handle 504 overpass error code like 429

[matkoniecz]

In my experience (see for example tilezen/vector-datasource#1336) 504 error code also may happen, though it is quite rare and may be handled like 429.

Unfortunately this code is not tested as it is not possible to easily reproduce this situation.

[westnordost]

Hm, I find it dangerous to simply retry and retry when the server is clearly saying "ahrgh, stooop!"

[matkoniecz]

Hm, I find it dangerous to simply retry and retry when the server is clearly saying "ahrgh, stooop!"

From my understanding of code 429 is currently handled in SC by waiting till slots for overpass queries are available.

AFAIK checking till /status call reports free slots should be correct handling also for 504.

I never noticed 504 errors reported multiple times in row, but maybe I am wrong and 504 should be handled differently.

[westnordost]

429 is a client-error. The client overstepped his quota. The client will then patiently wait for the duration shown in /status until the next slot is free.

504 has nothing to do with slots. The server can very well show that slots are free, but still return a 504. This is independent of what the experience in dealing with an overpass server shows (i.e. "will not report this several times in a row"), the server's reply should be respected and the download process be cancelled.

As the documentation indicates, a new download process can always be started later.