libfetchers/tarball: Lock on effectiveUrl

Basically, if a tarball URL is used as a flake input, and the URL leads
to a redirect, the final redirect destination would be recorded as the
locked URL.

This allows tarballs under https://nixos.org/channels to be used as
flake inputs. If we, as before, lock on to the original URL it would
break every time the channel updates.

3 files changed, 24 insertions, 9 deletions

diff --git a/src/libfetchers/fetchers.hh b/src/libfetchers/fetchers.hh
index a72cfafa4..c6b219c02 100644
--- a/src/libfetchers/fetchers.hh
+++ b/src/libfetchers/fetchers.hh
@@ -145,7 +145,13 @@ DownloadFileResult downloadFile(
     bool immutable,
     const Headers & headers = {});
 
-std::pair<Tree, time_t> downloadTarball(
+struct DownloadTarballMeta
+{
+    time_t lastModified;
+    std::string effectiveUrl;
+};
+
+std::pair<Tree, DownloadTarballMeta> downloadTarball(
     ref<Store> store,
     const std::string & url,
     const std::string & name,
diff --git a/src/libfetchers/github.cc b/src/libfetchers/github.cc
index 8352ef02d..3e5ad75a8 100644
--- a/src/libfetchers/github.cc
+++ b/src/libfetchers/github.cc
@@ -207,16 +207,16 @@ struct GitArchiveInputScheme : InputScheme
 
         auto url = getDownloadUrl(input);
 
-        auto [tree, lastModified] = downloadTarball(store, url.url, "source", true, url.headers);
+        auto [tree, meta] = downloadTarball(store, url.url, "source", true, url.headers);
 
-        input.attrs.insert_or_assign("lastModified", uint64_t(lastModified));
+        input.attrs.insert_or_assign("lastModified", uint64_t(meta.lastModified));
 
         getCache()->add(
             store,
             immutableAttrs,
             {
                 {"rev", rev->gitRev()},
-                {"lastModified", uint64_t(lastModified)}
+                {"lastModified", uint64_t(meta.lastModified)}
             },
             tree.storePath,
             true);
diff --git a/src/libfetchers/tarball.cc b/src/libfetchers/tarball.cc
index b8d7d2c70..bd05bb2f1 100644
--- a/src/libfetchers/tarball.cc
+++ b/src/libfetchers/tarball.cc
@@ -109,7 +109,7 @@ DownloadFileResult downloadFile(
     };
 }
 
-std::pair<Tree, time_t> downloadTarball(
+std::pair<Tree, DownloadTarballMeta> downloadTarball(
     ref<Store> store,
     const std::string & url,
     const std::string & name,
@@ -127,7 +127,10 @@ std::pair<Tree, time_t> downloadTarball(
     if (cached && !cached->expired)
         return {
             Tree(store->toRealPath(cached->storePath), std::move(cached->storePath)),
-            getIntAttr(cached->infoAttrs, "lastModified")
+            {
+                .lastModified = time_t(getIntAttr(cached->infoAttrs, "lastModified")),
+                .effectiveUrl = maybeGetStrAttr(cached->infoAttrs, "effectiveUrl").value_or(url),
+            },
         };
 
     auto res = downloadFile(store, url, name, immutable, headers);
@@ -152,6 +155,7 @@ std::pair<Tree, time_t> downloadTarball(
 
     Attrs infoAttrs({
         {"lastModified", uint64_t(lastModified)},
+        {"effectiveUrl", res.effectiveUrl},
         {"etag", res.etag},
     });
 
@@ -164,7 +168,10 @@ std::pair<Tree, time_t> downloadTarball(
 
     return {
         Tree(store->toRealPath(*unpackedStorePath), std::move(*unpackedStorePath)),
-        lastModified,
+        {
+            .lastModified = lastModified,
+            .effectiveUrl = res.effectiveUrl,
+        },
     };
 }
 
@@ -223,9 +230,11 @@ struct TarballInputScheme : InputScheme
         return true;
     }
 
-    std::pair<Tree, Input> fetch(ref<Store> store, const Input & input) override
+    std::pair<Tree, Input> fetch(ref<Store> store, const Input & _input) override
     {
-        auto tree = downloadTarball(store, getStrAttr(input.attrs, "url"), "source", false).first;
+        Input input(_input);
+        auto [tree, meta] = downloadTarball(store, getStrAttr(input.attrs, "url"), "source", false);
+        input.attrs.insert_or_assign("url", meta.effectiveUrl);
         return {std::move(tree), input};
     }
 };