[BUG] Experimental.Traversals.topological_sort not working for non-connected sub-graphs

[h-spiess]

Description of bug
The topological_sort in LightGraphs.Experimental.Traversals is not working when non-connected sub-graphs exist in a graph, i.e., graph consists of multiple non-connected graphs.

How to reproduce

using LightGraphs
using LightGraphs.Experimental.Traversals
Adj = [0 0 0 0; 0 0 0 1; 0 1 0 0; 0 0 0 0]
graph = SimpleDiGraph(Adj)
is_cyclic(graph) # false

topological_sort(graph) # incorrect: 3, 2, 4, 2, 4, 1
topological_sort_by_dfs(graph) # correct: 3, 2, 4, 1

Expected behavior
topological_sort_by_dfs is correct. In topological_sort nodes are probably traversed multiple times.

Actual behavior
See above.

Code demonstrating bug
See reproduction.

Version information

Julia Version 1.6.0
Commit f9720dc2eb (2021-03-24 12:55 UTC)
Platform Info:
  OS: Linux (x86_64-pc-linux-gnu)
  CPU: Intel(R) Xeon(R) CPU E5-2630 0 @ 2.30GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-11.0.1 (ORCJIT, sandybridge)

LightGraphs v1.3.5

Additional context
Hey, I'm aware that this happens in code that is experimental but I think the bug is worth reporting.

BTW, i really like your package :) Thanks a lot for your great work :)

[sbromberger]

Thanks for your comments. Since this function is in Experimental, I think the best we can do is to try to find someone willing to troubleshoot it and work on the bug. I don't know who originally wrote the function; the history shows that I committed it but I'm not sure it's mine.