glibcLocales: re-enable parallel build

This was originally turned off in #245360 because the approach was
non-reproducible, i.e. all N concurrent jobs were racing to update
`locale-archive`.

glibc's localedef allows to separate the build from the install, i.e.
`localedef --no-archive` allows to only perform the build of the locale
which can be parallelized again. After that, a single process adds all
newly built locales into the archive.

This isn't supported by the Makefiles in upstream glibc yet[1], hence
the additional patches.

For me, this reduces the build-time of `pkgs.glibcLocales` from ~7min to
slightly under 1min.

[1] https://sourceware.org/pipermail/libc-alpha/2025-October/170803.html

pkgs/development/libraries/glibc/0001-localedata-allow-reproducible-parallel-install-of-lo.patch

@@ -0,0 +1,50 @@
+From 38e9742ed794febbc8caa4177df14d097607a2d9 Mon Sep 17 00:00:00 2001
+From: Maximilian Bosch <maximilian@mbosch.me>
+Date: Sun, 5 Oct 2025 12:45:25 +0200
+Subject: [PATCH 1/2] localedata: allow reproducible & parallel install of
+ locale-archive
+
+The problem with the current approach is that N processes are compiling
+a locale and updating the locale-archive at the same time. Hence,
+there's a race which process gets to update the archive first and thus
+the build is not reproducible[1].
+
+This patch changes that by adding the `--no-archive` flag to the
+`$(LOCALEDEF)` invocation in `build-one-locale` to parallelize the build
+process in a race-free manner. A single `localedef --add-to-archive`
+then adds all previously built locales to the archive.
+
+[1] https://github.com/NixOS/nixpkgs/issues/245360
+---
+ localedata/Makefile | 6 ++++++
+ 1 file changed, 6 insertions(+)
+
+diff --git a/localedata/Makefile b/localedata/Makefile
+index 01b32348d4..13f7d3a2c4 100644
+--- a/localedata/Makefile
++++ b/localedata/Makefile
+@@ -538,8 +538,13 @@ LOCALEDEF=I18NPATH=. GCONV_PATH=$(common-objpfx)iconvdata LC_ALL=C \
+ $(rtld-prefix) $(common-objpfx)locale/localedef
+ install-locales: install-locale-archive
+ 
++define install-all-locales
++  $(LOCALEDEF) $$flags --add-to-archive `find $(inst_complocaledir) -maxdepth 1 -mindepth 1 -type d | sort`
++endef
++
+ # Create and install the locale-archive file.
+ install-locale-archive: $(INSTALL-SUPPORTED-LOCALE-ARCHIVE)
++	$(install-all-locales)
+ # Create and install the locales individually (no archive).
+ install-locale-files: $(INSTALL-SUPPORTED-LOCALE-FILES)
+ 
+@@ -565,6 +570,7 @@ define build-one-locale
+ 	$(LOCALEDEF) $$flags --alias-file=../intl/locale.alias \
+ 		     -i locales/$$input -f charmaps/$$charset \
+ 		     $(addprefix --prefix=,$(install_root)) $$locale \
++		     --no-archive \
+ 	&& echo ' done';
+ endef
+ 
+-- 
+2.50.1
+

pkgs/development/libraries/glibc/0002-Makeconfig-make-inst_complocaledir-overridable.patch

@@ -0,0 +1,42 @@
+From 887cb6906afa2b2d8abf35ccfc33ce7b8dc902f8 Mon Sep 17 00:00:00 2001
+From: Maximilian Bosch <maximilian@mbosch.me>
+Date: Sun, 5 Oct 2025 12:48:43 +0200
+Subject: [PATCH 2/2] Makeconfig: make inst_complocaledir overridable
+
+On NixOS, we build the locales in a separate package from glibc itself.
+Also, packages aren't installed into a common location such as `/usr`,
+but each package has its own unique prefix below `/nix/store`.
+
+For us, the current state is a problem because `localedef` is built
+in a different package and expects DESTDIR to be the unique
+package-prefix of `glibc` itself. Our current workaround is to
+invoke `localedef` with `--prefix=$TMPDIR` and copy the contents
+to the correct prefix. This was doable without patching when
+`localedef` was doing build & locale-archive creation at the
+same time.
+
+With the previous patch, the Makefile uses `$(inst_complocaledir)` to
+tell localedef where the previously built locales are. In NixOS, we now
+need to override `$(inst_complocaledir)` to point to
+`$TMPDIR/$(glibc.out)`. Hence, this patch makes the variable overridable
+from env.
+---
+ Makeconfig | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+diff --git a/Makeconfig b/Makeconfig
+index 9eda4fa528..43d88325bc 100644
+--- a/Makeconfig
++++ b/Makeconfig
+@@ -197,7 +197,7 @@ inst_zonedir = $(install_root)$(zonedir)
+ ifndef complocaledir
+ complocaledir = $(libdir)/locale
+ endif
+-inst_complocaledir = $(install_root)$(complocaledir)
++inst_complocaledir ?= $(install_root)$(complocaledir)
+ 
+ # Where to install the message catalog data files (which are
+ # machine-independent).
+-- 
+2.50.1
+

pkgs/development/libraries/glibc/common.nix

@@ -111,6 +111,10 @@ stdenv.mkDerivation (
         & https://github.com/NixOS/nixpkgs/pull/188492#issuecomment-1233802991
       */
       ./reenable_DT_HASH.patch
+
+      # enable parallel & reproducible build of glibcLocales
+      ./0001-localedata-allow-reproducible-parallel-install-of-lo.patch
+      ./0002-Makeconfig-make-inst_complocaledir-overridable.patch
     ]
     /*
       NVCC does not support ARM intrinsics. Since <math.h> is pulled in by almost

pkgs/development/libraries/glibc/locales.nix

@@ -61,7 +61,8 @@
           # $TMPDIR/nix/store/...-glibc-.../lib/locale/locale-archive.
           LOCALEDEF_FLAGS+=" --prefix=$TMPDIR"
 
-          mkdir -p $TMPDIR/"${buildPackages.glibc.out}/lib/locale"
+          export inst_complocaledir="$TMPDIR/"${buildPackages.glibc.out}/lib/locale
+          mkdir -p "$inst_complocaledir"
 
           echo 'C.UTF-8/UTF-8 \' >> ../glibc-2*/localedata/SUPPORTED
 
@@ -88,12 +89,6 @@
           echo SUPPORTED-LOCALES='${toString locales}' > ../glibc-2*/localedata/SUPPORTED
         '';
 
-      # Current `nixpkgs` way of building locales is not compatible with
-      # parallel install. `locale-archive` is updated in parallel with
-      # multiple `localedef` processes and causes non-deterministic result:
-      #   https://github.com/NixOS/nixpkgs/issues/245360
-      enableParallelBuilding = false;
-
       makeFlags = (previousAttrs.makeFlags or [ ]) ++ [
         "localedata/install-locales"
         "localedir=${placeholder "out"}/lib/locale"