OpenExchangeRate being hit 2x as often as `retrieve_every` configuration.

[dbernheisel]

The ExchangeRate Retriever seems to have duplicate timers scheduled whenever retrieve_every is configured, this results in the OpenExchangeRates (of whichever configured api module) to fetch rates 2x as often.

I believe it's happening here:

money/lib/money/exchange_rates/exchange_rates_retriever.ex

Lines 263 to 267 in 7fab9d5

	if is_integer(config.retrieve_every) do
	log(config, :info, log_init_message(config.retrieve_every))
	schedule_work(0)
	schedule_work(config.retrieve_every)
	end

Which both land here:

money/lib/money/exchange_rates/exchange_rates_retriever.ex

Lines 329 to 333 in 7fab9d5

	def handle_info(:latest_rates, config) do
	retrieve_latest_rates(config)
	schedule_work(config.retrieve_every)
	{:noreply, config}
	end

Let's try to track it, and let's assume retrieve_every is 1000ms.

1. 0ms - We initialize the Retriever genserver.
2. 0ms - schedule_work(0) casts a message to immediately fetch exchange rates. This is put into mailbox. Expected to process immediately after initializing.
3. 0ms - schedule_work(1000) casts another message to fetch exchange rates 1000ms from now.
4. 1ms - Genserver has initialized
5. 2ms - The first :latest_rates message is processed in the mailbox, the API is immediately hit and now we schedule_work(1000) again, expected around 1002ms (now + 1000ms).
6. 1000ms - The second message in the mailbox is now processed, we hit the API again and schedule another timer for 1000ms, expected at 2000ms (now + 1000ms).
7. 1002ms - The third message in the mailbox is now processed, and we hit the API yet again and schedule another timer for 100ms, expected at 2002ms (now + 1000ms).
8. and so forth. We have 2 duplicate timers at this point past startup.

One solution I've done to avoid this issue is to store a reference of the timer in the genserver state, and whenever scheduling work, you cancel the timer and reset it.

[kipcole9]

@dbernheisel thanks again for a fabulously detailed report. I will dig into this later today. Clearly not acceptable to be consuming at twice the expected rate.

[kipcole9]

I changed

   schedule_work(0) 
   schedule_work(config.retrieve_every)

to be simply:

   schedule_work(0) 

And then let it run for a few iterations and I believe there is now only a single event loop.

Would you consider trying this by configuring ex_money from github? If you see the correct behaviour then I'll publish a new release right away.

Of course please re-open if the issue isn't fixed as you see it.

[dbernheisel]

Yep. I'll report back on Monday

[dbernheisel]

Confirmed fixed. Thanks so much.

[kipcole9]

I've published ex_money version 5.15.1 with the following changelog entry:

Bug Fixes

• Fixes the exchange rate retriever, removing the double retrieval loop. Thanks to @dbernheisel for the report. Closes #152.

Thanks again for the report!