src/nix/prefetch: fix prefetch containing current directory instead o…

…f tarball

When --unpack was used the nix would add the current directory to the
nix store instead of the content of unpacked.
The reason for this is that std::distance already consumes the iterator.
To fix this we re-instantiate the directory iterator in case the
directory only contains a single entry.

src/nix/prefetch.cc

@@ -114,14 +114,15 @@ std::tuple<StorePath, Hash> prefetchFile(
             createDirs(unpacked);
             unpackTarfile(tmpFile.string(), unpacked);
 
+            auto entries = std::filesystem::directory_iterator{unpacked};
             /* If the archive unpacks to a single file/directory, then use
                that as the top-level. */
-            auto entries = std::filesystem::directory_iterator{unpacked};
-            auto file_count = std::distance(entries, std::filesystem::directory_iterator{});
-            if (file_count == 1)
-                tmpFile = entries->path();
-            else
+            tmpFile = entries->path();
+            unsigned fileCount = std::distance(entries, std::filesystem::directory_iterator{});
+            if (fileCount != 1) {
+                /* otherwise, use the directory itself */
                 tmpFile = unpacked;
+            }
         }
 
         Activity act(*logger, lvlChatty, actUnknown,

tests/functional/tarball.sh

@@ -54,6 +54,18 @@ test_tarball() {
     # with the content-addressing
     (! nix-instantiate --eval -E "fetchTree { type = \"tarball\"; url = file://$tarball; narHash = \"$hash\"; name = \"foo\"; }")
 
+    store_path=$(nix store prefetch-file --json "file://$tarball" | jq -r .storePath)
+    if ! cmp -s "$store_path" "$tarball"; then
+        echo "prefetched tarball differs from original: $store_path vs $tarball" >&2
+        exit 1
+    fi
+    store_path2=$(nix store prefetch-file --json --unpack "file://$tarball" | jq -r .storePath)
+    diffOutput=$(diff -r $"$store_path2" "$tarroot")
+    if [ -n "$diffOutput" ]; then
+        echo "prefetched tarball differs from original: $store_path2 vs $tarroot" >&2
+        echo "$diffOutput"
+        exit 1
+    fi
 }
 
 test_tarball '' cat