build: optionally build and install with meson

This commit adds several meson.build, which successfully build and
install Lix executables, libraries, and headers. Meson does not yet
build docs, Perl bindings, or run tests, which will be added in
following commits. As such, this commit does not remove the existing
build system, or make it the default, and also as such, this commit has
several FIXMEs and TODOs as notes for what should be done before the
existing autoconf + make buildsystem can be removed and Meson made the
default. This commit does not modify any source files.

A Meson-enabled build is also added as a Hydra job, and to
`nix flake check`.

Change-Id: I667c8685b13b7bab91e281053f807a11616ae3d4

15 files changed, 1201 insertions, 12 deletions

diff --git a/flake.nix b/flake.nix
index cd5727415..5ab36408b 100644
--- a/flake.nix
+++ b/flake.nix
@@ -204,6 +204,16 @@
         # Binary package for various platforms.
         build = forAllSystems (system: self.packages.${system}.nix);
 
+        # FIXME(Qyriad): remove this when the migration to Meson has been completed.
+        mesonBuild = forAllSystems (system: self.packages.${system}.nix.override {
+          buildWithMeson = true;
+        });
+        mesonBuildClang = forAllSystems (system:
+            nixpkgsFor.${system}.stdenvs.clangStdenvPackages.nix.override {
+              buildWithMeson = true;
+            }
+        );
+
         # Perl bindings for various platforms.
         perlBindings = forAllSystems (system: nixpkgsFor.${system}.native.nix.perl-bindings);
 
@@ -262,6 +272,9 @@
       };
 
       checks = forAllSystems (system: {
+        # FIXME(Qyriad): remove this when the migration to Meson has been completed.
+        mesonBuild = self.hydraJobs.mesonBuild.${system};
+        mesonBuildClang = self.hydraJobs.mesonBuildClang.${system};
         binaryTarball = self.hydraJobs.binaryTarball.${system};
         perlBindings = self.hydraJobs.perlBindings.${system};
         nixpkgsLibTests = self.hydraJobs.tests.nixpkgsLibTests.${system};
@@ -328,7 +341,12 @@
                   # for some reason that seems accidental and was changed in
                   # NixOS 24.05-pre, clang-tools is pinned to LLVM 14 when
                   # default LLVM is newer.
-                  (pkgs.buildPackages.clang-tools.override { inherit (pkgs.buildPackages) llvmPackages; });
+                  (pkgs.buildPackages.clang-tools.override { inherit (pkgs.buildPackages) llvmPackages; })
+                ++ [
+                  # FIXME(Qyriad): remove once the migration to Meson is complete.
+                  pkgs.buildPackages.meson
+                  pkgs.buildPackages.ninja
+                ];
 
               src = null;
 
