Relevant telegraf.conf:

[agent]
    flush_interval = "1s"

[[inputs.influxdb_listener]]
    database_tag = "__database__"
    retention_policy_tag = "__retention__"

[[outputs.influxdb]]
    urls = ["http://influxdb:8086"]
    skip_database_creation = true
    database_tag = "__database__"
    exclude_database_tag = true
    retention_policy_tag = "__retention__"
    exclude_retention_policy_tag = true

[[outputs.file]]
    files = ["stdout"]

Reproduce the issue

with the telegraf.conf file, execute the following to run an influxdb and telegraf in docker:

docker run -d --rm --name influxdb -p 8086:8086 influxdb:1.8.3
curl "http://localhost:8086/query" -d "q=CREATE DATABASE telegraf WITH DURATION 1d"

docker run --rm -ti --link influxdb -v $(pwd):/etc/telegraf -p 8186:8186 telegraf:1.18.1

# to cleanup later, run: docker stop influxdb

now metrics can be sent like this

curl "http://localhost:8186/write?db=telegraf" -d "test value=1"
curl "http://localhost:8186/write?db=telegraf&rp=autogen" -d "test value=1"

Problem 1

writing to an invalid database:

curl "http://localhost:8186/write?db=invalid" -d "test value=1"

results in this error repeating forever

2021-04-28T12:37:11Z E! [outputs.influxdb] When writing to [http://influxdb:8086]: 404 Not Found: database not found: "invalid"
2021-04-28T12:37:11Z E! [agent] Error writing to outputs.influxdb: could not write any address

Problem 2

writing to an invalid retention:

curl "http://localhost:8186/write?db=telegraf&rp=invalid" -d "test value=1"

results in this error repeating forever

2021-04-28T12:37:32Z E! [outputs.influxdb] When writing to [http://influxdb:8086]: 500 Internal Server Error: retention policy not found: invalid
2021-04-28T12:37:32Z E! [agent] Error writing to outputs.influxdb: could not write any address

Expected behavior:

it would be expected, that when these errors occur, the metrics get dropped and don't get retried forever (while the telegraf buffer is filling up more and more)

Actual behavior:

metrics fill up the buffer of telegraf and writing to influx is retried forever