))
     , env1AllocCache(std::allocate_shared<void *>(traceable_allocator<void *>(), nullptr))
     , baseEnvP(std::allocate_shared<Env *>(traceable_allocator<Env *>(), &allocEnv(BASE_ENV_SIZE)))
@@ -2812,7 +2812,7 @@ bool EvalState::eqValues(Value & v1, Value & v2, const PosIdx pos, std::string_v
 }
 
 bool EvalState::fullGC() {
-#if HAVE_BOEHMGC
+#if NIX_USE_BOEHMGC
     GC_gcollect();
     // Check that it ran. We might replace this with a version that uses more
     // of the boehm API to get this reliably, at a maintenance cost.
@@ -2831,7 +2831,7 @@ void EvalState::maybePrintStats()
 
     if (showStats) {
         // Make the final heap size more deterministic.
-#if HAVE_BOEHMGC
+#if NIX_USE_BOEHMGC
         if (!fullGC()) {
             warn("failed to perform a full GC before reporting stats");
         }
@@ -2853,7 +2853,7 @@ void EvalState::printStatistics()
     uint64_t bValues = nrValues * sizeof(Value);
     uint64_t bAttrsets = nrAttrsets * sizeof(Bindings) + nrAttrsInAttrsets * sizeof(Attr);
 
-#if HAVE_BOEHMGC
+#if NIX_USE_BOEHMGC
     GC_word heapSize, totalBytes;
     GC_get_heap_usage_safe(&heapSize, 0, 0, 0, &totalBytes);
     double gcFullOnlyTime = ({
@@ -2875,7 +2875,7 @@ void EvalState::printStatistics()
 #ifndef _WIN32 // TODO implement
         {"cpu", cpuTime},
 #endif
-#if HAVE_BOEHMGC
+#if NIX_USE_BOEHMGC
         {GC_is_incremental_mode() ? "gcNonIncremental" : "gc", gcFullOnlyTime},
 #ifndef _WIN32 // TODO implement
         {GC_is_incremental_mode() ? "gcNonIncrementalFraction" : "gcFraction", gcFullOnlyTime / cpuTime},
@@ -2919,7 +2919,7 @@ void EvalState::printStatistics()
     topObj["nrLookups"] = nrLookups;
     topObj["nrPrimOpCalls"] = nrPrimOpCalls;
     topObj["nrFunctionCalls"] = nrFunctionCalls;
-#if HAVE_BOEHMGC
+#if NIX_USE_BOEHMGC
     topObj["gc"] = {
         {"heapSize", heapSize},
         {"totalBytes", totalBytes},

src/libexpr/include/nix/eval-gc.hh

@@ -3,7 +3,10 @@
 
 #include <cstddef>
 
-#if HAVE_BOEHMGC
+// For `NIX_USE_BOEHMGC`, and if that's set, `GC_THREADS`
+#include "nix/expr-config.hh"
+
+#if NIX_USE_BOEHMGC
 
 #  define GC_INCLUDE_NEW
 
@@ -43,7 +46,7 @@ void initGC();
  */
 void assertGCInitialized();
 
-#ifdef HAVE_BOEHMGC
+#if NIX_USE_BOEHMGC
 /**
  * The number of GC cycles since initGC().
  */

src/libexpr/include/nix/eval-inline.hh

@@ -6,6 +6,9 @@
 #include "nix/eval-error.hh"
 #include "nix/eval-settings.hh"
 
+// For `NIX_USE_BOEHMGC`, and if that's set, `GC_THREADS`
+#include "nix/expr-config.hh"
+
 namespace nix {
 
 /**
@@ -15,7 +18,7 @@ namespace nix {
 inline void * allocBytes(size_t n)
 {
     void * p;
-#if HAVE_BOEHMGC
+#if NIX_USE_BOEHMGC
     p = GC_MALLOC(n);
 #else
     p = calloc(n, 1);
@@ -28,7 +31,7 @@ inline void * allocBytes(size_t n)
 [[gnu::always_inline]]
 Value * EvalState::allocValue()
 {
-#if HAVE_BOEHMGC
+#if NIX_USE_BOEHMGC
     /* We use the boehm batch allocator to speed up allocations of Values (of which there are many).
        GC_malloc_many returns a linked list of objects of the given size, where the first word
        of each object is also the pointer to the next object in the list. This also means that we
@@ -60,7 +63,7 @@ Env & EvalState::allocEnv(size_t size)
 
     Env * env;
 
-#if HAVE_BOEHMGC
+#if NIX_USE_BOEHMGC
     if (size == 1) {
         /* see allocValue for explanations. */
         if (!*env1AllocCache) {

src/libexpr/include/nix/eval.hh

@@ -16,6 +16,9 @@
 #include "nix/repl-exit-status.hh"
 #include "nix/ref.hh"
 
+// For `NIX_USE_BOEHMGC`, and if that's set, `GC_THREADS`
+#include "nix/expr-config.hh"
+
 #include <map>
 #include <optional>
 #include <functional>
@@ -369,7 +372,7 @@ private:
      */
     std::shared_ptr<RegexCache> regexCache;
 
-#if HAVE_BOEHMGC
+#if NIX_USE_BOEHMGC
     /**
      * Allocation cache for GC'd Value objects.
      */
@@ -596,7 +599,7 @@ public:
      */
     SingleDerivedPath coerceToSingleDerivedPath(const PosIdx pos, Value & v, std::string_view errorCtx);
 
-#if HAVE_BOEHMGC
+#if NIX_USE_BOEHMGC
     /** A GC root for the baseEnv reference. */
     std::shared_ptr<Env *> baseEnvP;
 #endif

src/libexpr/include/nix/meson.build

@@ -2,12 +2,12 @@
 
 include_dirs = [include_directories('..')]
 