diff --git a/meson.build b/meson.build
new file mode 100644
index 000000000..73c13582b
--- /dev/null
+++ b/meson.build
@@ -0,0 +1,287 @@
+#
+# OUTLINE:
+#
+# The top-level meson.build file (this file) handles general logic for build options,
+# generation of config.h (which is put in the build directory, not the source root
+# like the previous, autoconf-based build system did), the mechanism for header
+# generation, and the few global C++ compiler arguments that are added to all targets in Lix.
+#
+# src/meson.build coordinates each of Lix's subcomponents (the lib dirs in ./src),
+# which each have their own meson.build. Lix's components depend on each other,
+# so each of `src/lib{util,store,fetchers,expr,main,cmd}/meson.build` rely on variables
+# set in earlier `meson.build` files. Each of these also defines the install targets for
+# their headers.
+#
+# src/meson.build also collects the miscellaneous source files that are in further subdirectories
+# that become part of the final Nix command (things like `src/nix-build/*.cc`).
+#
+# Finally, src/nix/meson.build defines the Nix command itself, relying on all prior meson files.
+
+project('lix', 'cpp',
+  version : run_command('bash', '-c', 'echo -n $(cat ./.version)$VERSION_SUFFIX', check : true).stdout().strip(),
+  default_options : [
+    'cpp_std=c++2a',
+    # TODO(Qyriad): increase the warning level
+    'warning_level=1',
+    'debug=true',
+    'optimization=2',
+  ],
+)
+
+fs = import('fs')
+
+prefix = get_option('prefix')
+# For each of these paths, assume that it is relative to the prefix unless
+# it is already an absolute path (which is the default for store-dir, state-dir, and log-dir).
+path_opts = [
+  # Meson built-ins.
+  'datadir',
+  'sysconfdir',
+  'bindir',
+  'mandir',
+  'libdir',
+  'includedir',
+  # Homecooked Lix directories.
+  'store-dir',
+  'state-dir',
+  'log-dir',
+]
+foreach optname : path_opts
+  varname = optname.replace('-', '_')
+  path = get_option(optname)
+  if fs.is_absolute(path)
+    set_variable(varname, path)
+  else
+    set_variable(varname, prefix / path)
+  endif
+endforeach
+
+cxx = meson.get_compiler('cpp')
+
+host_system = host_machine.cpu_family() + '-' + host_machine.system()
+message('canonical Nix system name:', host_system)
+
+is_linux = host_machine.system() == 'linux'
+is_x64 = host_machine.cpu_family() == 'x86_64'
+
+deps = [ ]
+configdata = { }
+
+#
+# Dependencies
+#
+
+boehm = dependency('bdw-gc', required : get_option('gc'))
+if boehm.found()
+  deps += boehm
+endif
+configdata += {
+  'HAVE_BOEHMGC': boehm.found().to_int(),
+}
+
+boost = dependency('boost', required : true, modules : ['context', 'coroutine', 'container'])
+deps += boost
+
+# cpuid only makes sense on x86_64
+cpuid_required = is_x64 ? get_option('cpuid') : false
+cpuid = dependency('libcpuid', 'cpuid', required : cpuid_required)
+configdata += {
+  'HAVE_LIBCPUID': cpuid.found().to_int(),
+}
+deps += cpuid
+
+# seccomp only makes sense on Linux
+seccomp_required = is_linux ? get_option('seccomp-sandboxing') : false
+seccomp = dependency('libseccomp', 'seccomp', required : seccomp_required)
+configdata += {
+  'HAVE_SECCOMP': seccomp.found().to_int(),
+}
+
+libarchive = dependency('libarchive', required : true)
+deps += libarchive
+
+brotli = [
+  dependency('libbrotlicommon', required : true),
+  dependency('libbrotlidec', required : true),
+  dependency('libbrotlienc', required : true),
+]
+deps += brotli
+
+openssl = dependency('libcrypto', 'openssl', required : true)
+deps += openssl
+
+aws_sdk = dependency('aws-cpp-sdk-core', required : false)
+if aws_sdk.found()
+  # The AWS pkg-config adds -std=c++11.
+  # https://github.com/aws/aws-sdk-cpp/issues/2673
+  aws_sdk = aws_sdk.partial_dependency(
+    compile_args : false,
+    includes : true,
+    link_args : true,
+    links : true,
+    sources : true,
+  )
+  deps += aws_sdk
+  s = aws_sdk.version().split('.')
+  configdata += {
+    'AWS_VERSION_MAJOR': s[0].to_int(),
+    'AWS_VERSION_MINOR': s[1].to_int(),
+    'AWS_VERSION_PATCH': s[2].to_int(),
+  }
+  aws_sdk_transfer = dependency('aws-cpp-sdk-transfer', required : true).partial_dependency(
+    compile_args : false,
+    includes : true,
+    link_args : true,
+    links : true,
+    sources : true,
+  )
+endif
+
+aws_s3 = dependency('aws-cpp-sdk-s3', required : false)
+if aws_s3.found()
+  # The AWS pkg-config adds -std=c++11.
+  # https://github.com/aws/aws-sdk-cpp/issues/2673
+  aws_s3 = aws_s3.partial_dependency(
+    compile_args : false,
+    includes : true,
+    link_args : true,
+    links : true,
+    sources : true,
+  )
+  deps += aws_s3
+endif
+
+configdata += {
+  'ENABLE_S3': aws_s3.found().to_int(),
+}
+
+sqlite = dependency('sqlite3', 'sqlite', version : '>=3.6.19', required : true)
+deps += sqlite
+
+sodium = dependency('libsodium', 'sodium', required : true)
+deps += sodium
+
+curl = dependency('libcurl', 'curl', required : true)
+deps += curl
+
+editline = dependency('libeditline', 'editline', version : '>=1.14', required : true)
+deps += editline
+
+lowdown = dependency('lowdown', version : '>=0.9.0', required : true)
+deps += lowdown
+
+rapidcheck = dependency('rapidcheck', required : false)
+deps += rapidcheck
+
+gtest = dependency('gtest', required : false)
+deps += gtest
+
+#
+# Build-time tools
+#
+bash = find_program('bash')
+
+# Used to workaround https://github.com/mesonbuild/meson/issues/2320 in src/nix/meson.build.
+installcmd = find_program('install')
+
+sandbox_shell = get_option('sandbox-shell')
+# Consider it required if we're on Linux and the user explicitly specified a non-default value.
+sandbox_shell_required = sandbox_shell != 'busybox' and host_machine.system() == 'linux'
+# NOTE(Qyriad): package.nix puts busybox in buildInputs for Linux.
+# Most builds should not require setting this.
+busybox = find_program(sandbox_shell, required : sandbox_shell_required, native : false)
+if not busybox.found() and host_machine.system() == 'linux' and sandbox_shell_required
+  warning('busybox not found and other sandbox shell was specified')
+  warning('a sandbox shell is recommended on Linux -- configure with -Dsandbox-shell=/path/to/shell to set')
+endif
+# FIXME(Qyriad): the autoconf system checks that busybox has the "standalone" feature, indicating
+# that busybox sh won't run busybox applets as builtins (which would break our sandbox).
+
+lsof = find_program('lsof')
+bison = find_program('bison')
+flex = find_program('flex')
+
+# This is how Nix does generated headers...
+# FIXME(Qyriad): do we really need to use the shell for this?
+gen_header = generator(
+  bash,
