fix(oauth2-proxy): gate startup on Keycloak discovery, or a cold boot CrashLoops the proxy

Exposed by the 2026-10-17 whole-cluster shutdown. oauth2-proxy performs OIDC
discovery ONCE at startup and exits 1 if it fails. On a cold boot Keycloak is
not serving yet, the shared Gateway answers 503, and the proxy dies:

  [provider.go:55] Performing OIDC Discovery...
  [main.go:73] ERROR: Failed to initialise OAuth2 Proxy: ... failed to discover
               OIDC configuration: unexpected status "503": no healthy upstream

It then sits in exponential CrashLoopBackOff long after Keycloak is healthy --
one replica was still down minutes later and only a manual pod delete cleared
it. There is no retry flag, a readinessProbe cannot help (the container is
already dead), and CrashLoopBackOff backoff is not tunable per-pod.

Add an extraInitContainers gate (wait-for-idp) that polls the discovery doc
until it answers. A pod in Init accrues no CrashLoopBackOff backoff and retries
promptly. BOUNDED and FAILS OPEN: after ~5m it starts the proxy anyway, so the
worst case is exactly today's behaviour and never something new.

extraInitContainers is the chart's OWN hook for this -- its commented example is
literally `- name: wait-for-idp` -- and the key was CONFIRMED against the pinned
chart (helm show values, 10.7.1) rather than assumed, because a wrong Helm values
key is silently ignored. No new base image: the chart already runs an alpine init
container for its built-in waitForRedis gate; pinned to 3.22 (verified upstream)
rather than the chart's `latest`.

Rejected: Flux postRenderers (no precedent in this tree, and the chart has a
hook); --skip-oidc-discovery (removes the fetch entirely, but needs four
hardcoded endpoints and I could not verify lazy-JWKS offline -- this component
gates SSO for seven apps, so the additive option wins).

Shell vars are written BARE with no braced form: platform declares
postBuild.substituteFrom, and envsubst would blank a braced shell variable and
silently break the loop. Same convention as the gitea/nextcloud OIDC hooks.

UNAPPLIED; the init container has never run, and the race was not re-created to
prove the fix. Check the init container's log on first apply. See the handoff.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

