From c59bac08bd6fe608eac0083a68ec96819a32662d Mon Sep 17 00:00:00 2001
From: Timothy Gallion <timbama@gmail.com>
Date: Sun, 30 Mar 2025 01:52:11 -0400
Subject: [PATCH] python3: Fixing excessive memory usage for overriden python

An attempt to address #338231. The current release of nixpkgs takes over
20GB of memory to evaluate python when the interpreter is overriden.
This adds memoization to the spliced python packages to avoid
evaluating all the commbinations of overrides.
---
 .../interpreters/python/cpython/default.nix   | 35 ++++++++++++++-----
 1 file changed, 27 insertions(+), 8 deletions(-)

diff --git a/pkgs/development/interpreters/python/cpython/default.nix b/pkgs/development/interpreters/python/cpython/default.nix
index 977dd577fc90..56c4fe09fdce 100644
--- a/pkgs/development/interpreters/python/cpython/default.nix
+++ b/pkgs/development/interpreters/python/cpython/default.nix
@@ -55,6 +55,7 @@
   pkgsBuildTarget,
   pkgsHostHost,
   pkgsTargetTarget,
+  __splices ? { },
 
   # build customization
   sourceVersion,
@@ -145,10 +146,28 @@ let
       # When we override the interpreter we also need to override the spliced versions of the interpreter
       # bluez is excluded manually to break an infinite recursion.
       inputs' = lib.filterAttrs (n: v: n != "bluez" && n != "passthruFun" && !lib.isDerivation v) inputs;
+      # Memoization of the splices to avoid re-evaluating this function for all combinations of splices e.g.
+      # python3.pythonOnBuildForHost.pythonOnBuildForTarget == python3.pythonOnBuildForTarget by consuming
+      # __splices as an arg and using the cache if populated.
+      splices = {
+        pythonOnBuildForBuild = override pkgsBuildBuild.${pythonAttr};
+        pythonOnBuildForHost = override pkgsBuildHost.${pythonAttr};
+        pythonOnBuildForTarget = override pkgsBuildTarget.${pythonAttr};
+        pythonOnHostForHost = override pkgsHostHost.${pythonAttr};
+        pythonOnTargetForTarget = lib.optionalAttrs (lib.hasAttr pythonAttr pkgsTargetTarget) (
+          override pkgsTargetTarget.${pythonAttr}
+        );
+      } // __splices;
       override =
         attr:
         let
-          python = attr.override (inputs' // { self = python; });
+          python = attr.override (
+            inputs'
+            // {
+              self = python;
+              __splices = splices;
+            }
+          );
         in
         python;
     in
@@ -160,13 +179,13 @@ let
       pythonVersion = with sourceVersion; "${major}.${minor}";
       sitePackages = "lib/${libPrefix}/site-packages";
       inherit hasDistutilsCxxPatch pythonAttr;
-      pythonOnBuildForBuild = override pkgsBuildBuild.${pythonAttr};
-      pythonOnBuildForHost = override pkgsBuildHost.${pythonAttr};
-      pythonOnBuildForTarget = override pkgsBuildTarget.${pythonAttr};
-      pythonOnHostForHost = override pkgsHostHost.${pythonAttr};
-      pythonOnTargetForTarget = lib.optionalAttrs (lib.hasAttr pythonAttr pkgsTargetTarget) (
-        override pkgsTargetTarget.${pythonAttr}
-      );
+      inherit (splices)
+        pythonOnBuildForBuild
+        pythonOnBuildForHost
+        pythonOnBuildForTarget
+        pythonOnHostForHost
+        pythonOnTargetForTarget
+        ;
     };
 
   version = with sourceVersion; "${major}.${minor}.${patch}${suffix}";