+  arguments : [
+    '-c',
+    'echo \'R"__NIX_STR(\' | cat - @INPUT@ && echo \')__NIX_STR"\'',
+  ],
+  capture : true,
+  output : '@PLAINNAME@.gen.hh',
+)
+
+#
+# Configuration
+#
+
+run_command('ln', '-s',
+  meson.project_build_root() / '__nothing_link_target',
+  meson.project_build_root() / '__nothing_symlink',
+  check : true,
+)
+can_link_symlink = run_command('ln',
+  meson.project_build_root() / '__nothing_symlink',
+  meson.project_build_root() / '__nothing_hardlink',
+  check : false,
+).returncode() == 0
+run_command('rm', '-f',
+  meson.project_build_root() / '__nothing_symlink',
+  meson.project_build_root() / '__nothing_hardlink',
+  check : true,
+)
+summary('can hardlink to symlink', can_link_symlink, bool_yn : true)
+configdata += { 'CAN_LINK_SYMLINK': can_link_symlink.to_int() }
+
+
+# Check for each of these functions, and create a define like `#define HAVE_LCHOWN 1`.
+check_funcs = [
+  'lchown',
+  'lutimes',
+  'pipe2',
+  'posix_fallocate',
+  'statvfs',
+  'strsignal',
+  'sysconf',
+]
+foreach funcspec : check_funcs
+  define_name = 'HAVE_' + funcspec.underscorify().to_upper()
+  define_value = cxx.has_function(funcspec).to_int()
+  configdata += {
+    define_name: define_value,
+  }
+endforeach
+
+config_h = configure_file(
+  configuration : {
+    'PACKAGE_NAME': '"' + meson.project_name() + '"',
+    'PACKAGE_VERSION': '"' + meson.project_version() + '"',
+    'PACKAGE_TARNAME': '"' + meson.project_name() + '"',
+    'PACKAGE_STRING': '"' + meson.project_name() + ' ' + meson.project_version() + '"',
+    'HAVE_STRUCT_DIRENT_D_TYPE': 1, # FIXME: actually check this for solaris
+    'SYSTEM': '"' + host_system + '"',
+  } + configdata,
+  output : 'config.h',
+)
+
+install_headers(config_h, subdir : 'nix')
+
+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', 'config.h',
+  '-Wno-deprecated-declarations',
+  '-Wimplicit-fallthrough',
+  '-Werror=switch',
+  '-Werror=switch-enum',
+  language : 'cpp',
+)
+
+add_project_link_arguments('-pthread', language : 'cpp')
+if cxx.get_linker_id() in ['ld.bfd', 'ld.gold']
+  add_project_link_arguments('-Wl,--no-copy-dt-needed-entries', language : 'cpp')
+endif
+
+subdir('src')
diff --git a/meson.options b/meson.options
new file mode 100644
index 000000000..50b1d8553
--- /dev/null
+++ b/meson.options
@@ -0,0 +1,32 @@
+# vim: filetype=meson
+option('gc', type : 'feature',
+  description : 'enable garbage collection in the Nix expression evaluator (requires Boehm GC)',
+)
+# TODO(Qyriad): is this feature maintained?
+option('embedded-sandbox-shell', type : 'feature',
+  description : 'include the sandbox shell in the Nix binary',
+)
+
+option('cpuid', type : 'feature',
+  description : 'determine microarchitecture levels with libcpuid (only relevant on x86_64)',
+)
+
+option('seccomp-sandboxing', type : 'feature',
+  description : 'build support for seccomp sandboxing (recommended unless your arch doesn\'t support libseccomp, only relevant on Linux)',
+)
+
+option('sandbox-shell', type : 'string', value : 'busybox',
+  description : 'path to a statically-linked shell to use as /bin/sh in sandboxes (usually busybox)',
+)
+
+option('store-dir', type : 'string', value : '/nix/store',
+  description : 'path of the Nix store',
+)
+
+option('state-dir', type : 'string', value : '/nix/var/nix',
+  description : 'path to store state in for Nix',
+)
+
+option('log-dir', type : 'string', value : '/nix/var/log',
+  description : 'path to store logs in for Nix',
+)
diff --git a/meson/cleanup-install.bash b/meson/cleanup-install.bash
new file mode 100755
index 000000000..3e229cbc6
--- /dev/null
+++ b/meson/cleanup-install.bash
@@ -0,0 +1,50 @@
+#!/usr/bin/env bash
+# Meson will call this with an absolute path to Bash.
+# The shebang is just for convenience.
+
+# The parser and lexer tab are generated via custom Meson targets in src/libexpr/meson.build,
+# but Meson doesn't support marking only part of a target for install. The generation creates
+# both headers (parser-tab.hh, lexer-tab.hh) and source files (parser-tab.cc, lexer-tab.cc),
+# and we definitely want the former installed, but not the latter. This script is added to
+# Meson's install steps to correct this, as the logic for it is just complex enough to
+# warrant separate and careful handling, because both Meson's configured include directory
+# may or may not be an absolute path, and DESTDIR may or may not be set at all, but can't be
+# manipulated in Meson logic.
+
+set -euo pipefail
+
+echo "cleanup-install: removing Meson-placed C++ sources from dest includedir"
+
+if [[ "${1/--help/}" != "$1" ]]; then
+	echo "cleanup-install: this script should only be called from the Meson build system"
+	exit 1
+fi
+
+# Ensure the includedir was passed as the first argument
+# (set -u will make this fail otherwise).
+includedir="$1"
+# And then ensure that first argument is a directory that exists.
+if ! [[ -d "$1" ]]; then
+	echo "cleanup-install: this script should only be called from the Meson build system"
+	echo "argv[1] (${1@Q}) is not a directory"
+	exit 2
+fi
+
+# If DESTDIR environment variable is set, prepend it to the include dir.
+# Unfortunately, we cannot do this on the Meson side. We do have an environment variable
+# `MESON_INSTALL_DESTDIR_PREFIX`, but that will not refer to the include directory if
+# includedir has been set separately, which Lix's split-output derivation does.
+# We also cannot simply do an inline bash conditional like "${DESTDIR:=}" or similar,
+# because we need to specifically *join* DESTDIR and includedir with a slash, and *not*
+# have a slash if DESTDIR isn't set at all, since $includedir could be a relative directory.
+# Finally, DESTDIR is only available to us as an environment variable in these install scripts,
+# not in Meson logic.
+# Therefore, our best option is to have Meson pass this script the configured includedir,
+# and perform this dance with it and $DESTDIR.
+if [[ -n "${DESTDIR:-}" ]]; then
+	includedir="$DESTDIR/$includedir"
+fi
+
+# Intentionally not using -f.
+# If these files don't exist then our assumptions have been violated and we should fail.
+rm -v "$includedir/nix/parser-tab.cc" "$includedir/nix/lexer-tab.cc"
diff --git a/package.nix b/package.nix
index 8d33759b7..4b4c35731 100644
--- a/package.nix
+++ b/package.nix
@@ -24,10 +24,13 @@
   libcpuid,
   libseccomp,
   libsodium,
+  lsof,
   lowdown,
   mdbook,
   mdbook-linkcheck,
   mercurial,
+  meson,
+  ninja,
   openssl,
   pkg-config,
   rapidcheck,
@@ -47,6 +50,10 @@
   # Avoid setting things that would interfere with a functioning devShell
   forDevShell ? false,
 