decisions/2026-10-17-oauth2-proxy-oidc-startup-race.md

@@ -0,0 +1,99 @@
+---
+date: 2026-10-17
+branch: fix/oauth2-proxy-oidc-startup-race
+title: "oauth2-proxy dies on a cold start because OIDC discovery runs once, before Keycloak serves"
+---
+
+## What changed
+
+Exposed by the 2026-10-17 whole-cluster shutdown for the UPS move. oauth2-proxy performs OIDC
+discovery **once, at startup**, and `os.Exit(1)`s if it fails. On a cold boot Keycloak is not
+serving yet, so the shared Gateway answers 503 and the proxy dies:
+
+```
+[provider.go:55] Performing OIDC Discovery...
+[main.go:73] ERROR: Failed to initialise OAuth2 Proxy: error initialising provider: ...
+             failed to discover OIDC configuration: unexpected status "503": no healthy upstream
+```
+
+It then sits in **exponential CrashLoopBackOff long after Keycloak is healthy** — one of the two
+replicas was still down minutes later and only a manual `kubectl delete pod` cleared it. There is no
+retry flag; the process does not survive a cold start. A readinessProbe cannot help (the container
+is already dead), and CrashLoopBackOff backoff is not tunable per-pod.
+
+`platform/oauth2-proxy/helmrelease.yaml` gains an `extraInitContainers` gate (`wait-for-idp`) that
+polls Keycloak's discovery doc until it answers, then lets the proxy start. A pod in `Init` does not
+accrue CrashLoopBackOff backoff and retries promptly, which is the point.
+
+**Bounded, and fails open.** After ~5 minutes it gives up and starts the proxy anyway, so the worst
+case is exactly today's behaviour and never something new. An unbounded wait would trade a CrashLoop
+for a pod wedged in `Init` forever, which is not an improvement.
+
+## Why this shape and not the alternatives
+
+* **`extraInitContainers` is the chart's own documented hook for precisely this.** Its commented
+  example in `values.yaml` is literally `- name: wait-for-idp`. The key was **confirmed against the
+  pinned chart** (`helm show values oauth2-proxy/oauth2-proxy --version 10.7.1` → `extraInitContainers: []`
+  at line 428), not assumed — a wrong Helm values key is silently ignored, which is the
+  "reviewed, merged, apparently done, still broken" shape this repo keeps getting bitten by.
+* **No new base image.** The chart already runs an `alpine` init container for its built-in
+  `initContainers.waitForRedis` gate, so alpine is a base this chart uses regardless. Pinned to
+  `3.22` (verified to exist upstream) rather than the chart's own `latest`.
+* **Flux `postRenderers` rejected:** it would work independently of the chart schema, but nothing in
+  this tree uses postRenderers, so it would introduce a mechanism with no precedent for a problem
+  the chart already has a hook for.
+* **`--skip-oidc-discovery` rejected:** it removes the startup fetch entirely, which is arguably
+  more elegant, but it means hardcoding four Keycloak endpoints and I could not verify offline that
+  oauth2-proxy then fetches JWKS lazily rather than at startup. oauth2-proxy gates SSO for PiHole,
+  the \*arr apps, qBittorrent, Homepage, Prometheus, Alertmanager and Hubble — a wrong guess locks
+  the operator out of all of them, so the reversible, additive option wins.
+
+The `$$`-free convention matters here: shell variables are written bare (`$i`, `$url`, `$(( ))`)
+with **no braced form anywhere**, because `platform` declares `postBuild.substituteFrom cluster-vars`
+and envsubst consumes braced expansions — a braced shell variable renders to the empty string and
+silently breaks the loop. `${BASE_DOMAIN}` is braced on purpose. Same hazard and same convention as
+the gitea and nextcloud OIDC bootstrap hooks, and `ci: flux-postbuild-substitution` enforces it.
+
+## What I verified
+
+* **The failure is measured, not theorised.** The log above is from the live pod
+  (`oauth2-proxy-5d6c74876b-bzb2p`, `lastState.terminated: {reason: Error, exitCode: 1}`), captured
+  with `--previous` after the cold boot. The surviving replica's log shows the same startup
+  succeeding once Keycloak was up, which is what identifies this as a race rather than a config
+  fault.
+* **Deleting the pod is a sufficient workaround** — done on prod 2026-10-17, the replacement came up
+  clean and both replicas have been healthy since.
+* **The chart key is real**, at the pinned version range: `helm search repo` resolves
+  `>=10.0.0 <11.0.0` to chart `10.7.1` (app `7.15.5`), whose values carry `extraInitContainers: []`,
+  `extraContainers: []` and the built-in `initContainers.waitForRedis` (which uses `alpine`).
+* **`docker.io/library/alpine:3.22` exists** — confirmed against the registry tag list rather than
+  assumed from memory.
+* The edited HelmRelease parses as YAML.
+
+## What I did NOT verify
+
+* **UNAPPLIED. The init container has never run.** I did not reconcile the HelmRelease, so I have not
+  seen `wait-for-idp` execute, exit 0, or gate the proxy. Applying it rolls both oauth2-proxy
+  replicas, which is a mutating action (CLAUDE.md §5) — and it rolls the component that gates SSO for
+  seven apps, so **do it when you can watch it**, not unattended.
+* **I did not re-create the cold-start race to prove the fix.** That needs Keycloak down while
+  oauth2-proxy starts; the honest test is the next whole-cluster boot. Until then this is reasoned
+  from the measured failure, not demonstrated against it.
+* **busybox `wget`'s TLS behaviour here is assumed, not tested.** The claim is that alpine's
+  `ca-certificates-bundle` lets it validate the Gateway's LE-prod certificate, so a genuine TLS
+  failure exits non-zero rather than passing falsely. If that is wrong in either direction the gate
+  degrades safely — a never-succeeding probe just burns the 5-minute budget and then starts the proxy
+  anyway — but the fast path would be silently dead and this would be doing nothing. **Check the
+  init container's log on first apply**: it should print "Keycloak discovery reachable" within
+  seconds on a warm cluster, not the give-up message.
+* **Whether a pod can reach the Gateway hostname at all from this namespace is inherited, not
+  re-proven.** The main container does exactly this fetch today and succeeds, so the path is known
+  good — but note the standing trap that an arbitrary pod gets 403 from every Gateway hostname
+  (`decisions/2026-10-04-plex-advertise-plexdirect.md`); oauth2-proxy is exempt only because
+  Keycloak's ingress policy admits it. An init container in the same pod inherits that identity, so
+  this should hold — unverified.
+* **60 attempts x 5s is not tuned.** It is "comfortably longer than Keycloak's observed cold start",
+  not a measured bound.
+* **No runtime conformance.** `vars.CLUSTER_AVAILABLE` is unset, so CI proves the L0/L1 offline
+  subset only.
+* I did not re-measure branch protection or runner state (CLAUDE.md §2b).