-config_h = configure_file(
+config_pub_h = configure_file(
-  configuration : configdata,
+  configuration : configdata_pub,
-  output : 'config-expr.hh',
+  output : 'expr-config.hh',
 )
 
-headers = [config_h] + files(
+headers = [config_pub_h] + files(
   'attr-path.hh',
   'attr-set.hh',
   'eval-cache.hh',

src/libexpr/meson.build

@@ -14,7 +14,8 @@ cxx = meson.get_compiler('cpp')
 
 subdir('nix-meson-build-support/deps-lists')
 
-configdata = configuration_data()
+configdata_pub = configuration_data()
+configdata_priv = configuration_data()
 
 deps_private_maybe_subproject = [
 ]
@@ -26,6 +27,16 @@ deps_public_maybe_subproject = [
 subdir('nix-meson-build-support/subprojects')
 subdir('nix-meson-build-support/big-objs')
 
+# Check for each of these functions, and create a define like `#define HAVE_LCHOWN 1`.
+check_funcs = [
+  'sysconf',
+]
+foreach funcspec : check_funcs
+  define_name = 'HAVE_' + funcspec.underscorify().to_upper()
+  define_value = cxx.has_function(funcspec).to_int()
+  configdata_priv.set(define_name, define_value)
+endforeach
+
 boost = dependency(
   'boost',
   modules : ['container', 'context'],
@@ -47,11 +58,13 @@ if bdw_gc.found()
   ]
     define_name = 'HAVE_' + funcspec.underscorify().to_upper()
     define_value = cxx.has_function(funcspec).to_int()
-    configdata.set(define_name, define_value)
+    configdata_priv.set(define_name, define_value)
   endforeach
-  configdata.set('GC_THREADS', 1)
+  # Affects ABI, because it changes what bdw_gc itself does!
+  configdata_pub.set('GC_THREADS', 1)
 endif
-configdata.set('HAVE_BOEHMGC', bdw_gc.found().to_int())
+# Used in public header. Affects ABI!
+configdata_pub.set('NIX_USE_BOEHMGC', bdw_gc.found().to_int())
 
 toml11 = dependency(
   'toml11',
@@ -61,14 +74,9 @@ toml11 = dependency(
 )
 deps_other += toml11
 
-add_project_arguments(
+config_priv_h = configure_file(
-  # TODO(Qyriad): Yes this is how the autoconf+Make system did it.
+  configuration : configdata_priv,
-  # It would be nice for our headers to be idempotent instead.
+  output : 'expr-config-private.hh',
-  '-include', 'nix/config-util.hh',
-  '-include', 'nix/config-store.hh',
-  # '-include', 'nix_api_fetchers_config.h',
-  '-include', 'nix/config-expr.hh',
-  language : 'cpp',
 )
 
 subdir('nix-meson-build-support/common')
@@ -158,6 +166,7 @@ subdir('nix-meson-build-support/windows-version')
 this_library = library(
   'nixexpr',
   sources,
+  config_priv_h,
   parser_tab,
   lexer_tab,
   generated_headers,

src/libfetchers-tests/meson.build

@@ -34,15 +34,6 @@ deps_private += gtest
 libgit2 = dependency('libgit2')
 deps_private += libgit2
 
-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.
-  '-include', 'nix/config-util.hh',
-  '-include', 'nix/config-store.hh',
-  # '-include', 'nix_api_fetchers_config.h',
-  language : 'cpp',
-)
-
 subdir('nix-meson-build-support/common')
 
 sources = files(

src/libfetchers/meson.build

@@ -30,15 +30,6 @@ deps_public += nlohmann_json
 libgit2 = dependency('libgit2')
 deps_private += libgit2
 
-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.
-  '-include', 'nix/config-util.hh',
-  '-include', 'nix/config-store.hh',
-  # '-include', 'nix_api_fetchers_config.h',
-  language : 'cpp',
-)
-
 subdir('nix-meson-build-support/common')
 
 sources = files(

src/libflake-c/meson.build

@@ -27,20 +27,6 @@ deps_public_maybe_subproject = [
 ]
 subdir('nix-meson-build-support/subprojects')
 
-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', 'nix/config-util.hh',
-  '-include', 'nix/config-store.hh',
-  '-include', 'nix/config-expr.hh',
-  # not generated (yet?)
-  # '-include', 'nix/config-flake.hh',
-
-  language : 'cpp',
-)
-
 subdir('nix-meson-build-support/common')
 
 sources = files(

src/libflake-tests/meson.build

@@ -32,15 +32,6 @@ deps_private += rapidcheck
 gtest = dependency('gtest', main : true)
 deps_private += gtest
 
-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.
-  '-include', 'nix/config-util.hh',
-  '-include', 'nix/config-store.hh',
-  '-include', 'nix/config-expr.hh',
-  language : 'cpp',
-)
-
 subdir('nix-meson-build-support/common')
 
 sources = files(

src/libflake/meson.build

@@ -27,16 +27,6 @@ subdir('nix-meson-build-support/subprojects')
 nlohmann_json = dependency('nlohmann_json', version : '>= 3.9')
 deps_public += nlohmann_json
 
-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.
-  '-include', 'nix/config-util.hh',
-  '-include', 'nix/config-store.hh',
-  # '-include', 'nix_api_fetchers_config.h',
-  '-include', 'nix/config-expr.hh',
-  language : 'cpp',
-)
-
 subdir('nix-meson-build-support/common')
 
 subdir('nix-meson-build-support/generate-header')

src/libmain-c/meson.build

@@ -25,17 +25,6 @@ deps_public_maybe_subproject = [
 ]
 subdir('nix-meson-build-support/subprojects')
 
-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', 'nix/config-util.hh',
-  '-include', 'nix/config-store.hh',
-
-  language : 'cpp',
-)
-
 subdir('nix-meson-build-support/common')
 
 sources = files(

src/libmain/include/nix/meson.build

@@ -2,12 +2,7 @@
 
 include_dirs = [include_directories('..')]
 
