digraph / digraph_utils: Adding edges to a subgraph fails unexpectedly

[al2o3cr]

Describe the bug

(discovered while solving AoC 2024 Day 5)

Adding edges to a graph that was created with digraph_utils:subgraph can fail with bad_edge unexpectedly.

The cause: when digraph_utils:subgraph copies edges into the result, it does not copy $eid in ntab so edge names are reused.

To Reproduce

%% simple reproduction for subgraph bug
main([]) ->
  G = digraph:new(),
  digraph:add_vertex(G, 1),
  digraph:add_vertex(G, 2),
  digraph:add_edge(G, 1, 2),

  SG = digraph_utils:subgraph(G, [1,2]),
  Result1 = digraph:add_edge(SG, 2, 1),
  io:format("~p~n", [Result1]),
  Result2 = digraph:add_edge(SG, 2, 1),
  io:format("~p~n", [Result2]).

prints (on OTP 27.1.2):

mattjones@Matts-Mac-Studio day5 % escript bug.erl
{error,{bad_edge,[2,1]}}
['$e'|1]

Expected behavior

The first call to digraph:add_edge should succeed.

Affected versions
Tested on 27.1.2, but digraph_utils.erl has not changed much over the years so likely many previous versions.