libflake: Fix flake id flake refs with revisions

Starting from c436b7a32a
this used to lead to assertion failures like:

> std::string nix::ParsedURL::renderAuthorityAndPath() const: Assertion `path.empty() || path.front().empty()' failed.

This has the bugfix for the issue and regressions tests
so that this gets properly tested in the future.

src/libflake-tests/flakeref.cc

@@ -2,6 +2,7 @@
 
 #include "nix/fetchers/fetch-settings.hh"
 #include "nix/flake/flakeref.hh"
+#include "nix/fetchers/attrs.hh"
 
 namespace nix {
 
@@ -90,6 +91,90 @@ TEST(parseFlakeRef, GitArchiveInput)
     }
 }
 
+struct InputFromURLTestCase
+{
+    std::string url;
+    fetchers::Attrs attrs;
+    std::string description;
+    std::string expectedUrl = url;
+};
+
+class InputFromURLTest : public ::testing::WithParamInterface<InputFromURLTestCase>, public ::testing::Test
+{};
+
+TEST_P(InputFromURLTest, attrsAreCorrectAndRoundTrips)
+{
+    experimentalFeatureSettings.experimentalFeatures.get().insert(Xp::Flakes);
+    fetchers::Settings fetchSettings;
+
+    const auto & testCase = GetParam();
+
+    auto flakeref = parseFlakeRef(fetchSettings, testCase.url);
+
+    EXPECT_EQ(flakeref.toAttrs(), testCase.attrs);
+    EXPECT_EQ(flakeref.to_string(), testCase.expectedUrl);
+
+    auto input = fetchers::Input::fromURL(fetchSettings, flakeref.to_string());
+
+    EXPECT_EQ(input.toURLString(), testCase.expectedUrl);
+    EXPECT_EQ(input.toAttrs(), testCase.attrs);
+
+    // Round-trip check.
+    auto input2 = fetchers::Input::fromURL(fetchSettings, input.toURLString());
+    EXPECT_EQ(input, input2);
+    EXPECT_EQ(input.toURLString(), input2.toURLString());
+}
+
+using fetchers::Attr;
+
+INSTANTIATE_TEST_SUITE_P(
+    InputFromURL,
+    InputFromURLTest,
+    ::testing::Values(
+        InputFromURLTestCase{
+            .url = "flake:nixpkgs",
+            .attrs =
+                {
+                    {"id", Attr("nixpkgs")},
+                    {"type", Attr("indirect")},
+                },
+            .description = "basic_indirect",
+        },
+        InputFromURLTestCase{
+            .url = "flake:nixpkgs/branch",
+            .attrs =
+                {
+                    {"id", Attr("nixpkgs")},
+                    {"type", Attr("indirect")},
+                    {"ref", Attr("branch")},
+                },
+            .description = "basic_indirect_branch",
+        },
+        InputFromURLTestCase{
+            .url = "nixpkgs/branch",
+            .attrs =
+                {
+                    {"id", Attr("nixpkgs")},
+                    {"type", Attr("indirect")},
+                    {"ref", Attr("branch")},
+                },
+            .description = "flake_id_ref_branch",
+            .expectedUrl = "flake:nixpkgs/branch",
+        },
+        InputFromURLTestCase{
+            .url = "nixpkgs/branch/2aae6c35c94fcfb415dbe95f408b9ce91ee846ed",
+            .attrs =
+                {
+                    {"id", Attr("nixpkgs")},
+                    {"type", Attr("indirect")},
+                    {"ref", Attr("branch")},
+                    {"rev", Attr("2aae6c35c94fcfb415dbe95f408b9ce91ee846ed")},
+                },
+            .description = "flake_id_ref_branch_trailing_slash",
+            .expectedUrl = "flake:nixpkgs/branch/2aae6c35c94fcfb415dbe95f408b9ce91ee846ed",
+        }),
+    [](const ::testing::TestParamInfo<InputFromURLTestCase> & info) { return info.param.description; });
+
 TEST(to_string, doesntReencodeUrl)
 {
     fetchers::Settings fetchSettings;

src/libflake/flakeref.cc

@@ -198,7 +198,7 @@ parseFlakeIdRef(const fetchers::Settings & fetchSettings, const std::string & ur
     if (std::regex_match(url, match, flakeRegex)) {
         auto parsedURL = ParsedURL{
             .scheme = "flake",
-            .authority = ParsedURL::Authority{},
+            .authority = std::nullopt,
             .path = splitString<std::vector<std::string>>(match[1].str(), "/"),
         };