top-level: Make overridePackages extend rather than replace existing overrides

doc/functions.xml

@@ -52,6 +52,20 @@ in ...</programlisting>
     It's equivalent to <varname>pkgs</varname> in the above example.
   </para>
 
+  <para>
+    Note that in previous versions of nixpkgs, this method replaced any changes from <link 
+    linkend="sec-modify-via-packageOverrides">config.packageOverrides</link>,
+    along with that from previous calls if this function was called repeatedly.
+    Now those previous changes will be preserved so this function can be "chained" meaningfully.
+    To recover the old behavior, make sure <varname>config.packageOverrides<varname> is unset,
+    and call this only once off a "freshly" imported nixpkgs:
+
+    <programlisting>let
+  pkgs = import &lt;nixpkgs&gt; { config: {}; };
+  newpkgs = pkgs.overridePackages ...;
+in ...</programlisting>
+  </para>
+
 </section>
 
 <section xml:id="sec-pkg-override">

lib/trivial.nix

@@ -69,9 +69,13 @@ rec {
   #
   #     nix-repl> obj
   #     { __unfix__ = «lambda»; bar = "bar"; extend = «lambda»; foo = "foo + "; foobar = "foo + bar"; }
-  makeExtensible = rattrs:
+  makeExtensible = makeExtensibleWithCustomName "extend";
+
+  # Same as `makeExtensible` but the name of the extending attribute is
+  # customized.
+  makeExtensibleWithCustomName = extenderName: rattrs:
     fix' rattrs // {
-      extend = f: makeExtensible (extends f rattrs);
+      ${extenderName} = f: makeExtensibleWithCustomName extenderName (extends f rattrs);
    };
 
   # Flip the order of the arguments of a binary function.

pkgs/top-level/all-packages.nix

@@ -6,7 +6,6 @@
  * Hint: ### starts category names.
  */
 { system, bootStdenv, noSysDirs, config, crossSystem, platform, lib
-, pkgsWithOverrides
 , ... }:
 self: pkgs:
 
@@ -35,19 +34,6 @@ in
 
   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: pkgsWithOverrides f;
-
   # Override system. This is useful to build i686 packages on x86_64-linux.
   forceSystem = system: kernel: (import ../..) {
     inherit system;

pkgs/top-level/default.nix

@@ -61,20 +61,6 @@ let
     inherit system bootStdenv noSysDirs config crossSystem platform lib;
   };
 
-  # Allow packages to be overridden globally via the `packageOverrides'
-  # configuration option, which must be a function that takes `pkgs'
-  # as an argument and returns a set of new or overridden packages.
-  # The `packageOverrides' function is called with the *original*
-  # (un-overridden) set of packages, allowing packageOverrides
-  # attributes to refer to the original attributes (e.g. "foo =
-  # ... pkgs.foo ...").
-  pkgs = pkgsWithOverrides (self: config.packageOverrides or (super: {}));
-
-  # Return the complete set of packages, after applying the overrides
-  # returned by the `overrider' function (see above).  Warning: this
-  # function is very expensive!
-  pkgsWithOverrides = overrider:
-    let
   stdenvAdapters = self: super:
     let res = import ../stdenv/adapters.nix self; in res // {
       stdenvAdapters = res;
@@ -87,9 +73,8 @@ let
 
   stdenvDefault = self: super: (import ./stdenv.nix topLevelArguments) {} pkgs;
 
-      allPackagesArgs = topLevelArguments // { inherit pkgsWithOverrides; };
   allPackages = self: super:
-        let res = import ./all-packages.nix allPackagesArgs res self;
+    let res = import ./all-packages.nix topLevelArguments res self;
     in res;
 
   aliases = self: super: import ./aliases.nix super;
@@ -105,17 +90,41 @@ let
     lib.optionalAttrs (crossSystem == null && super.stdenv ? overrides)
       (super.stdenv.overrides super);
 
-      customOverrides = self: super:
-        lib.optionalAttrs (bootStdenv == null) (overrider self super);
-    in
-      lib.fix' (
-        lib.extends customOverrides (
-          lib.extends stdenvOverrides (
-            lib.extends aliases (
-              lib.extends allPackages (
-                lib.extends stdenvDefault (
-                  lib.extends trivialBuilders (
-                    lib.extends stdenvAdapters (
-                      self: {}))))))));
-in
-  pkgs
+  # Allow packages to be overridden globally via the `packageOverrides'
+  # configuration option, which must be a function that takes `pkgs'
+  # as an argument and returns a set of new or overridden packages.
+  # The `packageOverrides' function is called with the *original*
+  # (un-overridden) set of packages, allowing packageOverrides
+  # attributes to refer to the original attributes (e.g. "foo =
+  # ... pkgs.foo ...").
+  configOverrides = self: super:
+    lib.optionalAttrs (bootStdenv == null)
+      ((config.packageOverrides or (super: {})) super);
+
+  # The complete chain of package set builders, applied from bottom to top
+  toFix = lib.fold lib.extends (self: {}) [
+    configOverrides
+    stdenvOverrides
+    aliases
+    allPackages
+    stdenvDefault
+    trivialBuilders
+    stdenvAdapters
+  ];
+
+  # Use `overridePackages` to 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.
+
+  # Return the complete set of packages. Warning: this function is very
+  # expensive!
+  pkgs = lib.makeExtensibleWithCustomName "overridePackages" toFix;
+in pkgs