NodeAccessorPlugin spec does not deduplicate

[jcgsville]

Summary

When using the xByNodeId queries, the plan does not correctly deduplicate equivalent constants.

Steps to reproduce

Setup Postgraphile v5 with the relay preset. Add a table with the ability to do a root query to fetch a single row of that table by its node ID. e.g.

create table foo (id int primary key, a text);

Set graphile.explain to true in your preset

Execute a query in Ruru using aliases to query the same row twice. e.g.

query MyFoos {
  foo1: fooById(id="asdfasdf") {
    id
  }
  foo2: fooById(id="asdfasdf") {
    id
  }
}

See the diagram of the plan is similar to the screenshot above

If necessary, I can create a minimal repro of this, but it's probably not necessary, because I'd like to tackle the issue, and I have a repro on my project 🙂

Expected results

I'd expect to see only a single Lambda step

Actual results

I see two Lambda steps

Additional context

Discussed here

https://discord.com/channels/489127045289476126/1325736426587033652/1325877404559740959

Possible Solution

According to @benjie, the specForHandler function here incorrectly returns a new function each time. Thus the lambda() here cannot deduplicate because the functions are not equivalent. We could potentially cache the function in a Map such that the functions are equivalent, and lambda() can deduplicate