Ethers.js: Events matching a filter are fired by the square of the number of filters

[asselstine]

Ok, found some weird behaviour that may need an explanation, or is a bug.

When I listen for an event in multiple places, i.e.:

const contract = new ethers.Contract(...etc, ethersProvider)

var filter = contract.filters.TestEvent()

ethersProvider.on(filter, console.log) // listen in one place

ethersProvider.on(filter, console.log) // listen in another place

When the smart contract emits a TestEvent, the above code prints out the event four times.

If I added a third ethersProvider.on(filter, console.log) then it prints out the event nine times.

I think I have an explanation:

1. Looking at the BaseProvider class, it appears that the filters are stored in a _events array using the _addEventListener() function.

2. Events are broadcast using a polling function. When a new block comes in, it loops through all of the _events to see what needs to be emitted from the block.

For each log that matches the _event, it is emitted using the emit() function

1. Finally, the emit() function dispatches the log to every _event that was listening to it.

Now, this would explain the (event listeners)^2 results I was getting above, because:

1. poll(): For each listener, find the block logs that match the listener and call emit()
2. emit(): Receive log, find each listener that is listening to the log and dispatch it

Thoughts?

[ricmoo]

Weird and quite possible; if so it is a bug. I will investigate, but I think your explanation would make sense.

[ricmoo]

I've confirmed this is a bug and have a consistent script to reproduce it. I'm working on it now, but have to jet soon, so it may not be fixed until tomorrow.

[asselstine]

Fantastic! I'm glad it's getting fixed. We actually switched to Ethers from web3 1.0 because it was having a similar issue.

Thanks for being so on top of it!

[ricmoo]

I have a fix in place, if you want to pull the latest code you can try it locally. Travis CI takes about 35 minutes to run all the test cases, and I will probably sleep on this fix so I can mull it over, but seems like this is the right thing to do. :)

[asselstine]

Tested the latest code- the fix works for provider.on() but fails for contract.on()

My test was:

const filter = contract.filters.Test()

provider.on(filter, console.log)
provider.on(filter, console.log)

I then transacted with Ganache for one event and there was two console.logs. Correct!

However, this test:

const filter = contract.filters.Test()

contract.on(filter, console.log)
contract.on(filter, console.log)

The above test still prints four console.logs, so for some reason the contract specific one is still not working.

[ricmoo]

Awesome! Glad I held off publishing. :)

I’ll investigate further tonight... May be similar code in the Contract... it was hard to generalize that functionality...

[ricmoo]

I've identified the issue; very similar, and it will require some non-trivial changes. I will get to this soon, but it will require much more testing and thinking things through, so it might be a bit longer before it is ready to be published. :s

[ricmoo]

Just following up to make sure you know I haven't forgotten about this. It is next on my plate. :)

[asselstine]

Excellent! Thank you

[krzkaczor]

@ricmoo should this be closed?

[ricmoo]

@krzkaczor Yes, I'll close this now. It is still an issue in v4, but seems like a dangerous and involved change for v4. It works fine in v5, and this required a fairly substantial re-write of how events work, which is how I know it is risky for v4.

Thanks!

[asselstine]

All the more reason to upgrade to v5! Thank you.