DataLoader batching doesn't work properly with lists

Hello,

There seems to be still some batching issues with DataLoader and DataLoaderDispatcherInstrumentation, even after #764 was merged.

I created a simple example in https://gist.github.com/arlampin/183d40ab7e8eb2507917fe8c46730189 that demonstrates the issue.

In the example, there is a simple tree of 7 items, like so:

      1
    /   \
   2     3
  / \   / \
 4   5 6   7

Using following query query Q { root { id childNodes { id childNodes { id childNodes { id }}}}} (where ‘root’ is node 1) results in output:

BatchLoader called for [1] -> got [[2, 3]]
BatchLoader called for [2, 3] -> got [[4, 5], [6, 7]]
BatchLoader called for [4, 5] -> got [[], []]
BatchLoader called for [6, 7] -> got [[], []]

If I’ve understood correctly the idea behind DataLoader, there should only be three calls, one for each level in tree, and calls to [4, 5] and [6, 7] should have been merged into a single call.

This example doesn’t actually do any true async execution, but replacing CompletableFuture.completedFuture with an async call doesn’t affect the results.

[andimarek]

@arlampin yes, we actually managed to ship it in 9.0.

[andimarek]

closing it again, because it is fixed in 9.0.