+  # FIXME(Qyriad): build Lix using Meson instead of autoconf and make.
+  # This flag will be removed when the migration to Meson is complete.
+  buildWithMeson ? false,
+
   # Not a real argument, just the only way to approximate let-binding some
   # stuff for argument defaults.
   __forDefaults ? {
@@ -86,12 +93,16 @@
     ./README.md
   ];
 
-  topLevelBuildFiles = fileset.unions [
+  topLevelBuildFiles = fileset.unions ([
     ./local.mk
     ./Makefile
     ./Makefile.config.in
     ./mk
-  ];
+  ] ++ lib.optionals buildWithMeson [
+    ./meson.build
+    ./meson.options
+    ./meson/cleanup-install.bash
+  ]);
 
  functionalTestFiles = fileset.unions [
     ./tests/functional
@@ -126,6 +137,11 @@ in stdenv.mkDerivation (finalAttrs: {
 
   dontBuild = false;
 
+  # FIXME(Qyriad): see if this is still needed once the migration to Meson is completed.
+  mesonFlags = lib.optionals (buildWithMeson && stdenv.hostPlatform.isLinux) [
+    "-Dsandbox-shell=${lib.getBin busybox-sandbox-shell}/bin/busybox"
+  ];
+
   nativeBuildInputs = [
     bison
     flex
@@ -134,17 +150,21 @@ in stdenv.mkDerivation (finalAttrs: {
     mdbook
     mdbook-linkcheck
     autoconf-archive
-    autoreconfHook
+  ] ++ lib.optional (!buildWithMeson) autoreconfHook ++ [
     pkg-config
 
     # Tests
     git
     mercurial
     jq
+    lsof
   ] ++ lib.optional stdenv.hostPlatform.isLinux util-linuxMinimal
     ++ lib.optional (!officialRelease && buildUnreleasedNotes) changelog-d
     ++ lib.optional internalApiDocs doxygen
-  ;
+    ++ lib.optionals buildWithMeson [
+      meson
+      ninja
+  ];
 
   buildInputs = [
     curl
@@ -159,7 +179,7 @@ in stdenv.mkDerivation (finalAttrs: {
     lowdown
     libsodium
   ]
-    ++ lib.optionals stdenv.isLinux [ libseccomp ]
+    ++ lib.optionals stdenv.hostPlatform.isLinux [ libseccomp busybox-sandbox-shell ]
     ++ lib.optional stdenv.hostPlatform.isx86_64 libcpuid
     # There have been issues building these dependencies
     ++ lib.optional (stdenv.hostPlatform == stdenv.buildPlatform) aws-sdk-cpp-nix
@@ -177,6 +197,13 @@ in stdenv.mkDerivation (finalAttrs: {
     boost
   ];
 
+  # Needed for Meson to find Boost.
+  # https://github.com/NixOS/nixpkgs/issues/86131.
+  env = lib.optionalAttrs (buildWithMeson || forDevShell) {
+    BOOST_INCLUDEDIR = "${lib.getDev boost}/include";
+    BOOST_LIBRARYDIR = "${lib.getLib boost}/lib";
+  };
+
   preConfigure = lib.optionalString (!finalAttrs.dontBuild && !stdenv.hostPlatform.isStatic) ''
     # Copy libboost_context so we don't get all of Boost in our closure.
     # https://github.com/NixOS/nixpkgs/issues/45462
@@ -214,6 +241,8 @@ in stdenv.mkDerivation (finalAttrs: {
     ++ [ (lib.enableFeature internalApiDocs "internal-api-docs") ]
     ++ lib.optional (!forDevShell) "--sysconfdir=/etc";
 
+  mesonBuildType = lib.optional (buildWithMeson || forDevShell) "debugoptimized";
+
   installTargets = lib.optional internalApiDocs "internal-api-html";
 