-config_h = configure_file(
+headers = files(
-  configuration : configdata,
-  output : 'config-main.hh',
-)
-
-headers = [config_h] + files(
   'common-args.hh',
   'loggers.hh',
   'plugin.hh',

src/libmain/meson.build

@@ -42,13 +42,9 @@ configdata.set(
   description: 'Optionally used for buffering on standard error'
 )
 
-add_project_arguments(
+config_priv_h = configure_file(
-  # TODO(Qyriad): Yes this is how the autoconf+Make system did it.
+  configuration : configdata,
-  # It would be nice for our headers to be idempotent instead.
+  output : 'main-config-private.hh',
-  '-include', 'nix/config-util.hh',
-  '-include', 'nix/config-store.hh',
-  '-include', 'nix/config-main.hh',
-  language : 'cpp',
 )
 
 subdir('nix-meson-build-support/common')
@@ -75,7 +71,7 @@ subdir('nix-meson-build-support/windows-version')
 this_library = library(
   'nixmain',
   sources,
-  config_h,
+  config_priv_h,
   dependencies : deps_public + deps_private + deps_other,
   include_directories : include_dirs,
   link_args: linker_export_flags,

src/libmain/shared.cc

@@ -25,6 +25,8 @@
 #include "nix/exit.hh"
 #include "nix/strings.hh"
 
+#include "main-config-private.hh"
+
 namespace nix {
 
 char * * savedArgv;
@@ -297,7 +299,7 @@ void printVersion(const std::string & programName)
     std::cout << fmt("%1% (Nix) %2%", programName, nixVersion) << std::endl;
     if (verbosity > lvlInfo) {
         Strings cfg;
-#if HAVE_BOEHMGC
+#if NIX_USE_BOEHMGC
         cfg.push_back("gc");
 #endif
         cfg.push_back("signed-caches");

src/libstore-c/meson.build

@@ -23,17 +23,6 @@ deps_public_maybe_subproject = [
 ]
 subdir('nix-meson-build-support/subprojects')
 
-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', 'nix/config-util.hh',
-  '-include', 'nix/config-store.hh',
-
-  language : 'cpp',
-)
-
 subdir('nix-meson-build-support/common')
 
 sources = files(

src/libstore-test-support/meson.build

@@ -27,14 +27,6 @@ subdir('nix-meson-build-support/subprojects')
 rapidcheck = dependency('rapidcheck')
 deps_public += rapidcheck
 
-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.
-  '-include', 'nix/config-util.hh',
-  '-include', 'nix/config-store.hh',
-  language : 'cpp',
-)
-
 subdir('nix-meson-build-support/common')
 
 sources = files(

src/libstore-tests/meson.build

@@ -37,17 +37,17 @@ deps_private += rapidcheck
 gtest = dependency('gtest', main : true)
 deps_private += gtest
 
+configdata = configuration_data()
+configdata.set_quoted('PACKAGE_VERSION', meson.project_version())
+
+config_priv_h = configure_file(
+  configuration : configdata,
+  output : 'store-tests-config.hh',
+)
+
 gtest = dependency('gmock')
 deps_private += gtest
 
-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.
-  '-include', 'nix/config-util.hh',
-  '-include', 'nix/config-store.hh',
-  language : 'cpp',
-)
-
 subdir('nix-meson-build-support/common')
 
 sources = files(
@@ -84,6 +84,7 @@ include_dirs = [include_directories('.')]
 this_exe = executable(
   meson.project_name(),
   sources,
+  config_priv_h,
   dependencies : deps_private_subproject + deps_private + deps_other,
   include_directories : include_dirs,
   # TODO: -lrapidcheck, see ../libutil-support/build.meson

src/libstore-tests/nix_api_store.cc

@@ -6,6 +6,8 @@
 #include "nix/tests/nix_api_store.hh"
 #include "nix/tests/string_callback.hh"
 
+#include "store-tests-config.hh"
+
 namespace nixC {
 
 std::string PATH_SUFFIX = "/g1w7hy3qg1w7hy3qg1w7hy3qg1w7hy3q-name";

src/libstore/gc.cc

@@ -6,6 +6,8 @@
 #include "nix/signals.hh"
 #include "nix/posix-fs-canonicalise.hh"
 
+#include "store-config-private.hh"
+
 #if !defined(__linux__)
 // For shelling out to lsof
 #  include "nix/processes.hh"

src/libstore/globals.cc

@@ -6,6 +6,7 @@
 #include "nix/abstract-setting-to-json.hh"
 #include "nix/compute-levels.hh"
 #include "nix/signals.hh"
+#include "nix/strings.hh"
 
 #include <algorithm>
 #include <map>
@@ -35,7 +36,8 @@
 #include <sys/sysctl.h>
 #endif
 
-#include "nix/strings.hh"
+#include "store-config-private.hh"
+
 
 namespace nix {
 
@@ -202,7 +204,7 @@ StringSet Settings::getDefaultExtraPlatforms()
 {
     StringSet extraPlatforms;
 
