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 +114,19 @@ std::tuple<StorePath, Hash> prefetchFile(
             createDirs(unpacked);
             unpackTarfile(tmpFile.string(), unpacked);
 
+            unsigned fileCount = 0;
             /* 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
+            for (auto & entry : std::filesystem::directory_iterator{unpacked}) {
+                fileCount++;
+                if (fileCount > 1) {
+                    break;
+                }
+                tmpFile = entry.path();
+            }
+            if (fileCount != 1) {
                 tmpFile = unpacked;
+            }
         }
 
         Activity act(*logger, lvlChatty, actUnknown,

tests/functional/tarball.sh

@@ -54,6 +54,10 @@ test_tarball() {
     # with the content-addressing
     (! nix-instantiate --eval -E "fetchTree { type = \"tarball\"; url = file://$tarball; narHash = \"$hash\"; name = \"foo\"; }")
 
+    data=$(nix store prefetch-file --json "file://$tarball")
+    diff "$(echo "$data" | jq -r .storePath)" "$tarball"
+    data2=$(nix store prefetch-file --json --unpack "file://$tarball")
+    diff -r "$(echo "$data2" | jq -r .storePath)" "$tarroot"
 }
 
 test_tarball '' cat