Update the component builder

Author: ElvishJerricco

nix/default.nix

@@ -16,31 +16,49 @@
 
   haskellLib = import ./lib.nix { inherit lib haskellLib; };
 
-  plan =
-    let p = planFunc hackageConfigs;
-    in p // {
-      packages = lib.mapAttrs
-        (_: pkg: self:
-          lib.recursiveUpdate (pkg.revision self) { flags = pkg.flags or {}; }
-        ) p.packages;
-    };
   hackageConfigs =
+    lib.flip lib.mapAttrs hackage
+      (pname: lib.mapAttrs
+        (vnum: version: version // {
+          revisions =
+            let
+              rev2Config = rev: {
+                inherit (version) sha256;
+                inherit (rev) outPath;
+                revision = rev.revNum;
+                revisionSha256 = rev.sha256;
+              };
+              f = rev: acc: acc // {
+                # If there's a collision (e.g. a revision was
+                # reverted), pick the one with the smaller
+                # revNum. They're identical, but if the smaller one is
+                # r0 then we don't have to download a cabal file.
+                ${rev.sha256} = if lib.hasAttr rev.sha256 acc && acc.${rev.sha256}.revNum < rev.revNum
+                  then acc.${rev.sha256}
+                  else rev;
+              };
+              contentAddressedRevs = lib.foldr f {} (builtins.attrValues version.revisions);
+            in lib.mapAttrs (_: rev2Config) (version.revisions // contentAddressedRevs);
+        }));
+  plan =
     let
       args = {
         inherit hsPkgs compiler system pkgconfPkgs;
         pkgs = adjustedPkgs;
       };
-      handleVer = vdata:
-        let
-          revs = builtins.removeAttrs vdata ["sha256"];
-          rev2HashedConfig = name: rev: { name = rev.cabalSha256; value = revConfigs.${name}; };
-          revConfigs = lib.mapAttrs (_: rev2config vdata.sha256) revs;
-        in revConfigs // lib.mapAttrs' rev2HashedConfig revs;
-      rev2config = sha256: rev: self: import rev (args // { inherit (self) flags; }) // {
-        inherit sha256;
-        inherit (rev) cabalFile;
-      };
-    in lib.mapAttrs (_: lib.mapAttrs (_: handleVer)) hackage;
+      p = planFunc hackageConfigs;
+    in p // {
+      packages = lib.mapAttrs
+        (pname: { revision, flags ? {} }: self:
+          # Set the flags with the rhs of the recursiveUpdate, but
+          # pass the final choice in flags using open recursion.
+          lib.recursiveUpdate (import revision (args // { inherit (self) flags; })) {
+            inherit flags;
+            inherit (revision) sha256 revision revisionSha256;
+          }
+        )
+        p.packages;
+    };
 
   new-builder = weakCallPackage pkgs ./new-builder.nix {
     inherit haskellLib ghc weakCallPackage;

nix/new-builder.nix

@@ -7,10 +7,12 @@
 , name ? "${package.identifier.name}-${package.identifier.version}"
 , sha256 ? null
 , src ? fetchurl { url = "mirror://hackage/${name}.tar.gz"; inherit sha256; }
-, cabalFile ? null
+, revision ? null
+, revisionSha256 ? null
 }@config:
 
 let
+  cabalFile = null;
   defaultSetupSrc = builtins.toFile "Setup.hs" ''
     import Distribution.Simple
     main = defaultMain