-    if (std::string{SYSTEM} == "x86_64-linux" && !isWSL1())
+    if (std::string{NIX_LOCAL_SYSTEM} == "x86_64-linux" && !isWSL1())
         extraPlatforms.insert("i686-linux");
 
 #if __linux__
@@ -214,7 +216,7 @@ StringSet Settings::getDefaultExtraPlatforms()
     // machines. Note that we can’t force processes from executing
     // x86_64 in aarch64 environments or vice versa since they can
     // always exec with their own binary preferences.
-    if (std::string{SYSTEM} == "aarch64-darwin" &&
+    if (std::string{NIX_LOCAL_SYSTEM} == "aarch64-darwin" &&
         runProgram(RunOptions {.program = "arch", .args = {"-arch", "x86_64", "/usr/bin/true"}, .mergeStderrToStdout = true}).first == 0)
         extraPlatforms.insert("x86_64-darwin");
 #endif

src/libstore/include/nix/globals.hh

@@ -1,16 +1,18 @@
 #pragma once
 ///@file
 
+#include <map>
+#include <limits>
+
+#include <sys/types.h>
+
 #include "nix/types.hh"
 #include "nix/config.hh"
 #include "nix/environment-variables.hh"
 #include "nix/experimental-features.hh"
 #include "nix/users.hh"
 
-#include <map>
+#include "nix/store-config.hh"
-#include <limits>
-
-#include <sys/types.h>
 
 namespace nix {
 
@@ -181,7 +183,7 @@ public:
     bool readOnlyMode = false;
 
     Setting<std::string> thisSystem{
-        this, SYSTEM, "system",
+        this, NIX_LOCAL_SYSTEM, "system",
         R"(
           The system type of the current Nix installation.
           Nix will only build a given [store derivation](@docroot@/glossary.md#gloss-store-derivation) locally when its `system` attribute equals any of the values specified here or in [`extra-platforms`](#conf-extra-platforms).
@@ -1089,7 +1091,7 @@ public:
         )"};
 #endif
 
-#if HAVE_ACL_SUPPORT
+#if NIX_SUPPORT_ACL
     Setting<StringSet> ignoredAcls{
         this, {"security.selinux", "system.nfs4_acl", "security.csm"}, "ignored-acls",
         R"(

src/libstore/include/nix/meson.build

@@ -4,12 +4,12 @@ include_dirs = [
   include_directories('..'),
 ]
 
-config_h = configure_file(
+config_pub_h = configure_file(
-  configuration : configdata,
+  configuration : configdata_pub,
-  output : 'config-store.hh',
+  output : 'store-config.hh',
 )
 
-headers = [config_h] + files(
+headers = [config_pub_h] + files(
   'binary-cache-store.hh',
   'build-result.hh',
   'build/derivation-goal.hh',

src/libstore/linux/include/nix/fchmodat2-compat.hh

@@ -1,3 +1,5 @@
+#include "store-config-private.hh"
+
 /*
  * Determine the syscall number for `fchmodat2`.
  *

src/libstore/linux/personality.cc

@@ -15,7 +15,7 @@ void setPersonality(std::string_view system)
         struct utsname utsbuf;
         uname(&utsbuf);
         if ((system == "i686-linux"
-                && (std::string_view(SYSTEM) == "x86_64-linux"
+                && (std::string_view(NIX_LOCAL_SYSTEM) == "x86_64-linux"
                     || (!strcmp(utsbuf.sysname, "Linux") && !strcmp(utsbuf.machine, "x86_64"))))
             || system == "armv7l-linux"
             || system == "armv6l-linux"

src/libstore/local-store.cc

@@ -54,6 +54,8 @@
 
 #include "nix/strings.hh"
 
+#include "store-config-private.hh"
+
 
 namespace nix {
 

src/libstore/meson.build

@@ -15,12 +15,20 @@ cxx = meson.get_compiler('cpp')
 
 subdir('nix-meson-build-support/deps-lists')
 
-configdata = configuration_data()
+configdata_pub = configuration_data()
+configdata_priv = configuration_data()
 
 # TODO rename, because it will conflict with downstream projects
-configdata.set_quoted('PACKAGE_VERSION', meson.project_version())
+configdata_priv.set_quoted('PACKAGE_VERSION', meson.project_version())
 
-configdata.set_quoted('SYSTEM', host_machine.cpu_family() + '-' + host_machine.system())
+# Used in public header.
+configdata_pub.set_quoted(
+  'NIX_LOCAL_SYSTEM',
+  host_machine.cpu_family() + '-' + host_machine.system(),
+  description :
+    'This is the system name Nix expects for local running instance of Nix.\n\n'
+    + 'See the "system" setting for additional details',
+)
 
 deps_private_maybe_subproject = [
 ]
@@ -47,28 +55,30 @@ run_command('rm', '-f',
   check : true,
 )
 summary('can hardlink to symlink', can_link_symlink, bool_yn : true)
-configdata.set('CAN_LINK_SYMLINK', can_link_symlink.to_int())
+configdata_priv.set('CAN_LINK_SYMLINK', can_link_symlink.to_int())
 