   enableParallelBuilding = true;
@@ -231,10 +260,12 @@ in stdenv.mkDerivation (finalAttrs: {
     mkdir -p $out/nix-support
     echo "file binary-dist $out/bin/nix" >> $out/nix-support/hydra-build-products
   '' + lib.optionalString stdenv.isDarwin ''
-    install_name_tool \
-      -change ${boost}/lib/libboost_context.dylib \
-      $out/lib/libboost_context.dylib \
-      $out/lib/libnixutil.dylib
+    for lib in libnixutil.dylib libnixexpr.dylib; do
+      install_name_tool \
+        -change "${lib.getLib boost}/lib/libboost_context.dylib" \
+        "$out/lib/libboost_context.dylib" \
+        "$out/lib/$lib"
+    done
   '' + lib.optionalString internalApiDocs ''
     mkdir -p $out/nix-support
     echo "doc internal-api-docs $out/share/doc/nix/internal-api/html" >> "$out/nix-support/hydra-build-products"
diff --git a/src/libcmd/meson.build b/src/libcmd/meson.build
new file mode 100644
index 000000000..e033bb1cd
--- /dev/null
+++ b/src/libcmd/meson.build
@@ -0,0 +1,58 @@
+libcmd_sources = files(
+  'built-path.cc',
+  'command-installable-value.cc',
+  'command.cc',
+  'common-eval-args.cc',
+  'editor-for.cc',
+  'installable-attr-path.cc',
+  'installable-derived-path.cc',
+  'installable-flake.cc',
+  'installable-value.cc',
+  'installables.cc',
+  'legacy.cc',
+  'markdown.cc',
+  'repl.cc',
+  'repl-interacter.cc',
+)
+
+libcmd_headers = files(
+  'built-path.hh',
+  'command-installable-value.hh',
+  'command.hh',
+  'common-eval-args.hh',
+  'editor-for.hh',
+  'installable-attr-path.hh',
+  'installable-derived-path.hh',
+  'installable-flake.hh',
+  'installable-value.hh',
+  'installables.hh',
+  'legacy.hh',
+  'markdown.hh',
+  'repl-interacter.hh',
+  'repl.hh',
+)
+
+libcmd = library(
+  'nixcmd',
+  libcmd_sources,
+  dependencies : [
+    liblixutil,
+    liblixstore,
+    liblixexpr,
+    liblixfetchers,
+    liblixmain,
+    boehm,
+    editline,
+    lowdown,
+  ],
+  install : true,
+  # FIXME(Qyriad): is this right?
+  install_rpath : libdir,
+)
+
+install_headers(libcmd_headers, subdir : 'nix', preserve_path : true)
+
+liblixcmd = declare_dependency(
+  include_directories : '.',
+  link_with : libcmd,
+)
diff --git a/src/libexpr/meson.build b/src/libexpr/meson.build
new file mode 100644
index 000000000..baac9bb24
--- /dev/null
+++ b/src/libexpr/meson.build
@@ -0,0 +1,153 @@
+parser_tab = custom_target(
+  input : 'parser.y',
+  output : [
+    'parser-tab.cc',
+    'parser-tab.hh',
+  ],
+  command : [
+    'bison',
+    '-v',
+    '-o',
+    '@OUTPUT0@',
+    '@INPUT@',
+    '-d',
+  ],
+  # NOTE(Qyriad): Meson doesn't support installing only part of a custom target, so we add
+  # an install script below which removes parser-tab.cc.
+  install : true,
+  install_dir : includedir / 'nix',
+)
+
+lexer_tab = custom_target(
+  input : [
+    'lexer.l',
+    parser_tab,
+  ],
+  output : [
+    'lexer-tab.cc',
+    'lexer-tab.hh',
+  ],
+  command : [
+    'flex',
+    '--outfile',
+    '@OUTPUT0@',
+    '--header-file=' + '@OUTPUT1@',
+    '@INPUT0@',
+  ],
+  # NOTE(Qyriad): Meson doesn't support installing only part of a custom target, so we add
+  # an install script below which removes lexer-tab.cc.
+  install : true,
+  install_dir : includedir / 'nix',
+)
+
+# TODO(Qyriad): When the parser and lexer are rewritten this should be removed.
+# NOTE(Qyriad): We do this this way instead of an inline bash or rm command
+# due to subtleties in Meson. Check the comments in cleanup-install.bash for details.
+meson.add_install_script(
+  bash,
+  meson.project_source_root() / 'meson/cleanup-install.bash',
+  '@0@'.format(includedir),
+)
+
+imported_drv_to_derivation_gen = gen_header.process('imported-drv-to-derivation.nix')
+fetchurl_gen = gen_header.process('fetchurl.nix')
+derivation_gen = gen_header.process('primops/derivation.nix', preserve_path_from : meson.current_source_dir())
+call_flake_gen = gen_header.process('flake/call-flake.nix')
+
+libexpr_sources = files(
+  'attr-path.cc',
