Add support for patches

[fooker]

niv should allow to apply local patches for sources. This would avoid to maintain a tree for sources that exist only to track patches.

I can think of multiple ways to implement this:

• By providing a wrapper for each source derivation that allow to apply patches (something like sources.nixpkgs.patched [ ./path/to.patch ]
• By adding a config option listing the patches to be applied

I would prefer the first option and would be happy to work on an implementation if this is an acceptable approach.

[jskrzypek]

Is this using the applyPatches approach? I can imagine having also a patches.json for remote patches as well that specify patches for fetchurl, and follow a similar pattern to sources.json. niv could also add a niv patch add --source nixpkgs --pull XXX to point to a for a PR and find the correct patch-diff url & sha256 for the patch.

[jskrzypek]

For any followers, if you need to support patches, you can apply the patch below to your nix/sources.nix file, and add a nix/patches.json file that contains entries like:

{
    "nixpkgs-pull-346698":{
        "source": "nixpkgs",
        "owner": "NixOS",
        "repo": "nixpkgs",
        "pull": "346698",
        "type": "file",
        "builtin": true,
        "issue_url": "https://github.com/NixOS/nixpkgs/pull/346698",
        "sha256": "1g079rghwgccj3gs4dhi9qn7c1hkjfd46pl9yq2bskx15mdl1998",
        "title": "deskflow: init at 1.17.1",
        "url": "https://patch-diff.githubusercontent.com/NixOS/nixpkgs/pull/346698.patch",
        "url_template": "https://patch-diff.githubusercontent.com/<owner>/<repo>/pull/<pull>.patch"
    }
}

diff --git i/nix/sources.nix w/nix/sources.nix
--- i/nix/sources.nix
+++ w/nix/sources.nix
@@ -222,27 +222,66 @@ let
     else
       fetchurl attrs;
 
+  mkPatched = config: name: src: patches:
+    config.pkgs.applyPatches {
+      name = name + "-patched";
+      inherit patches src;
+    };
+
+  assertNoCollisions = specType: names: spec:
+    if builtins.any (n: builtins.hasAttr n spec) names
+    then
+      abort
+        "The values in ${specType}.json should not have attributes with names ${names}."
+    else
+      spec;
+
   # Create the final "sources" from the config
   mkSources = config:
-    mapAttrs
+    let
+      patchesBySource =
+        builtins.mapAttrs
+          (_: v: builtins.map
+            (name: {inherit name; value = config.patches.${name};})
+            v)
+          (builtins.groupBy
+            (n: config.patches.${n}.source)
+            (builtins.attrNames config.patches));
+    in mapAttrs
       (
         name: spec:
-          if builtins.hasAttr "outPath" spec
-          then
-            abort
-              "The values in sources.json should not have an 'outPath' attribute"
-          else
-            spec // { outPath = replace name (fetch config.pkgs name spec); }
+          (assertNoCollisions 
+            "sources"
+            [ "outPath" "patched" "patches" ]
+            spec) // rec {
+            outPath = replace name (fetch config.pkgs name spec);
+            patched = mkPatched config name outPath;
+          } // (
+              let
+                patchesForSource = patchesBySource."${name}" or [];
+              in optionalAttrs
+                (builtins.length patchesForSource != 0)
+                {
+                  patches = builtins.map
+                    ({name, value}: value // {outPath = replace name (fetch config.pkgs name value);})
+                    patchesForSource;
+                }
+          )
       )
       config.sources;
 
   # The "config" used by the fetchers
   mkConfig =
-    { sourcesFile ? if builtins.pathExists ./sources.json then ./sources.json else null
+    { patchesFile ? if builtins.pathExists ./patches.json then ./patches.json else null
+    , patches ? if patchesFile == null then { } else builtins.fromJSON (builtins.readFile patchesFile)
+    , sourcesFile ? if builtins.pathExists ./sources.json then ./sources.json else null
     , sources ? if sourcesFile == null then { } else builtins.fromJSON (builtins.readFile sourcesFile)
     , system ? builtins.currentSystem
     , pkgs ? mkPkgs sources system
-    }: rec {
+    }: {
+      # The patches, i.e. the attribute set of patch name to patch spec
+      inherit patches;
+
       # The sources, i.e. the attribute set of spec name to spec
       inherit sources;