Split tarball-specific logic from GitFileSystemObjectSink

src/libfetchers/git-utils.cc

@@ -486,6 +486,24 @@ struct GitRepoImpl : GitRepo, std::enable_shared_from_this<GitRepoImpl>
 
         return narHash;
     }
+
+    Hash dereferenceSingletonDirectory(const Hash & oid_) override
+    {
+        auto oid = hashToOID(oid_);
+
+        /* If the root directory contains  */
+        auto _tree = lookupObject(*this, oid, GIT_OBJECT_TREE);
+        auto tree = (const git_tree *) &*_tree;
+
+        if (git_tree_entrycount(tree) == 1) {
+            auto entry = git_tree_entry_byindex(tree, 0);
+            auto mode = git_tree_entry_filemode(entry);
+            if (mode == GIT_FILEMODE_BLOB || mode == GIT_FILEMODE_TREE)
+                oid = *git_tree_entry_id(entry);
+        }
+
+        return toHash(oid);
+    }
 };
 
 ref<GitRepo> GitRepo::openRepo(const std::filesystem::path & path, bool create, bool bare)
@@ -991,21 +1009,6 @@ struct GitFileSystemObjectSinkImpl : GitFileSystemObjectSink
 
         auto [oid, _name] = popBuilder();
 
-        /* If the root directory contains a single entry that is a
-           directory or a non-executable regular file, return that as
-           the top-level object. We don't do this for executables
-           because they don't have a tree hash in the Git object
-           model. */
-        auto _tree = lookupObject(*repo, oid, GIT_OBJECT_TREE);
-        auto tree = (const git_tree *) &*_tree;
-
-        if (git_tree_entrycount(tree) == 1) {
-            auto entry = git_tree_entry_byindex(tree, 0);
-            auto mode = git_tree_entry_filemode(entry);
-            if (mode == GIT_FILEMODE_BLOB || mode == GIT_FILEMODE_TREE)
-                oid = *git_tree_entry_id(entry);
-        }
-
         return toHash(oid);
     }
 };

src/libfetchers/git-utils.hh

@@ -98,6 +98,17 @@ struct GitRepo
      * serialisation. This is memoised on-disk.
      */
     virtual Hash treeHashToNarHash(const Hash & treeHash) = 0;
+
+    /**
+     * If the specified Git object is a directory with a single entry
+     * that is a directory or a non-executable regular file, return
+     * the ID of that object.
+     *
+     * Note: We don't do this for executable files because they don't
+     * have a tree hash in the Git object model that distinguishes
+     * them from non-executable files.
+     */
+    virtual Hash dereferenceSingletonDirectory(const Hash & oid) = 0;
 };
 
 ref<GitRepo> getTarballCache();

src/libfetchers/github.cc

@@ -255,11 +255,12 @@ struct GitArchiveInputScheme : InputScheme
         });
 
         TarArchive archive { *source };
-        auto parseSink = getTarballCache()->getFileSystemObjectSink();
+        auto tarballCache = getTarballCache();
+        auto parseSink = tarballCache->getFileSystemObjectSink();
         auto lastModified = unpackTarfileToSink(archive, *parseSink);
 
         TarballInfo tarballInfo {
-            .treeHash = parseSink->sync(),
+            .treeHash = tarballCache->dereferenceSingletonDirectory(parseSink->sync()),
             .lastModified = lastModified
         };
 

src/libfetchers/tarball.cc

@@ -164,7 +164,8 @@ DownloadTarballResult downloadTarball(
                 TarArchive{path};
           })
         : TarArchive{*source};
-    auto parseSink = getTarballCache()->getFileSystemObjectSink();
+    auto tarballCache = getTarballCache();
+    auto parseSink = tarballCache->getFileSystemObjectSink();
     auto lastModified = unpackTarfileToSink(archive, *parseSink);
 
     auto res(_res->lock());
@@ -177,7 +178,8 @@ DownloadTarballResult downloadTarball(
         infoAttrs = cached->value;
     } else {
         infoAttrs.insert_or_assign("etag", res->etag);
-        infoAttrs.insert_or_assign("treeHash", parseSink->sync().gitRev());
+        infoAttrs.insert_or_assign("treeHash",
+            tarballCache->dereferenceSingletonDirectory(parseSink->sync()).gitRev());
         infoAttrs.insert_or_assign("lastModified", uint64_t(lastModified));
         if (res->immutableUrl)
             infoAttrs.insert_or_assign("immutableUrl", *res->immutableUrl);

tests/unit/libfetchers/git-utils.cc

@@ -77,7 +77,7 @@ TEST_F(GitUtilsTest, sink_basic)
 
     // sink->createHardlink("foo-1.1/links/foo-2", CanonPath("foo-1.1/hello"));
 
-    auto result = sink->sync();
+    auto result = repo->dereferenceSingletonDirectory(sink->sync());
     auto accessor = repo->getAccessor(result, false);
     auto entries = accessor->readDirectory(CanonPath::root);
     ASSERT_EQ(entries.size(), 5);