+  'attr-set.cc',
+  'eval-cache.cc',
+  'eval-error.cc',
+  'eval-settings.cc',
+  'eval.cc',
+  'function-trace.cc',
+  'get-drvs.cc',
+  'json-to-value.cc',
+  'nixexpr.cc',
+  'paths.cc',
+  'primops.cc',
+  'print-ambiguous.cc',
+  'print.cc',
+  'search-path.cc',
+  'value-to-json.cc',
+  'value-to-xml.cc',
+  'flake/config.cc',
+  'flake/flake.cc',
+  'flake/flakeref.cc',
+  'flake/lockfile.cc',
+  'primops/context.cc',
+  'primops/fetchClosure.cc',
+  'primops/fetchMercurial.cc',
+  'primops/fetchTree.cc',
+  'primops/fromTOML.cc',
+  'value/context.cc',
+)
+
+libexpr_headers = files(
+  'attr-path.hh',
+  'attr-set.hh',
+  'eval-cache.hh',
+  'eval-error.hh',
+  'eval-inline.hh',
+  'eval-settings.hh',
+  'eval.hh',
+  'flake/flake.hh',
+  'flake/flakeref.hh',
+  'flake/lockfile.hh',
+  'function-trace.hh',
+  'gc-small-vector.hh',
+  'get-drvs.hh',
+  'json-to-value.hh',
+  'nixexpr.hh',
+  'parser-state.hh',
+  'pos-idx.hh',
+  'pos-table.hh',
+  'primops.hh',
+  'print-ambiguous.hh',
+  'print-options.hh',
+  'print.hh',
+  'repl-exit-status.hh',
+  'search-path.hh',
+  'symbol-table.hh',
+  'value/context.hh',
+  'value-to-json.hh',
+  'value-to-xml.hh',
+  'value.hh',
+)
+
+libexpr = library(
+  'nixexpr',
+  libexpr_sources,
+  parser_tab,
+  lexer_tab,
+  imported_drv_to_derivation_gen,
+  fetchurl_gen,
+  derivation_gen,
+  call_flake_gen,
+  dependencies : [
+    liblixutil,
+    liblixstore,
+    liblixfetchers,
+    boehm,
+    boost,
+  ],
+  # for shared.hh
+  include_directories : [
+    '../libmain',
+  ],
+  install : true,
+  # FIXME(Qyriad): is this right?
+  install_rpath : libdir,
+)
+
+install_headers(
+  libexpr_headers,
+  subdir : 'nix',
+  preserve_path : true,
+)
+
+liblixexpr = declare_dependency(
+  include_directories : include_directories('.'),
+  link_with : libexpr,
+)
diff --git a/src/libfetchers/meson.build b/src/libfetchers/meson.build
new file mode 100644
index 000000000..5eacfd5c4
--- /dev/null
+++ b/src/libfetchers/meson.build
@@ -0,0 +1,42 @@
+libfetchers_sources = files(
+  'attrs.cc',
+  'cache.cc',
+  'fetch-settings.cc',
+  'fetch-to-store.cc',
+  'fetchers.cc',
+  'git.cc',
+  'github.cc',
+  'indirect.cc',
+  'mercurial.cc',
+  'path.cc',
+  'registry.cc',
+  'tarball.cc',
+)
+
+libfetchers_headers = files(
+  'attrs.hh',
+  'cache.hh',
+  'fetch-settings.hh',
+  'fetch-to-store.hh',
+  'fetchers.hh',
+  'registry.hh',
+)
+
+libfetchers = library(
+  'nixfetchers',
+  libfetchers_sources,
+  dependencies : [
+    liblixstore,
+    liblixutil,
+  ],
+  install : true,
+  # FIXME(Qyriad): is this right?
+  install_rpath : libdir,
+)
+
+install_headers(libfetchers_headers, subdir : 'nix', preserve_path : true)
+
+liblixfetchers = declare_dependency(
+  include_directories : include_directories('.'),
+  link_with : libfetchers,
+)
diff --git a/src/libmain/meson.build b/src/libmain/meson.build
new file mode 100644
index 000000000..54326c305
--- /dev/null
+++ b/src/libmain/meson.build
@@ -0,0 +1,33 @@
+libmain_sources = files(
+  'common-args.cc',
+  'loggers.cc',
+  'progress-bar.cc',
+  'shared.cc',
+  'stack.cc',
+)
+
+libmain_headers = files(
+  'common-args.hh',
+  'loggers.hh',
+  'progress-bar.hh',
+  'shared.hh',
+)
+
+libmain = library(
+  'nixmain',
+  libmain_sources,
+  dependencies : [
+    liblixutil,
+    liblixstore,
+  ],
+  install : true,
+  # FIXME(Qyriad): is this right?
+  install_rpath : libdir,
+)
+
+install_headers(libmain_headers, subdir : 'nix', preserve_path : true)
+
+liblixmain = declare_dependency(
+  include_directories : include_directories('.'),
+  link_with : libmain,
+)
diff --git a/src/libmain/shared.cc b/src/libmain/shared.cc
index 6d851d776..cd62b1398 100644
--- a/src/libmain/shared.cc
+++ b/src/libmain/shared.cc
@@ -114,10 +114,8 @@ static void sigHandler(int signo) { }
 void initNix()
 {
     /* Turn on buffering for cerr. */
-#if HAVE_PUBSETBUF
     static char buf[1024];
     std::cerr.rdbuf()->pubsetbuf(buf, sizeof(buf));
-#endif
 
