cuda-modules: fix and clean up multiplex builder package selection logic

pkgs/development/cuda-modules/generic-builders/multiplex.nix

@@ -30,16 +30,7 @@
   shimsFn ? (throw "shimsFn must be provided"),
 }:
 let
-  inherit (lib)
-    attrsets
-    lists
-    modules
-    strings
-    ;
-
-  inherit (stdenv) hostPlatform;
-
-  evaluatedModules = modules.evalModules {
+  evaluatedModules = lib.modules.evalModules {
     modules = [
       ../modules
       releasesModule
@@ -50,49 +41,55 @@ let
   # - Releases: ../modules/${pname}/releases/releases.nix
   # - Package: ../modules/${pname}/releases/package.nix
 
-  # FIXME: do this at the module system level
-  propagatePlatforms = lib.mapAttrs (
-    redistArch: packages: map (p: { inherit redistArch; } // p) packages
-  );
+  # redistArch :: String
+  # Value is `"unsupported"` if the platform is not supported.
+  redistArch = flags.getRedistArch stdenv.hostPlatform.system;
 
-  # All releases across all platforms
+  # Check whether a package supports our CUDA version.
+  # satisfiesCudaVersion :: Package -> Bool
+  satisfiesCudaVersion =
+    package:
+    lib.versionAtLeast cudaMajorMinorVersion package.minCudaVersion
+    && lib.versionAtLeast package.maxCudaVersion cudaMajorMinorVersion;
+
+  # Releases for our platform and CUDA version.
   # See ../modules/${pname}/releases/releases.nix
-  releaseSets = propagatePlatforms evaluatedModules.config.${pname}.releases;
+  # allPackages :: List Package
+  allPackages = lib.filter satisfiesCudaVersion (
+    evaluatedModules.config.${pname}.releases.${redistArch} or [ ]
+  );
 
   # Compute versioned attribute name to be used in this package set
   # Patch version changes should not break the build, so we only use major and minor
   # computeName :: Package -> String
-  computeName = { version, ... }: mkVersionedPackageName pname version;
-
-  # Check whether a package supports our CUDA version and platform.
-  # isSupported :: Package -> Bool
-  isSupported =
-    package:
-    redistArch == package.redistArch
-    && strings.versionAtLeast cudaMajorMinorVersion package.minCudaVersion
-    && strings.versionAtLeast package.maxCudaVersion cudaMajorMinorVersion;
+  computeName = package: mkVersionedPackageName pname package.version;
 
-  # Get all of the packages for our given platform.
-  # redistArch :: String
-  # Value is `"unsupported"` if the platform is not supported.
-  redistArch = flags.getRedistArch hostPlatform.system;
-
-  preferable =
-    p1: p2: (isSupported p2 -> isSupported p1) && (strings.versionOlder p2.version p1.version);
-
-  # All the supported packages we can build for our platform.
-  # perSystemReleases :: List Package
-  allReleases = lib.pipe releaseSets [
-    (lib.attrValues)
-    (lists.flatten)
-    (lib.groupBy (p: lib.versions.majorMinor p.version))
-    (lib.mapAttrs (_: builtins.sort preferable))
-    (lib.mapAttrs (_: lib.take 1))
-    (lib.attrValues)
-    (lib.concatMap lib.trivial.id)
-  ];
-
-  newest = builtins.head (builtins.sort preferable allReleases);
+  # The newest package for each major-minor version, with newest first.
+  # newestPackages :: List Package
+  newestPackages =
+    let
+      newestForEachMajorMinorVersion = lib.foldl' (
+        newestPackages: package:
+        let
+          majorMinorVersion = lib.versions.majorMinor package.version;
+          existingPackage = newestPackages.${majorMinorVersion} or null;
+        in
+        newestPackages
+        // {
+          ${majorMinorVersion} =
+            # Only keep the existing package if it is newer than the one we are considering.
+            if existingPackage != null && lib.versionOlder package.version existingPackage.version then
+              existingPackage
+            else
+              package;
+        }
+      ) { } allPackages;
+    in
+    # Sort the packages by version so the newest is first.
+    # NOTE: builtins.sort requires a strict weak ordering, so we must use versionOlder rather than versionAtLeast.
+    lib.sort (p1: p2: lib.versionOlder p2.version p1.version) (
+      lib.attrValues newestForEachMajorMinorVersion
+    );
 
   extension =
     final: _:
@@ -102,25 +99,24 @@ let
       buildPackage =
         package:
         let
-          shims = final.callPackage shimsFn {
-            inherit package;
-            inherit (package) redistArch;
-          };
+          shims = final.callPackage shimsFn { inherit package redistArch; };
           name = computeName package;
           drv = final.callPackage ./manifest.nix {
             inherit pname redistName;
             inherit (shims) redistribRelease featureRelease;
           };
         in
-        attrsets.nameValuePair name drv;
+        lib.nameValuePair name drv;
 
       # versionedDerivations :: AttrSet Derivation
-      versionedDerivations = builtins.listToAttrs (lists.map buildPackage allReleases);
+      versionedDerivations = builtins.listToAttrs (lib.map buildPackage newestPackages);
 
       defaultDerivation = {
-        ${pname} = (buildPackage newest).value;
+        ${pname} = (buildPackage (lib.head newestPackages)).value;
       };
     in
-    versionedDerivations // defaultDerivation;
+    # NOTE: Must condition on the length of newestPackages to avoid non-total function lib.head aborting if
+    # newestPackages is empty.
+    lib.optionalAttrs (lib.length newestPackages > 0) (versionedDerivations // defaultDerivation);
 in
 extension