 # Check for each of these functions, and create a define like `#define HAVE_LCHOWN 1`.
-#
-# Only need to do functions that deps (like `libnixutil`) didn't already
-# check for.
 check_funcs = [
   # Optionally used for canonicalising files from the build
   'lchown',
+  'posix_fallocate',
   'statvfs',
 ]
 foreach funcspec : check_funcs
   define_name = 'HAVE_' + funcspec.underscorify().to_upper()
   define_value = cxx.has_function(funcspec).to_int()
-  configdata.set(define_name, define_value)
+  configdata_priv.set(define_name, define_value)
 endforeach
 
 has_acl_support = cxx.has_header('sys/xattr.h') \
   and cxx.has_function('llistxattr') \
   and cxx.has_function('lremovexattr')
-# TODO: used in header - make proper public header and make sure it's included. Affects ABI!
+# Used in public header. Affects ABI!
-configdata.set('HAVE_ACL_SUPPORT', has_acl_support.to_int())
+configdata_pub.set(
+  'NIX_SUPPORT_ACL',
+  has_acl_support.to_int(),
+  description : 'FIXME: It\'s a bit peculiar that this needs to be exposed. The reason is that that it effects whether the settings struct in a header has a particular field. This is also odd, because it means when there is no ACL support one will just get an "unknown setting" warning from their configuration.',
+)
 
 if host_machine.system() == 'darwin'
   sandbox = cxx.find_library('sandbox')
@@ -104,7 +114,7 @@ seccomp = dependency('libseccomp', 'seccomp', required : seccomp_required, versi
 if is_linux and not seccomp.found()
   warning('Sandbox security is reduced because libseccomp has not been found! Please provide libseccomp if it supports your CPU architecture.')
 endif
-configdata.set('HAVE_SECCOMP', seccomp.found().to_int())
+configdata_priv.set('HAVE_SECCOMP', seccomp.found().to_int())
 deps_private += seccomp
 
 nlohmann_json = dependency('nlohmann_json', version : '>= 3.9')
@@ -116,7 +126,7 @@ deps_private += sqlite
 # AWS C++ SDK has bad pkg-config. See
 # https://github.com/aws/aws-sdk-cpp/issues/2673 for details.
 aws_s3 = dependency('aws-cpp-sdk-s3', required : false)
-configdata.set('ENABLE_S3', aws_s3.found().to_int())
+configdata_priv.set('ENABLE_S3', aws_s3.found().to_int())
 if aws_s3.found()
   aws_s3 = declare_dependency(
     include_directories: include_directories(aws_s3.get_variable('includedir')),
@@ -148,7 +158,7 @@ if get_option('embedded-sandbox-shell')
   # The path to busybox is passed as a -D flag when compiling this_library.
   # This solution is inherited from the old make buildsystem
   # TODO: do this differently?
-  configdata.set('HAVE_EMBEDDED_SANDBOX_SHELL', 1)
+  configdata_priv.set('HAVE_EMBEDDED_SANDBOX_SHELL', 1)
   hexdump = find_program('hexdump', native : true)
   embedded_sandbox_shell_gen = custom_target(
     'embedded-sandbox-shell.gen.hh',
@@ -166,12 +176,9 @@ if get_option('embedded-sandbox-shell')
   generated_headers += embedded_sandbox_shell_gen
 endif
 
-add_project_arguments(
+config_priv_h = configure_file(
-  # TODO(Qyriad): Yes this is how the autoconf+Make system did it.
+  configuration : configdata_priv,
-  # It would be nice for our headers to be idempotent instead.
+  output : 'store-config-private.hh',
-  '-include', 'nix/config-util.hh',
-  '-include', 'nix/config-store.hh',
-  language : 'cpp',
 )
 
 subdir('nix-meson-build-support/common')
@@ -347,6 +354,7 @@ this_library = library(
   'nixstore',
   generated_headers,
   sources,
+  config_priv_h,
   dependencies : deps_public + deps_private + deps_other,
   include_directories : include_dirs,
   cpp_args : cpp_args,

src/libstore/posix-fs-canonicalise.cc

@@ -1,13 +1,16 @@
-#if HAVE_ACL_SUPPORT
-# include <sys/xattr.h>
-#endif
-
 #include "nix/posix-fs-canonicalise.hh"
 #include "nix/file-system.hh"
 #include "nix/signals.hh"
 #include "nix/util.hh"
 #include "nix/globals.hh"
 #include "nix/store-api.hh"
+#include "nix/store-config.hh"
+
+#include "store-config-private.hh"
+
+#if NIX_SUPPORT_ACL
+# include <sys/xattr.h>
+#endif
 
 namespace nix {
 
@@ -72,7 +75,7 @@ static void canonicalisePathMetaData_(
     if (!(S_ISREG(st.st_mode) || S_ISDIR(st.st_mode) || S_ISLNK(st.st_mode)))
         throw Error("file '%1%' has an unsupported type", path);
 
-#if HAVE_ACL_SUPPORT
+#if NIX_SUPPORT_ACL
     /* Remove extended attributes / ACLs. */
     ssize_t eaSize = llistxattr(path.c_str(), nullptr, 0);
 

src/libstore/unix/build/local-derivation-goal.cc

@@ -22,6 +22,7 @@
 #include "nix/posix-fs-canonicalise.hh"
 #include "nix/posix-source-accessor.hh"
 #include "nix/restricted-store.hh"
+#include "nix/store-config.hh"
 
 #include <regex>
 #include <queue>
@@ -34,6 +35,8 @@
 #include <sys/resource.h>
 #include <sys/socket.h>
 
+#include "store-config-private.hh"
+
 #if HAVE_STATVFS
 #include <sys/statvfs.h>
 #endif
@@ -1785,7 +1788,7 @@ void setupSeccomp()
         seccomp_release(ctx);
     });
 
