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/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 {