[stable/nginx-ingress] Controller version 0.26.2 breaks external auth

[travisgroth]

Describe the bug
As in the title, external authentication does not function correctly with nginx 0.26.2.

See kubernetes/ingress-nginx#4872 for the outstanding issue.

Surfaced in Pomerium pomerium/pomerium#442 (ignore the parts about Traefik).

Version of Helm and Kubernetes:

Helm:
Client: &version.Version{SemVer:"v2.16.1", GitCommit:"bbdfe5e7803a12bbdf97e94cd847859890cf4050", GitTreeState:"clean"}

Server: &version.Version{SemVer:"v2.16.1", GitCommit:"bbdfe5e7803a12bbdf97e94cd847859890cf4050", GitTreeState:"clean"}

Kubernetes:
Client Version: version.Info{Major:"1", Minor:"16", GitVersion:"v1.16.2", GitCommit:"c97fe5036ef3df2967d086711e6c0c405941e14b", GitTreeState:"clean", BuildDate:"2019-10-15T19:18:23Z", GoVersion:"go1.12.10", Compiler:"gc", Platform:"darwin/amd64"}
Server Version: version.Info{Major:"1", Minor:"15", GitVersion:"v1.15.5", GitCommit:"20c265fef0741dd71a66480e35bd69f18351daea", GitTreeState:"clean", BuildDate:"2019-10-15T19:07:57Z", GoVersion:"go1.12.10", Compiler:"gc", Platform:"linux/amd64"}

Which chart:
stable/nginx-ingress version 1.28.2

What happened:
Upgraded the nginx-ingress chart in an otherwise functioning configuration. Endpoints using external auth via annotations cased working. Specifically, we wind up in a redirect loop. I do not fully understand the nature of the breakage but it seems like nginx may not be setting its own authentication cookie correctly during login flow.

What you expected to happen:
Existing Ingress to function as expected with nginx-ingress-1.28.2 / 0.26.2

How to reproduce it (as minimally and precisely as possible):
This is a known regression in 0.26.2. kubernetes/ingress-nginx#4872.

Reproduction requires setting up a cookie based auth endpoint such as pomerium and upgrading to nginx-ingress-1.27.2 or above. Downgrading to 1.27.1 or 0.26.1 fixes the issue.

Anything else we need to know:
I propose rolling back to 0.26.1 until the issue is resolved upstream. It has been open for some time with no progress. I will open a corresponding PR.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Any further update will cause the issue/pull request to no longer be considered stale. Thank you for your contributions.

[travisgroth]

Fixed with #20813