-    constexpr std::string_view nativeSystem = SYSTEM;
+    constexpr std::string_view nativeSystem = NIX_LOCAL_SYSTEM;
 
     if (nativeSystem == "x86_64-linux" &&
         seccomp_arch_add(ctx, SCMP_ARCH_X86) != 0)

src/libutil-c/meson.build

@@ -25,21 +25,11 @@ subdir('nix-meson-build-support/subprojects')
 
 configdata.set_quoted('PACKAGE_VERSION', meson.project_version())
 
-config_h = configure_file(
+config_priv_h = configure_file(
   configuration : configdata,
   output : 'nix_api_util_config.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', 'nix/config-util.hh',
-
-  language : 'cpp',
-)
-
 subdir('nix-meson-build-support/common')
 
 sources = files(
@@ -61,7 +51,7 @@ subdir('nix-meson-build-support/windows-version')
 this_library = library(
   'nixutilc',
   sources,
-  config_h,
+  config_priv_h,
   dependencies : deps_public + deps_private + deps_other,
   include_directories : include_dirs,
   link_args: linker_export_flags,

src/libutil-test-support/meson.build

@@ -25,13 +25,6 @@ subdir('nix-meson-build-support/subprojects')
 rapidcheck = dependency('rapidcheck')
 deps_public += rapidcheck
 
-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.
-  '-include', 'nix/config-util.hh',
-  language : 'cpp',
-)
-
 subdir('nix-meson-build-support/common')
 
 sources = files(

src/libutil-tests/meson.build

@@ -35,14 +35,9 @@ deps_private += gtest
 configdata = configuration_data()
 configdata.set_quoted('PACKAGE_VERSION', meson.project_version())
 
-config_h = configure_file(
+config_priv_h = configure_file(
   configuration : configdata,
-  output : 'config-util-tests.hh',
+  output : 'util-tests-config.hh',
-)
-
-add_project_arguments(
-  '-include', 'config-util-tests.hh',
-  language : 'cpp',
 )
 
 subdir('nix-meson-build-support/common')
@@ -84,7 +79,7 @@ include_dirs = [include_directories('.')]
 this_exe = executable(
   meson.project_name(),
   sources,
-  config_h,
+  config_priv_h,
   dependencies : deps_private_subproject + deps_private + deps_other,
   include_directories : include_dirs,
   # TODO: -lrapidcheck, see ../libutil-support/build.meson

src/libutil-tests/nix_api_util.cc

@@ -9,6 +9,8 @@
 
 #include <memory>
 
+#include "util-tests-config.hh"
+
 namespace nixC {
 
 TEST_F(nix_api_util_context, nix_context_error)

src/libutil/compute-levels.cc

@@ -1,8 +1,10 @@
 #include "nix/types.hh"
 
+#include "util-config-private.hh"
+
 #if HAVE_LIBCPUID
-#include <libcpuid/libcpuid.h>
+#  include <libcpuid/libcpuid.h>
-#include <map>
+#  include <map>
 #endif
 
 namespace nix {

src/libutil/file-system.cc

@@ -27,6 +27,8 @@
 
 #include "nix/strings-inline.hh"
 
+#include "util-config-private.hh"
+
 namespace nix {
 
 namespace fs { using namespace std::filesystem; }
@@ -632,62 +634,6 @@ void replaceSymlink(const fs::path & target, const fs::path & link)
     }
 }
 
-void setWriteTime(
-    const fs::path & path,
-    time_t accessedTime,
-    time_t modificationTime,
-    std::optional<bool> optIsSymlink)
-{
-#ifdef _WIN32
-    // FIXME use `fs::last_write_time`.
-    //
-    // Would be nice to use std::filesystem unconditionally, but
-    // doesn't support access time just modification time.
-    //
-    // System clock vs File clock issues also make that annoying.
-    warn("Changing file times is not yet implemented on Windows, path is %s", path);
-#elif HAVE_UTIMENSAT && HAVE_DECL_AT_SYMLINK_NOFOLLOW
-    struct timespec times[2] = {
-        {
-            .tv_sec = accessedTime,
-            .tv_nsec = 0,
-        },
-        {
-            .tv_sec = modificationTime,
-            .tv_nsec = 0,
-        },
-    };
-    if (utimensat(AT_FDCWD, path.c_str(), times, AT_SYMLINK_NOFOLLOW) == -1)
-        throw SysError("changing modification time of %s (using `utimensat`)", path);
-#else
-    struct timeval times[2] = {
-        {
-            .tv_sec = accessedTime,
-            .tv_usec = 0,
-        },
-        {
-            .tv_sec = modificationTime,
-            .tv_usec = 0,
-        },
-    };
-#if HAVE_LUTIMES
-    if (lutimes(path.c_str(), times) == -1)
-        throw SysError("changing modification time of %s", path);
-#else
-    bool isSymlink = optIsSymlink
-        ? *optIsSymlink
-        : fs::is_symlink(path);
-
-    if (!isSymlink) {
-        if (utimes(path.c_str(), times) == -1)
-            throw SysError("changing modification time of %s (not a symlink)", path);
-    } else {
-        throw Error("Cannot change modification time of symlink %s", path);
-    }
-#endif
-#endif
-}
-
 void setWriteTime(const fs::path & path, const struct stat & st)
 {
     setWriteTime(path, st.st_atime, st.st_mtime, S_ISLNK(st.st_mode));

src/libutil/fs-sink.cc

@@ -10,6 +10,8 @@
 # include "nix/windows-error.hh"
 #endif
 
+#include "util-config-private.hh"
+
 namespace nix {
 
 void copyRecursive(

src/libutil/include/nix/meson.build

@@ -2,12 +2,7 @@
 
 include_dirs = [include_directories('..')]
 
