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]>

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