platform/oauth2-proxy/helmrelease.yaml

@@ -63,6 +63,75 @@ spec:
       # gets oauth2-proxy's HTML sign-in page (HTTP 403) instead of an immediate redirect (docs/15 UX).
       skip-provider-button: "true"
       scope: "openid email profile"
+    # ── Cold-start gate: do not start the proxy until Keycloak's discovery doc answers ──────────
+    # MEASURED on prod 2026-10-17, during the UPS move: oauth2-proxy performs OIDC discovery ONCE at
+    # startup and EXITS 1 if it fails. On a whole-cluster cold boot Keycloak is not serving yet, so
+    # the Gateway answers 503 and the proxy dies:
+    #
+    #   [provider.go:55] Performing OIDC Discovery...
+    #   [main.go:73] ERROR: Failed to initialise OAuth2 Proxy: ... failed to discover OIDC
+    #                configuration: unexpected status "503": no healthy upstream
+    #
+    # It then sits in exponential CrashLoopBackOff long after Keycloak is healthy -- one replica was
+    # still down minutes later and only a manual pod delete cleared it. There is no retry flag; the
+    # process simply does not survive a cold start. A readinessProbe cannot help (the container is
+    # already dead), and CrashLoopBackOff backoff is not tunable per-pod.
+    #
+    # `extraInitContainers` is the chart's OWN documented hook for this -- its commented example in
+    # values.yaml is literally `- name: wait-for-idp` -- and the chart already uses an alpine init
+    # container for its built-in waitForRedis gate, so this adds no new base image to the platform.
+    # A pod in Init does not accrue CrashLoopBackOff backoff and retries promptly, which is the whole
+    # point.
+    #
+    # BOUNDED, AND FAILS OPEN. After ~5 minutes it gives up and starts the proxy anyway: the worst
+    # case is then exactly today's behaviour, never something new. An unbounded wait would trade a
+    # CrashLoop for a pod wedged in Init forever, which is not an improvement.
+    #
+    # SHELL VARIABLES ARE WRITTEN BARE ($i, $url, $(( ))), WITH NO BRACED FORM ANYWHERE. This file is
+    # rendered through Flux envsubst (platform declares postBuild.substituteFrom cluster-vars), which
+    # CONSUMES braced expansions -- so a braced shell variable renders to the empty string and
+    # silently breaks the loop. BASE_DOMAIN below is braced ON PURPOSE: it is a real cluster-vars
+    # knob and must be substituted. Same hazard, and the same convention, as the gitea and nextcloud
+    # OIDC bootstrap hooks. (ci: flux-postbuild-substitution enforces it.)
+    extraInitContainers:
+      - name: wait-for-idp
+        image: docker.io/library/alpine:3.22
+        imagePullPolicy: IfNotPresent
+        command: ["/bin/sh", "-c"]
+        args:
+          - |
+            set -u
+            url=https://keycloak.${BASE_DOMAIN}/realms/homelab/.well-known/openid-configuration
+            i=0
+            while [ "$i" -lt 60 ]; do
+              # busybox wget validates against alpine's ca-certificates-bundle, so a real TLS
+              # failure is a non-zero exit here rather than a false pass.
+              if wget -q -O /dev/null --timeout=5 "$url" 2>/dev/null; then
+                echo "[wait-for-idp] Keycloak discovery reachable; starting oauth2-proxy"
+                exit 0
+              fi
+              i=$(( i + 1 ))
+              sleep 5
+            done
+            echo "[wait-for-idp] discovery still unreachable after ~5m; starting anyway so this" >&2
+            echo "[wait-for-idp] gate can never be worse than the un-gated behaviour it replaces" >&2
+            exit 0
+        securityContext:
+          allowPrivilegeEscalation: false
+          runAsNonRoot: true
+          runAsUser: 65534
+          runAsGroup: 65534
+          readOnlyRootFilesystem: true
+          capabilities:
+            drop:
+              - ALL
+        resources:
+          requests:
+            cpu: 10m
+            memory: 16Mi
+          limits:
+            memory: 32Mi
+
     # Metrics for the ServiceMonitor (../oauth2-proxy servicemonitor in platform-config).
     metrics:
       enabled: true