nixpkgs-bootstrap: make the updater atomic, thereby allowing update of nixpkgs-bootstrap.master in parallel with anything else

pkgs/by-name/nixpkgs-bootstrap/mkNixpkgs.nix

@@ -40,13 +40,14 @@
   applyPatches ? null,
   fetchpatch2 ? null,
   fetchzip ? null,
+  nixpkgs-bootstrap-updater ? null,
   stdenv ? null,
-  unstableGitUpdater ? null,
 #VVV config
   localSystem ? if stdenv != null then stdenv.buildPlatform.system else builtins.currentSystem,  #< not available in pure mode
   system ? if stdenv != null then stdenv.hostPlatform.system else localSystem,
 }:
 let
+  optionalAttrs = cond: attrs: if cond then attrs else {};
   # nixpkgs' update-source-version (updateScript) calculates the new hash for a `src` by specifying this hardcoded bogus hash and then attempting to realize it.
   sentinelSha256 = "sha256-AzH1rZFqEH8sovZZfJykvsEmCedEZWigQFHWHl6/PdE=";
   fetchBootstrap = { url, pname, version, ... }@args: {
@@ -128,14 +129,16 @@ let
         in
           "${position.file}:${toString position.line}";
 
-        # while we could *technically* use `nixpkgs.<...>` updateScript
-        # that can force maaany rebuilds on staging/staging-next.
         # updateScript = nix-update-script {
         #   extraArgs = [ "--version=branch=${branch}" ];
         # };
-        updateScript = unstableGitUpdater {
+        # updateScript = unstableGitUpdater {
-          # else the update script tries to walk 10000's of commits to find a tag
+        #   # else the update script tries to walk 10000's of commits to find a tag
-          hardcodeZeroVersion = true;
+        #   hardcodeZeroVersion = true;
+        #   inherit branch;
+        # };
+      } // optionalAttrs (nixpkgs-bootstrap-updater != null) {
+        updateScript = nixpkgs-bootstrap-updater.makeUpdateScript {
           inherit branch;
         };
       };
@@ -146,11 +149,11 @@ let
         allowUnfree = true;  # NIXPKGS_ALLOW_UNFREE=1
         allowBroken = true;  # NIXPKGS_ALLOW_BROKEN=1
       };
-    } // (if (system != localSystem) then {
+    } // optionalAttrs (system != localSystem) {
       # XXX(2023/12/11): cache.nixos.org uses `system = ...` instead of `hostPlatform.system`, and that choice impacts the closure of every package.
       # so avoid specifying hostPlatform.system on non-cross builds, so i can use upstream caches.
       crossSystem = system;
-    } else {});
+    };
     nixpkgs = import patchedSrc nixpkgsArgs;
   in
     patchedSrc

pkgs/by-name/nixpkgs-bootstrap/nixpkgs-bootstrap-updater/default.nix

@@ -0,0 +1,15 @@
+{
+  lib,
+  static-nix-shell,
+}:
+let
+  self = static-nix-shell.mkBash {
+    pname = "nixpkgs-bootstrap-updater";
+    srcRoot = ./.;
+    pkgs = [ "common-updater-scripts" "curl" "jq" "nix-prefetch-github" ];
+    passthru.makeUpdateScript = { branch }: [
+      (lib.getExe self)
+      branch
+    ];
+  };
+in self

pkgs/by-name/nixpkgs-bootstrap/nixpkgs-bootstrap-updater/nixpkgs-bootstrap-updater

@@ -0,0 +1,28 @@
+#!/usr/bin/env nix-shell
+#!nix-shell -i bash -p bash -p common-updater-scripts -p curl -p jq -p nix-prefetch-github
+
+set -eu
+
+branch=$1
+
+if [ -z "$branch" ]; then
+  echo "USAGE: nixpkgsUpdater <branch>"
+  exit 1
+fi
+
+jsonGetField() {
+  local json=$1
+  local field=$2
+  echo "$json" | jq --exit-status --raw-output ".$field"
+}
+
+set -x
+prefetchData=$(nix-prefetch-github --json --meta --rev "$branch" NixOS nixpkgs)
+
+rev=$(jsonGetField "$prefetchData" src.rev)
+nixHash=$(jsonGetField "$prefetchData" src.hash)
+commitDate=$(jsonGetField "$prefetchData" meta.commitDate)
+
+update-source-version "sane.nixpkgs-bootstrap.$branch" "unstable-$commitDate" "$nixHash" \
+  --rev="$rev" \
+  --print-changes

pkgs/by-name/nixpkgs-bootstrap/package.nix

@@ -7,6 +7,7 @@
 lib.recurseIntoAttrs (lib.makeScope newScope (self: with self; {
   mkNixpkgs = callPackage ./mkNixpkgs.nix { };
   master = callPackage ./master.nix { };
+  nixpkgs-bootstrap-updater = callPackage ./nixpkgs-bootstrap-updater { };
   staging = callPackage ./staging.nix { };
   staging-next = callPackage ./staging-next.nix { };
 }))