Support registry entries that must match exactly

An example use is for pinning the "nixpkgs" entry the system-wide
registry to a particular store path. Inexact matches
(e.g. "nixpkgs/master") should still use the global registry.

src/libfetchers/registry.cc

@@ -43,11 +43,13 @@ std::shared_ptr<Registry> Registry::read(
                 extraAttrs.insert(*j);
                 toAttrs.erase(j);
             }
+            auto exact = i.find("exact");
             registry->entries.push_back(
                 Entry {
                     .from = inputFromAttrs(jsonToAttrs(i["from"])),
                     .to = inputFromAttrs(toAttrs),
                     .extraAttrs = extraAttrs,
+                    .exact = exact != i.end() && exact.value()
                 });
         }
     }
@@ -68,6 +70,8 @@ void Registry::write(const Path & path)
         obj["to"] = attrsToJson(entry.to->toAttrs());
         if (!entry.extraAttrs.empty())
             obj["to"].update(attrsToJson(entry.extraAttrs));
+        if (entry.exact)
+            obj["exact"] = true;
         arr.emplace_back(std::move(obj));
     }
 
@@ -185,6 +189,13 @@ std::pair<std::shared_ptr<const Input>, Attrs> lookupInRegistries(
     for (auto & registry : getRegistries(store)) {
         // FIXME: O(n)
         for (auto & entry : registry->entries) {
+            if (entry.exact) {
+                if (*entry.from == *input) {
+                    input = entry.to;
+                    extraAttrs = entry.extraAttrs;
+                    goto restart;
+                }
+            } else {
                 if (entry.from->contains(*input)) {
                     input = entry.to->applyOverrides(
                         !entry.from->getRef() && input->getRef() ? input->getRef() : std::optional<std::string>(),
@@ -194,6 +205,7 @@ std::pair<std::shared_ptr<const Input>, Attrs> lookupInRegistries(
                 }
             }
         }
+    }
 
     if (!input->isDirect())
         throw Error("cannot find flake '%s' in the flake registries", input->to_string());

src/libfetchers/registry.hh

@@ -23,6 +23,7 @@ struct Registry
         std::shared_ptr<const Input> from;
         std::shared_ptr<const Input> to;
         Attrs extraAttrs;
+        bool exact = false;
     };
 
     std::vector<Entry> entries;