Exclude 'dir' from the FlakeRef's URL

This fixes an issue where nix would try to check out invalid URLs,
because it would pass 'dir' to the HTTP endpoint.

For later versions this was fixed in
b2be6fed86. This is a backport of just the
relevant part.

See #12417

src/libflake/flake/flakeref.cc

@@ -234,15 +234,16 @@ std::optional<std::pair<FlakeRef, std::string>> parseURLFlakeRef(
         return std::nullopt;
     }
 
+    const auto subdir = getOr(parsedURL.query, "dir", "");
+    parsedURL.query.erase("dir");
+
     std::string fragment;
     std::swap(fragment, parsedURL.fragment);
 
     auto input = fetchers::Input::fromURL(fetchSettings, parsedURL, isFlake);
     input.parent = baseDir;
 
-    return std::make_pair(
+    return std::make_pair(FlakeRef(std::move(input), subdir), fragment);
-        FlakeRef(std::move(input), getOr(parsedURL.query, "dir", "")),
-        fragment);
 }
 
 std::pair<FlakeRef, std::string> parseFlakeRefWithFragment(

tests/unit/libflake/flakeref.cc

@@ -21,4 +21,27 @@ namespace nix {
         ASSERT_EQ(parsed, expected);
     }
 
+    /* ----------------------------------------------------------------------------
+     * parseFlakeRef
+     * --------------------------------------------------------------------------*/
+
+    TEST(parseFlakeRef, removesDirFromInputURL) {
+        fetchers::Settings fetchSettings;
+        auto s = "git+https://localhost:8181/test/test.git?dir=subdir";
+        auto flakeref = parseFlakeRef(fetchSettings, s);
+        auto expected = "git+https://localhost:8181/test/test.git";
+        auto inputURL = flakeref.input.toURLString();
+
+        ASSERT_EQ(inputURL, expected);
+    }
+
+    TEST(parseFlakeRef, setsSubdir) {
+        fetchers::Settings fetchSettings;
+        auto s = "git+https://localhost:8181/test/test.git?dir=subdir";
+        auto flakeref = parseFlakeRef(fetchSettings, s);
+        auto expected = "subdir";
+        auto flakerefSubdir = flakeref.subdir;
+
+        ASSERT_EQ(flakerefSubdir, expected);
+    }
 }