-config_h = configure_file(
+headers = files(
-  configuration : configdata,
-  output : 'config-util.hh',
-)
-
-headers = [config_h] + files(
   'abstract-setting-to-json.hh',
   'ansicolor.hh',
   'archive.hh',

src/libutil/meson.build

@@ -23,36 +23,20 @@ deps_public_maybe_subproject = [
 subdir('nix-meson-build-support/subprojects')
 
 # Check for each of these functions, and create a define like `#define
-# HAVE_LUTIMES 1`. The `#define` is unconditional, 0 for not found and 1
+# HAVE_POSIX_FALLOCATE 1`. The `#define` is unconditional, 0 for not
-# for found. One therefore uses it with `#if` not `#ifdef`.
+# found and 1 for found. One therefore uses it with `#if` not `#ifdef`.
 check_funcs = [
-  'close_range',
+  [
-  # Optionally used for changing the mtime of symlinks.
+    'posix_fallocate',
-  'lutimes',
+    'Optionally used to preallocate files to be large enough before writing to them.',
-  # Optionally used for creating pipes on Unix
+  ],
-  'pipe2',
-  # Optionally used to preallocate files to be large enough before
-  # writing to them.
-  # WARNING: define also used in libstore
-  'posix_fallocate',
-  # Optionally used to get more information about processes failing due
-  # to a signal on Unix.
-  'strsignal',
-  # Optionally used to try to close more file descriptors (e.g. before
-  # forking) on Unix.
-  # WARNING: also used in libexpr
-  'sysconf',
-  # Optionally used for changing the mtime of files and symlinks.
-  'utimensat',
 ]
 foreach funcspec : check_funcs
-  define_name = 'HAVE_' + funcspec.underscorify().to_upper()
+  define_name = 'HAVE_' + funcspec[0].underscorify().to_upper()
-  define_value = cxx.has_function(funcspec).to_int()
+  define_value = cxx.has_function(funcspec[0]).to_int()
-  configdata.set(define_name, define_value)
+  configdata.set(define_name, define_value, description: funcspec[1])
 endforeach
 
-configdata.set('HAVE_DECL_AT_SYMLINK_NOFOLLOW', cxx.has_header_symbol('fcntl.h', 'AT_SYMLINK_NOFOLLOW').to_int())
-
 subdir('nix-meson-build-support/libatomic')
 
 if host_machine.system() == 'windows'
@@ -116,16 +100,14 @@ deps_public += nlohmann_json
 
 cxx = meson.get_compiler('cpp')
 
-add_project_arguments(
+config_priv_h = configure_file(
-  # TODO(Qyriad): Yes this is how the autoconf+Make system did it.
+  configuration : configdata,
-  # It would be nice for our headers to be idempotent instead.
+  output : 'util-config-private.hh',
-  '-include', 'nix/config-util.hh',
-  language : 'cpp',
 )
 
 subdir('nix-meson-build-support/common')
 
-sources = files(
+sources = [config_priv_h] + files(
   'archive.cc',
   'args.cc',
   'canon-path.cc',

src/libutil/unix/file-descriptor.cc

@@ -7,6 +7,9 @@
 #include <unistd.h>
 #include <poll.h>
 
+#include "util-config-private.hh"
+#include "util-unix-config-private.hh"
+
 namespace nix {
 
 namespace {

src/libutil/unix/file-system.cc

@@ -1,10 +1,72 @@
+#include <cerrno>
+#include <climits>
+#include <cstdio>
+#include <cstdlib>
+
+#include <fcntl.h>
+#include <sys/types.h>
+#include <sys/time.h>
+#include <unistd.h>
+
 #include "nix/file-system.hh"
 
+#include "util-unix-config-private.hh"
+
 namespace nix {
 
+namespace fs {
+using namespace std::filesystem;
+}
+
 Descriptor openDirectory(const std::filesystem::path & path)
 {
     return open(path.c_str(), O_RDONLY | O_DIRECTORY);
 }
 
+void setWriteTime(const fs::path & path, time_t accessedTime, time_t modificationTime, std::optional<bool> optIsSymlink)
+{
+    // Would be nice to use std::filesystem unconditionally, but
+    // doesn't support access time just modification time.
+    //
+    // System clock vs File clock issues also make that annoying.
+#if HAVE_UTIMENSAT && HAVE_DECL_AT_SYMLINK_NOFOLLOW
+    struct timespec times[2] = {
+        {
+            .tv_sec = accessedTime,
+            .tv_nsec = 0,
+        },
+        {
+            .tv_sec = modificationTime,
+            .tv_nsec = 0,
+        },
+    };
+    if (utimensat(AT_FDCWD, path.c_str(), times, AT_SYMLINK_NOFOLLOW) == -1)
+        throw SysError("changing modification time of %s (using `utimensat`)", path);
+#else
+    struct timeval times[2] = {
+        {
+            .tv_sec = accessedTime,
+            .tv_usec = 0,
+        },
+        {
+            .tv_sec = modificationTime,
+            .tv_usec = 0,
+        },
+    };
+#  if HAVE_LUTIMES
+    if (lutimes(path.c_str(), times) == -1)
+        throw SysError("changing modification time of %s", path);
+#  else
+    bool isSymlink = optIsSymlink ? *optIsSymlink : fs::is_symlink(path);
+
+    if (!isSymlink) {
+        if (utimes(path.c_str(), times) == -1)
+            throw SysError("changing modification time of %s (not a symlink)", path);
+    } else {
+        throw Error("Cannot change modification time of symlink %s", path);
+    }
+#  endif
+#endif
+}
+
 }