     initLibStore();
 
diff --git a/src/libstore/meson.build b/src/libstore/meson.build
new file mode 100644
index 000000000..33e475f70
--- /dev/null
+++ b/src/libstore/meson.build
@@ -0,0 +1,188 @@
+schema_sql_gen = gen_header.process('schema.sql')
+ca_specific_schema_gen = gen_header.process('ca-specific-schema.sql')
+
+libstore_sources = files(
+  'binary-cache-store.cc',
+  'build-result.cc',
+  'common-protocol.cc',
+  'content-address.cc',
+  'crypto.cc',
+  'daemon.cc',
+  'derivations.cc',
+  'derived-path-map.cc',
+  'derived-path.cc',
+  'downstream-placeholder.cc',
+  'dummy-store.cc',
+  'export-import.cc',
+  'filetransfer.cc',
+  'gc.cc',
+  'globals.cc',
+  'http-binary-cache-store.cc',
+  'legacy-ssh-store.cc',
+  'local-binary-cache-store.cc',
+  'local-fs-store.cc',
+  'local-store.cc',
+  'lock.cc',
+  'log-store.cc',
+  'machines.cc',
+  'make-content-addressed.cc',
+  'misc.cc',
+  'names.cc',
+  'nar-accessor.cc',
+  'nar-info-disk-cache.cc',
+  'nar-info.cc',
+  'optimise-store.cc',
+  'outputs-spec.cc',
+  'parsed-derivations.cc',
+  'path-info.cc',
+  'path-references.cc',
+  'path-with-outputs.cc',
+  'path.cc',
+  'pathlocks.cc',
+  'profiles.cc',
+  'realisation.cc',
+  'remote-fs-accessor.cc',
+  'remote-store.cc',
+  's3-binary-cache-store.cc',
+  'serve-protocol.cc',
+  'sqlite.cc',
+  'ssh-store.cc',
+  'ssh.cc',
+  'store-api.cc',
+  'uds-remote-store.cc',
+  'worker-protocol.cc',
+  'build/derivation-goal.cc',
+  'build/drv-output-substitution-goal.cc',
+  'build/entry-points.cc',
+  'build/goal.cc',
+  'build/hook-instance.cc',
+  'build/local-derivation-goal.cc',
+  'build/personality.cc',
+  'build/substitution-goal.cc',
+  'build/worker.cc',
+  'builtins/buildenv.cc',
+  'builtins/fetchurl.cc',
+  'builtins/unpack-channel.cc',
+)
+
+
+libstore_headers = files(
+  'binary-cache-store.hh',
+  'build/derivation-goal.hh',
+  'build/drv-output-substitution-goal.hh',
+  'build/goal.hh',
+  'build/hook-instance.hh',
+  'build/local-derivation-goal.hh',
+  'build/personality.hh',
+  'build/substitution-goal.hh',
+  'build/worker.hh',
+  'build-result.hh',
+  'builtins/buildenv.hh',
+  'builtins.hh',
+  'common-protocol-impl.hh',
+  'common-protocol.hh',
+  'content-address.hh',
+  'crypto.hh',
+  'daemon.hh',
+  'derivations.hh',
+  'derived-path-map.hh',
+  'derived-path.hh',
+  'downstream-placeholder.hh',
+  'filetransfer.hh',
+  'fs-accessor.hh',
+  'gc-store.hh',
+  'globals.hh',
+  'indirect-root-store.hh',
+  'length-prefixed-protocol-helper.hh',
+  'local-fs-store.hh',
+  'local-store.hh',
+  'lock.hh',
+  'log-store.hh',
+  'machines.hh',
+  'make-content-addressed.hh',
+  'names.hh',
+  'nar-accessor.hh',
+  'nar-info-disk-cache.hh',
+  'nar-info.hh',
+  'outputs-spec.hh',
+  'parsed-derivations.hh',
+  'path-info.hh',
+  'path-references.hh',
+  'path-regex.hh',
+  'path-with-outputs.hh',
+  'path.hh',
+  'pathlocks.hh',
+  'profiles.hh',
+  'realisation.hh',
+  'remote-fs-accessor.hh',
+  'remote-store-connection.hh',
+  'remote-store.hh',
+  's3-binary-cache-store.hh',
+  's3.hh',
+  'serve-protocol-impl.hh',
+  'serve-protocol.hh',
+  'sqlite.hh',
+  'ssh-store-config.hh',
+  'ssh.hh',
+  'store-api.hh',
+  'store-cast.hh',
+  'uds-remote-store.hh',
+  'worker-protocol-impl.hh',
+  'worker-protocol.hh',
+)
+
+# These variables (aside from LSOF) are created pseudo-dynamically, near the beginning of
+# the top-level meson.build. Aside from prefix itself, each of these was
+# made into an absolute path by joining it with prefix, unless it was already
+# an absolute path (which is the default for store-dir, state-dir, and log-dir).
+cpp_str_defines = {
+  'LSOF': lsof.full_path(),
+  'NIX_PREFIX':    prefix,
+  'NIX_STORE_DIR': store_dir,
+  'NIX_DATA_DIR':  datadir,
+  'NIX_STATE_DIR': state_dir,
+  'NIX_LOG_DIR':   log_dir,
+  'NIX_CONF_DIR':  sysconfdir,
+  'NIX_BIN_DIR':   bindir,
+  'NIX_MAN_DIR':   mandir,
+}
+
+cpp_args = []
+
+foreach name, value : cpp_str_defines
+  cpp_args += [
+    '-D' + name + '=' + '"' + value + '"'
+  ]
+endforeach
+
+libstore = library(
+  'nixstore',
+  schema_sql_gen,
+  ca_specific_schema_gen,
+  libstore_sources,
+  dependencies : [
+    libarchive,
+    liblixutil, # Internal.
+    seccomp,
+    sqlite,
+    sodium,
+    seccomp,
+    curl,
+    openssl,
+    aws_sdk,
+    aws_s3,
+    aws_sdk_transfer,
+  ],
+  cpp_args : cpp_args,
+  install : true,
+  # FIXME(Qyriad): is this right?
+  install_rpath : libdir,
+)
+
+install_headers(libstore_headers, subdir : 'nix', preserve_path : true)
+
+# Used by libfetchers.
+liblixstore = declare_dependency(
+  include_directories : include_directories('.'),
+  link_with : libstore,
+)
diff --git a/src/libutil/meson.build b/src/libutil/meson.build
new file mode 100644
index 000000000..084d7ed11
--- /dev/null
+++ b/src/libutil/meson.build
@@ -0,0 +1,112 @@
+libutil_sources = files(
+  'archive.cc',
+  'args.cc',
+  'canon-path.cc',
+  'cgroup.cc',
+  'compression.cc',
+  'compute-levels.cc',
+  'config.cc',
+  'english.cc',
+  'error.cc',
+  'exit.cc',
+  'experimental-features.cc',
+  'filesystem.cc',
+  'git.cc',
+  'hash.cc',
+  'hilite.cc',
+  'json-utils.cc',
+  'logging.cc',
+  'namespaces.cc',
+  'position.cc',
+  'references.cc',
+  'serialise.cc',
+  'signals.cc',
+  'source-path.cc',
+  'suggestions.cc',
+  'tarfile.cc',
+  'thread-pool.cc',
+  'url.cc',
+  'util.cc',
+  'xml-writer.cc',
+)
+
+libutil_headers = files(
+  'abstract-setting-to-json.hh',
+  'ansicolor.hh',
+  'archive.hh',
+  'args.hh',
+  'box_ptr.hh',
+  'callback.hh',
+  'canon-path.hh',
+  'cgroup.hh',
+  'chunked-vector.hh',
+  'closure.hh',
+  'comparator.hh',
+  'compression.hh',
+  'compute-levels.hh',
+  'config-impl.hh',
+  'config.hh',
+  'english.hh',
+  'error.hh',
+  'exit.hh',
+  'experimental-features.hh',
+  'finally.hh',
+  'fmt.hh',
+  'git.hh',
+  'hash.hh',
+  'hilite.hh',
+  'input-accessor.hh',
+  'json-impls.hh',
+  'json-utils.hh',
+  'logging.hh',
+  'lru-cache.hh',
+  'monitor-fd.hh',
+  'namespaces.hh',
+  'pool.hh',
+  'position.hh',
+  'ref.hh',
+  'references.hh',
