fix(fetch.py): when fetch is run, first update unstable channel

This way we won’t forget to do it for months at a time.

You’d hope that stuff like this would be possible without jumping
through hoops, but this is nix after all. Flakes our salvation or
something.

fetch.py

@@ -11,6 +11,29 @@ def curl_latest_release():
     url = "https://api.github.com/repos/dhall-lang/dhall-haskell/releases/latest"
     return json.loads(sub.check_output(["curl", url]))
 
+# fetch the latest nixos unstable version
+def curl_latest_nixos_unstable():
+    url = "https://nixos.org/channels/nixos-unstable/git-revision"
+    return sub.check_output(["curl", "--location",  url]).strip().decode()
+
+def update_nixpkgs(lockfile_path, new_hash):
+    # --unpack produces the hash required by `builtins.fetchTarball`
+    github_archive = "https://github.com/NixOS/nixpkgs/archive/{}.tar.gz".format(new_hash)
+    hash = sub.check_output([
+        "nix-prefetch-url", '--unpack', github_archive
+    ]).strip().decode()
+    date = sub.check_output([
+        "date", "--iso-8601=minutes"
+    ]).strip().decode()
+    with open(lockfile_path, 'w') as f:
+        json.dump({
+            "comment": "autogenerated by fetch.py",
+            "url": github_archive,
+            "sha256": hash,
+            "date": date
+        }, f, indent=2)
+
+
 # call nix-prefetch-url on each asset to get their hashes
 def prefetch_binaries(release):
     res = []
@@ -39,11 +62,17 @@ def postprocess(fetched):
 
 
 if __name__ == "__main__":
+    print("updating nixpkgs to latest unstable", file=sys.stderr)
+    nixos_hash = curl_latest_nixos_unstable()
+    update_nixpkgs("./nixpkgs.json", nixos_hash)
+
     release = curl_latest_release()
     version = release['tag_name']
+
     print("updating to release {}".format(version), file=sys.stderr)
     fetched = prefetch_binaries(release)
     res = postprocess(fetched)
+
     print("writing ./release.json", file=sys.stderr)
     with open("./release.json", mode='w') as f:
         json.dump(res, f, indent=2)

nixpkgs.json

@@ -0,0 +1,6 @@
+{
+  "comment": "autogenerated by fetch.py",
+  "url": "https://github.com/NixOS/nixpkgs/archive/24eb3f87fc610f18de7076aee7c5a84ac5591e3e.tar.gz",
+  "sha256": "1ca14hhbinnz1ylbqnhwinjdbm6nn859j4gmyamg2kr7jl6611s0",
+  "date": "2020-11-30T23:28+01:00"
+}

nixpkgs.nix

@@ -1,6 +1,6 @@
-import (
-  builtins.fetchTarball {
-    url = "https://github.com/nixos/nixpkgs/archive/a7bf8161fa834a602278c15fcbdd955656b3aed8.tar.gz";
-    sha256 = "0lzd35niw0j8qy4mhglvqwv5qwyvdb9mhv8hpmn73ym57r92i4p1";
-  }
-)
+import (fetchTarball {
+  inherit (builtins.fromJSON (builtins.readFile ./nixpkgs.json))
+    url
+    sha256
+    ;
+})