feat(3p/nixpkgs): Plumb through externalArgs.nixpkgsBisectPath

This lets the import of the depot root accept an additional argument
called `externalArgs`, which can be used to pass additional arguments
into a depot package set.

This is used in //third_party/nixpkgs for replacing the source of the
nixos-unstable channel with a path. With this we can bisect the
nixpkgs used in third_party easily.

Change-Id: I4f65eb3d6b521ed9f437649b7b068f1e6ab8210f
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2925
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

default.nix

@@ -5,7 +5,7 @@
 # This makes packages accessible via the Nixery instance that is configured to
 # use this repository as its nixpkgs source.
 
-{ ... }@args:
+{ nixpkgsBisectPath ? null, ... }@args:
 
 with builtins;
 
@@ -24,6 +24,13 @@ let
     # Pass third_party as 'pkgs' (for compatibility with external
     # imports for certain subdirectories)
     pkgs = depot.third_party.nixpkgs;
+
+    # Pass arguments passed to the entire depot through, for packages
+    # that would like to add functionality based on this.
+    #
+    # Note that it is intended for exceptional circumstance, such as
+    # debugging by bisecting nixpkgs.
+    externalArgs = args;
   };
 
   readTree' = import ./nix/readTree {};

third_party/nixpkgs/default.nix

@@ -6,7 +6,7 @@
 # in //default.nix passes this attribute as the `pkgs` argument to all
 # readTree derivations.
 
-{ depot, ... }:
+{ depot, externalArgs, ... }:
 
 let
   # This provides the sources of nixpkgs. We track both
@@ -25,10 +25,16 @@ let
     sha256 = "073327ris0frqa3kpid3nsjr9w8yx2z83xpsc24w898mrs9r7d5v";
   };
 
-  nixpkgsSrc = fetchTarball {
+  # import the nixos-unstable package set, or optionally use the
+  # source (e.g. a path) specified by the `nixpkgsBisectPath`
+  # argument. This is intended for use-cases where the depot is
+  # bisected against nixpkgs to find the root cause of an issue in a
+  # channel bump.
+  nixpkgsSrc = externalArgs.nixpkgsBisectPath or (fetchTarball {
     url = "https://github.com/NixOS/nixpkgs/archive/${unstableHashes.commit}.tar.gz";
     sha256 = unstableHashes.sha256;
-  };
+  });
+
   stableNixpkgsSrc = fetchTarball {
     url = "https://github.com/NixOS/nixpkgs/archive/${stableHashes.commit}.tar.gz";
     sha256 = stableHashes.sha256;