Add an example of adding nixpkgs overlays and some work toward obtaining libchallenge-bypass-ristretto.

Also, go back to plain Obelisk develop.

.obelisk/impl/github.json

@@ -1,8 +1,8 @@
 {
   "owner": "obsidiansystems",
   "repo": "obelisk",
-  "branch": "cg/user-nixpkgs-overlays",
+  "branch": "develop",
   "private": false,
-  "rev": "c1c1e7e0aedefdf39497f4dda91cda76df1a05a9",
-  "sha256": "1xsp20j12sg6mg9wl640b709ksl3zs3bbkylm559gyvfbvll74p6"
+  "rev": "1c4b07ec67639356316b610142d3be8d302c07cc",
+  "sha256": "1j8zjdwv912s7gxzfi3bfy1kh840j2bvhqycxgi7v4pc0mi54fav"
 }

default.nix

@@ -3,41 +3,12 @@
 , obelisk ? import ./.obelisk/impl {
     inherit system;
     iosSdkVersion = "13.2";
-    # This argument (which is new, but was easy to add to Obelisk, thankfully), allows one to specify nixpkgs overlays.
-    # We'll use it to ensure we have the appropriate version of botan.
-    nixpkgsOverlays =
-      [
-        (self: super: rec {
-            botan2 =
-              (super.botan2.overrideAttrs (old: {
-                # Get rid of some patches that nixpkgs was applying to botan2 and which didn't apply to the branch.
-                patches = [];
-                # The --with-openssl flag didn't work for some reason, might need further figuring out.
-                configurePhase = builtins.replaceStrings [ "--with-openssl" ] [ " " ] old.configurePhase;
-                # Here, we use nix-thunk to get the source of the correct branch of the botan repo via a thunk.
-                # Nix thunks are essentially references to git repositories that can be unpacked to their
-                # source in-place when working on the project, or packed up into a few small files.
-                # After installing the nix-thunk command from https://github.com/obsidiansystems/nix-thunk
-                # you can run:
-                # nix-thunk unpack dep/botan
-                # from the top level of this repo to clone a copy of the botan git repo at the appropriate
-                # commit, and work on it from there. Similarly,
-                # nix-thunk pack dep/botan
-                # will pack it back up again, provided that the changes have been pushed somewhere.
-                # Note: there's a bug in the current version of Obelisk which occasionally gives it some trouble
-                # if certain repos are unpacked. If you have any trouble running an ob command
-                # (ob run, ob repl, etc.) with a thunk unpacked, try adding the flag  --no-interpret dep
-                # and hopefully that will sort it out.
-                src = nix-thunk.thunkSource ./dep/botan;
-              })).override (old: {
-                # Also, turn on debugging.
-                extraConfigureFlags = "--debug-mode";
-              });
-            # For whatever reason, it seems callCabal2nix below wants to use the botan package rather than botan2.
-            # We could override the pkgconfigDepends of the resulting package, but this is easier.
-            botan = self.botan2;
-        })
-      ];
+    # Here we insert our own nixpkgs overlays. Since it's a bit awkward, a future version of Obelisk will make
+    # this more straightforward, but for now, the way to do this is by modifying the reflex-platform function
+    # to modify its nixpkgsOverlays argument. Be sure to also have a look at ./nixpkgs-overlay.nix for the botan2
+    # overlay.
+    reflex-platform-func = (args: import (nix-thunk.thunkSource ./.obelisk/impl + "/dep/reflex-platform")
+      (args // { nixpkgsOverlays = args.nixpkgsOverlays ++ [(import ./nixpkgs-overlay.nix { inherit nix-thunk; })]; } ) );
 
     # You must accept the Android Software Development Kit License Agreement at
     # https://developer.android.com/studio/terms in order to build Android apps.
@@ -58,7 +29,10 @@ project ./. ({ pkgs, hackGet, ... }: {
   ios.bundleName = "Obelisk Minimal Example";
   overrides = self: super:
     with pkgs.haskell.lib; {
-      # Here, we get the tahoe-chk package from the chk.hs thunk, and use callCabal2nix to get a nix derivation to build it.
+      # Here, we get the tahoe-chk package from the chk.hs thunk,
+      # and use callCabal2nix to get a nix derivation to build it.
+      # See the comment in ./nixpkgs-overlay.nix for a description
+      # of how to interact with nix thunks.
       tahoe-chk = self.callCabal2nix "tahoe-chk" (nix-thunk.thunkSource ./dep/chkhs) {};
       # We also ended up needing an override of the base32 library, which we obtain from Hackage.
       base32 = self.callHackageDirect {

dep/python-challenge-bypass-ristretto/default.nix

@@ -0,0 +1,2 @@
+# DO NOT HAND-EDIT THIS FILE
+import (import ./thunk.nix)