src/libutil/unix/meson.build

@@ -1,3 +1,53 @@
+include_dirs += include_directories('.')
+
+configdata_unix = configuration_data()
+
+configdata_unix.set(
+  'HAVE_DECL_AT_SYMLINK_NOFOLLOW',
+  cxx.has_header_symbol('fcntl.h', 'AT_SYMLINK_NOFOLLOW').to_int(),
+  description : 'Optionally used for changing the files and symlinks.'
+)
+
+# Check for each of these functions, and create a define like `#define
+# HAVE_CLOSE_RANGE 1`.
+check_funcs_unix = [
+  [
+    'close_range',
+    'For closing many file descriptors after forking.',
+  ],
+  [
+    'lutimes',
+    'Optionally used for changing the mtime of symlinks.',
+  ],
+  [
+    'pipe2',
+    'Optionally used for creating pipes on Unix.',
+  ],
+  [
+    'strsignal',
+    'Optionally used to get more information about processes failing due to a signal on Unix.',
+  ],
+  [
+    'sysconf',
+    'Optionally used to try to close more file descriptors (e.g. before forking) on Unix.',
+  ],
+  [
+    'utimensat',
+    'Optionally used for changing the mtime of files and symlinks.',
+  ],
+]
+foreach funcspec : check_funcs_unix
+  define_name = 'HAVE_' + funcspec[0].underscorify().to_upper()
+  define_value = cxx.has_function(funcspec[0]).to_int()
+  configdata_unix.set(define_name, define_value, description: funcspec[1])
+endforeach
+
+config_unix_priv_h = configure_file(
+  configuration : configdata_unix,
+  output : 'util-unix-config-private.hh',
+)
+sources += config_unix_priv_h
+
 sources += files(
   'environment-variables.cc',
   'file-descriptor.cc',

src/libutil/unix/processes.cc

@@ -28,6 +28,9 @@
 # include <sys/mman.h>
 #endif
 
+#include "util-config-private.hh"
+#include "util-unix-config-private.hh"
+
 
 namespace nix {
 

src/libutil/windows/file-system.cc

@@ -3,6 +3,21 @@
 #ifdef _WIN32
 namespace nix {
 
+namespace fs {
+using namespace std::filesystem;
+}
+
+void setWriteTime(const fs::path & path, time_t accessedTime, time_t modificationTime, std::optional<bool> optIsSymlink)
+{
+    // FIXME use `fs::last_write_time`.
+    //
+    // Would be nice to use std::filesystem unconditionally, but
+    // doesn't support access time just modification time.
+    //
+    // System clock vs File clock issues also make that annoying.
+    warn("Changing file times is not yet implemented on Windows, path is %s", path);
+}
+
 Descriptor openDirectory(const std::filesystem::path & path)
 {
     return CreateFileW(

src/nix/main.cc

@@ -23,6 +23,7 @@
 
 #include "self-exe.hh"
 #include "crash-handler.hh"
+#include "cli-config-private.hh"
 
 #include <sys/types.h>
 #include <regex>

src/nix/meson.build

@@ -46,25 +46,15 @@ if not fs.is_absolute(bindir)
 endif
 configdata.set_quoted('NIX_BIN_DIR', bindir)
 
-config_h = configure_file(
+config_priv_h = configure_file(
   configuration : configdata,
-  output : 'config-nix-cli.hh',
+  output : 'cli-config-private.hh',
-)
-
-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.
-  '-include', 'nix/config-util.hh',
-  '-include', 'nix/config-store.hh',
-  '-include', 'nix/config-expr.hh',
-  '-include', 'config-nix-cli.hh',
-  language : 'cpp',
 )
 
 subdir('nix-meson-build-support/common')
 subdir('nix-meson-build-support/generate-header')
 
-nix_sources = [config_h] + files(
+nix_sources = [config_priv_h] + files(
   'add-to-store.cc',
   'app.cc',
   'self-exe.cc',

src/nix/self-exe.cc

@@ -1,7 +1,9 @@
 #include "nix/current-process.hh"
 #include "nix/file-system.hh"
 #include "nix/globals.hh"
+
 #include "self-exe.hh"
+#include "cli-config-private.hh"
 
 namespace nix {
 

src/perl/lib/Nix/Store.xs

@@ -1,6 +1,3 @@
-#include "nix/config-util.hh"
-#include "nix/config-store.hh"
-
 #include "EXTERN.h"
 #include "perl.h"
 #include "XSUB.h"

tests/functional/plugins/meson.build

@@ -1,13 +1,6 @@
 libplugintest = shared_module(
   'plugintest',
   'plugintest.cc',
-  cpp_args : [
-    # TODO(Qyriad): Yes this is how the autoconf+Make system did it.
-    # It would be nice for our headers to be idempotent instead.
-    '-include', 'nix/config-util.hh',
-    '-include', 'nix/config-store.hh',
-    '-include', 'nix/config-expr.hh',
-  ],
   dependencies : [
     dependency('nix-expr'),
   ],

tests/functional/test-libstoreconsumer/meson.build

@@ -1,12 +1,6 @@
 libstoreconsumer_tester = executable(
   'test-libstoreconsumer',
   'main.cc',
-  cpp_args : [
-    # TODO(Qyriad): Yes this is how the autoconf+Make system did it.
-    # It would be nice for our headers to be idempotent instead.
-    '-include', 'nix/config-util.hh',
-    '-include', 'nix/config-store.hh',
-  ],
   dependencies : [
     dependency('nix-store'),
   ],