Merge pull request #8490 from lethalman/overridepkgs

Add pkgs.overridePackages

pkgs/top-level/all-packages.nix

@@ -100,6 +100,8 @@ let
   # ... pkgs.foo ...").
   pkgs = applyGlobalOverrides (config.packageOverrides or (pkgs: {}));
 
+  mkOverrides = pkgsOrig: overrides: overrides //
+        (lib.optionalAttrs (pkgsOrig.stdenv ? overrides && crossSystem == null) (pkgsOrig.stdenv.overrides pkgsOrig));
 
   # Return the complete set of packages, after applying the overrides
   # returned by the `overrider' function (see above).  Warning: this
@@ -110,8 +112,7 @@ let
       # in the case of cross-building, or otherwise the basic
       # overrided packages will not be built with the crossStdenv
       # adapter.
-      overrides = overrider pkgsOrig //
-        (lib.optionalAttrs (pkgsOrig.stdenv ? overrides && crossSystem == null) (pkgsOrig.stdenv.overrides pkgsOrig));
+      overrides = mkOverrides pkgsOrig (overrider pkgsOrig);
 
       # The un-overriden packages, passed to `overrider'.
       pkgsOrig = pkgsFun pkgs {};
@@ -142,6 +143,22 @@ let
 
   newScope = extra: lib.callPackageWith (defaultScope // extra);
 
+  # Easily override this package set.
+  # Warning: this function is very expensive and must not be used
+  # from within the nixpkgs repository.
+  #
+  # Example:
+  #  pkgs.overridePackages (self: super: {
+  #    foo = super.foo.override { ... };
+  #  }
+  #
+  # The result is `pkgs' where all the derivations depending on `foo'
+  # will use the new version.
+  overridePackages = f:
+    let
+      newpkgs = pkgsFun newpkgs overrides;
+      overrides = mkOverrides pkgs (f newpkgs pkgs);
+    in newpkgs;
 
   # Override system. This is useful to build i686 packages on x86_64-linux.
   forceSystem = system: kernel: (import ./all-packages.nix) {