Reject fully-qualified URLs in 'from' argument of `nix registry add`

We previously allowed you to map any flake URL to any other flake URL,
including shorthand flakerefs, indirect flake URLs like `flake:nixpkgs`,
direct flake URLs like `github:NixOS/nixpkgs`, or local paths.

But flake registry entries mapping from direct flake URLs often come
from swapping the 'from' and 'to' arguments by accident, and even when
created intentionally, they may not actually work correctly.

This patch rejects those URLs (and fully-qualified flake: URLs), making
it harder to swap the arguments by accident.

Fixes #181.

Change-Id: I24713643a534166c052719b8770a4edfcfdb8cf3

1 files changed, 8 insertions, 0 deletions

diff --git a/src/nix/registry.cc b/src/nix/registry.cc
index f509ccae8..9619b85d4 100644
--- a/src/nix/registry.cc
+++ b/src/nix/registry.cc
@@ -5,6 +5,7 @@
 #include "flake/flake.hh"
 #include "store-api.hh"
 #include "fetchers.hh"
+#include "url-parts.hh"
 #include "registry.hh"
 
 using namespace nix;
@@ -109,7 +110,14 @@ struct CmdRegistryAdd : MixEvalArgs, Command, RegistryCommand
 
     void run() override
     {
+        std::smatch match;
+        if (!std::regex_match(fromUrl, match, flakeShorthandRegex)) {
+            throw UsageError("'from-url' argument must be a shorthand like 'nixpkgs' or 'nixpkgs/nixos-20.03'");
+        }
         auto fromRef = parseFlakeRef(fromUrl);
+        if (fromRef.input.direct) {
+            throw UsageError("'from-url' argument must be an indirect flakeref like 'nixpkgs' or 'flake:nixpkgs'");
+        }
         auto toRef = parseFlakeRef(toUrl);
         auto registry = getRegistry();
         fetchers::Attrs extraAttrs;