dep/python-challenge-bypass-ristretto/github.json

@@ -0,0 +1,8 @@
+{
+  "owner": "LeastAuthority",
+  "repo": "python-challenge-bypass-ristretto",
+  "branch": "master",
+  "private": false,
+  "rev": "30b47c2a0ab00edbc6e3142ff806c7ac7e84e715",
+  "sha256": "14206h5b80xna6y070pk54qh228v4h43c8z1b0ibiv32ns1dhq5w"
+}

dep/python-challenge-bypass-ristretto/thunk.nix

@@ -0,0 +1,12 @@
+# DO NOT HAND-EDIT THIS FILE
+let fetch = { private ? false, fetchSubmodules ? false, owner, repo, rev, sha256, ... }:
+  if !fetchSubmodules && !private then builtins.fetchTarball {
+    url = "https://github.com/${owner}/${repo}/archive/${rev}.tar.gz"; inherit sha256;
+  } else (import (builtins.fetchTarball {
+  url = "https://github.com/NixOS/nixpkgs/archive/3aad50c30c826430b0270fcf8264c8c41b005403.tar.gz";
+  sha256 = "0xwqsf08sywd23x0xvw4c4ghq0l28w2ki22h0bdn766i16z9q2gr";
+}) {}).fetchFromGitHub {
+    inherit owner repo rev sha256 fetchSubmodules private;
+  };
+  json = builtins.fromJSON (builtins.readFile ./github.json);
+in fetch json

nixpkgs-overlay.nix

@@ -0,0 +1,51 @@
+{ nix-thunk }: self: super: rec {
+  botan2 =
+    (super.botan2.overrideAttrs (old: {
+      # Get rid of some patches that nixpkgs was applying to botan2 and which didn't apply to the branch.
+      patches = [];
+      # The --with-openssl flag didn't work for some reason, might need further figuring out.
+      configurePhase = builtins.replaceStrings [ "--with-openssl" ] [ " " ] old.configurePhase;
+      # Here, we use nix-thunk to get the source of the correct branch of the botan repo via a thunk.
+      # Nix thunks are essentially references to git repositories that can be unpacked to their
+      # source in-place when working on the project, or packed up into a few small files.
+      # After installing the nix-thunk command from https://github.com/obsidiansystems/nix-thunk
+      # you can run:
+      # nix-thunk unpack dep/botan
+      # from the top level of this repo to clone a copy of the botan git repo at the appropriate
+      # commit, and work on it from there. Similarly,
+      # nix-thunk pack dep/botan
+      # will pack it back up again, provided that the changes have been pushed somewhere.
+      # Note: there's a bug in the current version of Obelisk which occasionally gives it some trouble
+      # if certain repos are unpacked. If you have any trouble running an ob command
+      # (ob run, ob repl, etc.) with a thunk unpacked, try adding the flag  --no-interpret dep
+      # and hopefully that will sort it out.
+      src = nix-thunk.thunkSource ./dep/botan;
+    })).override (old: {
+      # Also, turn on debugging.
+      extraConfigureFlags = "--debug-mode";
+    });
+  # For whatever reason, it seems callCabal2nix for tahoe-chk
+  # wants to use the botan package rather than botan2.
+  # We could override the pkgconfigDepends of the resulting
+  # package if we needed both, but this is easier.
+  botan = self.botan2;
+
+  # Next, we'll try to obtain challenge-bypass-ristretto-ffi, for which we have a nix flake.
+  # To use the flake, we'll obtain Eelco Dolstra's flake-compat.
+  flake-compat = import (fetchTarball {
+    url = "https://github.com/edolstra/flake-compat/archive/35bb57c0c8d8b62bbfd284272c928ceb64ddbde9.tar.gz";
+    sha256 = "sha256:1prd9b1xx8c0sfwnyzkspplh30m613j42l1k789s521f4kv4c2z2";
+  });
+  # This has the top level of the flake...
+  challenge-bypass-ristretto-ffi-flake =
+    (self.flake-compat { src = nix-thunk.thunkSource ./dep/python-challenge-bypass-ristretto ; }).defaultNix;
+  # ... from which we can extract whatever packages we need.
+  challenge-bypass-ristretto-ffi =
+    challenge-bypass-ristretto-ffi-flake.packages."${builtins.currentSystem}".libchallenge_bypass_ristretto_ffi;
+  challenge-bypass-ristretto-ffi-android =
+    challenge-bypass-ristretto-ffi-flake.legacyPackages."${builtins.currentSystem}".pkgsCross.aarch64-android.libchallenge_bypass_ristretto_ffi;
+  # We can branch on the target platform to choose between the different architecture libraries from the flake. Modifying the flake might be easier.
+  libchallenge_bypass_ristretto_ffi = if self.stdenv.targetPlatform.isAndroid
+    then challenge-bypass-ristretto-ffi-flake.legacyPackages.${builtins.currentSystem}.pkgsCross.aarch64-android.libchallenge_bypass_ristretto_ffi
+    else challenge-bypass-ristretto-ffi-flake.packages.${builtins.currentSystem}.libchallenge_bypass_ristretto_ffi;
+}