Accidentally leaked the solution to an assignment, what to do now? (I'm the prof)

Very simple actually. Abandon the assignment. Apologize for the error, but not for wasting people's time. Those who didn't find the solution and worked on it certainly learned something. Those who found the solution used their time otherwise and hopefully learned something else.

Mistakes happen.

If you use a large number of exercises in grading it is probably harmless to give everyone full marks. They will be happy and it won't really matter otherwise. But trying, in any way, to discriminate between various levels of "performance" on such an exercise is a minefield.

One thing to remember, however. Presumably you gave that assignment because students would be expected to learn some specific thing(s) by doing it. That may not have occurred, so you need to assure that you find some way to reinforce that lesson in some future activity.

Leave the solution visible. Comment on it to everyone (so it is fair). Still require everyone to turn in a solution, but cannot be verbatim copy (but they can copy the algorithm/ideas/etc).

Then, announce and include that same tool/problem solving technique in the final exam. Those who work the hardest on understanding (not just copying) will be rewarded for their effort. Those who do not, will not do as well on the final.

Naturally the final has less time to do the work, but they have already seen an explicit way to work out that type of problem. If it is too big, you could provide some pieces, and they have to add the remaining functions.

The net effect of leaking the solution will just that everyone studies it to learn the tool for their exam.

I would just give them all the 10% saying that you made the solution available by accident - they will laugh and forget in 10 minutes. And, yes, been there, done that... You are not alone.

Just rely on the other 90% to give you the grading curve - nothing wrong with those I hope.

Based on one comment below, Just for clarity for some, I am not suggesting forcing the results to some arbitrary grading curve - just that the results will have a « curve » some in the highest band, others in other bands.