tsort: Switch to BTreeMap and BTreeSet

[cazou]

Using HashMap and HashSet give a valid topological sort, but the output will change randomly at each run.

BTree based structures will guarantee that the output is always ordered in the same way.

This also makes the ouptut similar to the output of the C version of the tools, on which some applications rely.

An example of application that rely on this is the initramfs-update program on Debian that uses tsort to create the order of hooks execution. The floating nodes still need to be executed alphabetically to avoid having the busybox version of some tools in initramfs instead of the full versions. See this issue on Apertis.

Although the issue could be fixed in initramfs-update, other tools/programs could rely on this ordered output and a stable output for a given input makes sense.

[sylvestre]

oh, super interesting.
could you please add a test to make sure we don't regress in the future ? thanks

[tertsdiepraam]

While I think we should merge this because correctness is more important than speed, I was curious what the performance difference would be and this seems to make tsort twice as slow[1] (this makes us 4 times slower than GNU instead of 2 times). I bet there would be some people interested in figuring out how to speed this up if we open an issue for it.

[1]: On a test case generated with this Python script:

import random

N = 10000
for i in range(100*N):
    a = random.randint(0,N)
    b = random.randint(0,N)
    if a != b:
        print(f"{min(a, b)} {max(a, b)}")

[sylvestre]

needs a test

[anastygnome]

Although this PR fixes the correctness issue, wouldn't it be better to use a graph library such as petgraph that supports topological sort ? (MIT-licensed)

Otherwise, isn't there a linked hash set crate or datastructure in rust ? This way, the ordering would be constant.
Maybe we would get better performance than a tree structure.

[tertsdiepraam]

Maybe, but since this is a very small PR, we can accept this first and optimize later.

[cazou]

Any idea why inotify-dir-recreate would fail ?

[sylvestre]

unrelated
we have an intermittent issue on this one :(