+  'regex-combinators.hh',
+  'repair-flag.hh',
+  'serialise.hh',
+  'signals.hh',
+  'source-path.hh',
+  'split.hh',
+  'suggestions.hh',
+  'sync.hh',
+  'tarfile.hh',
+  'thread-pool.hh',
+  'topo-sort.hh',
+  'types.hh',
+  'url-parts.hh',
+  'url.hh',
+  'util.hh',
+  'variant-wrapper.hh',
+  'xml-writer.hh',
+)
+
+libutil = library(
+  'nixutil',
+  libutil_sources,
+  dependencies : [
+    aws_sdk,
+    aws_s3,
+    boehm,
+    boost,
+    cpuid,
+    seccomp,
+    libarchive,
+    brotli,
+    openssl,
+  ],
+  implicit_include_directories : true,
+  install : true,
+)
+
+install_headers(libutil_headers, subdir : 'nix', preserve_path : true)
+
+# Used by libstore and libfetchers.
+liblixutil = declare_dependency(
+  include_directories : include_directories('.'),
+  link_with : libutil
+)
diff --git a/src/meson.build b/src/meson.build
new file mode 100644
index 000000000..f97b66252
--- /dev/null
+++ b/src/meson.build
@@ -0,0 +1,65 @@
+# Subcomponents: these link into artifacts themselves, and have interdependencies.
+
+subdir('libutil')
+# Load-bearing order. libstore depends on libutil.
+subdir('libstore')
+# libfetchers depends on libstore
+subdir('libfetchers')
+# libexpr depends on all of the above
+subdir('libexpr')
+# libmain depends on libutil and libstore
+subdir('libmain')
+# libcmd depends on everything
+subdir('libcmd')
+
+
+# The rest of the subdirectories aren't separate components,
+# just source files in another directory, so we process them here.
+
+build_remote_sources = files(
+  'build-remote/build-remote.cc',
+)
+nix_build_sources = files(
+  'nix-build/nix-build.cc',
+)
+nix_channel_sources = files(
+  'nix-channel/nix-channel.cc',
+)
+unpack_channel_gen = gen_header.process('nix-channel/unpack-channel.nix')
+nix_collect_garbage_sources = files(
+  'nix-collect-garbage/nix-collect-garbage.cc',
+)
+nix_copy_closure_sources = files(
+  'nix-copy-closure/nix-copy-closure.cc',
+)
+nix_env_buildenv_gen = gen_header.process('nix-env/buildenv.nix')
+nix_env_sources = files(
+  'nix-env/nix-env.cc',
+  'nix-env/user-env.cc',
+)
+nix_instantiate_sources = files(
+  'nix-instantiate/nix-instantiate.cc',
+)
+nix_store_sources = files(
+  'nix-store/dotgraph.cc',
+  'nix-store/graphml.cc',
+  'nix-store/nix-store.cc',
+)
+
+# Hurray for Meson list flattening!
+nix2_commands_sources = [
+  build_remote_sources,
+  nix_build_sources,
+  nix_channel_sources,
+  unpack_channel_gen,
+  nix_collect_garbage_sources,
+  nix_copy_closure_sources,
+  nix_env_buildenv_gen,
+  nix_env_sources,
+  nix_instantiate_sources,
+  nix_store_sources,
+]
+
+# Finally, the nix command itself, which all of the other commands are implmented in terms of
+# as a multicall binary.
+subdir('nix')
diff --git a/src/nix/meson.build b/src/nix/meson.build
new file mode 100644
index 000000000..2585ba3c3
--- /dev/null
+++ b/src/nix/meson.build
@@ -0,0 +1,119 @@
+generate_manpage_gen = gen_header.process(meson.project_source_root() / 'doc/manual/generate-manpage.nix')
+
+utils_gen = gen_header.process(meson.project_source_root() / 'doc/manual/utils.nix')
+
+get_env_gen = gen_header.process('get-env.sh')
+
+# src/nix/profile.cc includes src/nix/profile.md, which includes "doc/files/profiles.md.gen.hh".
+# Unfortunately, https://github.com/mesonbuild/meson/issues/2320.
+# "docs/files" isn't a directory hierarchy that already exists somewhere in this source tree,
+# and Meson refuses to create targets with specific directory paths.
+# So run_command() it is.
+# NOTE(Qyriad): This corresponds to the previous buildsystem's `src/nix/doc/files/%.md` rule,
+# which as far as I can tell was only used for this file.
+run_command(
+  installcmd,
+  '-D',
+  meson.project_source_root() / 'doc/manual/src/command-ref/files/profiles.md',
+  meson.current_build_dir() / 'doc/files/profiles.md',
+  check : true,
+)
+profiles_md_gen = gen_header.process(
+  meson.current_build_dir() / 'doc/files/profiles.md',
+  preserve_path_from : meson.current_build_dir(),
+)
+
+nix_sources = files(
+  'add-to-store.cc',
+  'app.cc',
+  'build.cc',
+  'bundle.cc',
+  'cat.cc',
+  'copy.cc',
+  'daemon.cc',
+  'derivation-add.cc',
+  'derivation-show.cc',
+  'derivation.cc',
+  'develop.cc',
+  'diff-closures.cc',
+  'doctor.cc',
+  'dump-path.cc',
+  'edit.cc',
+  'eval.cc',
+  'flake.cc',
+  'fmt.cc',
+  'hash.cc',
+  'log.cc',
+  'ls.cc',
+  'main.cc',
+  'make-content-addressed.cc',
+  'nar.cc',
+  'optimise-store.cc',
+  'path-from-hash-part.cc',
+  'path-info.cc',
+  'ping-store.cc',
+  'prefetch.cc',
+  'profile.cc',
+  'realisation.cc',
+  'registry.cc',
+  'repl.cc',
+  'run.cc',
+  'search.cc',
+  'show-config.cc',
+  'sigs.cc',
+  'store-copy-log.cc',
+  'store-delete.cc',
+  'store-gc.cc',
+  'store-repair.cc',
+  'store.cc',
+  'upgrade-nix.cc',
+  'verify.cc',
+  'why-depends.cc',
+)
+
+nix = executable(
+  'nix',
+  nix_sources,
+  generate_manpage_gen,
+  utils_gen,
+  get_env_gen,
+  profiles_md_gen,
+  nix2_commands_sources,
+  dependencies : [
+    liblixcmd,
+    liblixutil,
+    liblixstore,
+    liblixexpr,
+    liblixfetchers,
+    liblixmain,
+    boehm,
+  ],
+  install : true,
+  # FIXME(Qyriad): is this right?
+  install_rpath : libdir,
+)
+
+nix_symlinks = [
+  'nix-build',
+  'nix-channel',
+  'nix-collect-garbage',
+  'nix-copy-closure',
+  'nix-daemon',
+  'nix-env',
+  'nix-hash',
+  'nix-instantiate',
+  'nix-prefetch-url',
+  'nix-shell',
+  'nix-store',
+]
+
+foreach linkname : nix_symlinks
+  install_symlink(
+    linkname,
+    # TODO(Qyriad): should these continue to be relative symlinks?
+    pointing_to : 'nix',
+    install_dir : bindir,
+    # The 'runtime' tag is what executables default to, which we want to emulate here.
+    install_tag : 'runtime'
+  )
+endforeach
diff --git a/src/toml11/toml.hpp b/src/toml11/toml.hpp
index f34cfccca..f9a7430f2 100644
--- a/src/toml11/toml.hpp
+++ b/src/toml11/toml.hpp
@@ -22,6 +22,9 @@
  * THE SOFTWARE.
  */
 
+// TODO(Qyriad): let's get vendored toml11 out of here.
+#pragma GCC system_header
+
 #ifndef TOML_FOR_MODERN_CPP
 #define TOML_FOR_MODERN_CPP