GODRIVER-3032 Don't retry writes on pre-4.4 mongos

[prestonvasquez]

GODRIVER-3032

Summary

For a pre-4.4 mongos response, we can trust that mongos will have already retried the operation if necessary. Drivers should not retry to avoid "excessive retrying".

Under similar circumstances, the specifications now clarify that drivers should not consider codes within writeErrors from a mongod or mongos response. Notably, the Go Driver does not currently do this.

Background & Motivation

From the comments on the related drivers ticket:

Pre and post 4.4 mongos will retry on retryable codes within the writeConcernError field (it chooses to retry based on the "effective status" from a command response - 4.2 code and current master - which will check the write concern status - 4.2 and master). So in both cases drivers should not retry because mongos would only return a retryable write concern error if it exhausts its retry attempts. The only difference pre and post 4.4 with regard to write concern errors is that mongos will not retry on WriteConcernFailed in post 4.4 (as called out by the retryable error label proposal), but I don't think that affects drivers behavior because according to their retryable writes spec, that code isn't considered retryable.

[mongodb-drivers-pr-bot]

API Change Report

./v2/x/mongo/driver

incompatible changes

##WriteCommandError.Retryable: changed from func(*./v2/x/mongo/driver/description.VersionRange) bool to func(./v2/x/mongo/driver/description.ServerKind, *./v2/x/mongo/driver/description.VersionRange) bool
##WriteConcernError.Retryable: changed from func() bool to func(./v2/x/mongo/driver/description.ServerKind, *./v2/x/mongo/driver/description.VersionRange) bool
WriteConcernError: no longer implements RetryablePoolError

[qingyang-hu]

👍 LGTM!