#!/usr/bin/env bash
#
# Make sure kubectl can talk to the cluster API server.
#
# The kubeconfig embeds the k3s server CA.  If the cluster's CA has been
# rotated since the secret was created, kubectl fails with:
#
#   tls: failed to verify certificate: x509: certificate signed by unknown authority
#
# which reads like a network problem rather than a stale secret.  This script
# refreshes the CA in place.
#
# The refresh is PINNED: the server's CA is fetched over an unverified
# connection, but it is only used if its SHA-256 fingerprint matches the value
# pinned in scripts/cluster-ca.sha256.  An attacker able to intercept the
# fetch cannot use it - their CA will not hash to the pinned fingerprint.
# The runner has cluster-admin credentials, so trusting an unverified CA here
# would be a privilege-escalation hole.
#
# The fingerprint is a PUBLIC value - it can be read straight off the
# certificate - so it lives in the repo rather than in a secret.  That way a
# change to it is a reviewable commit, and nobody with secret-write access
# can silently repoint the CI runner at a different CA.
#
# The expected fingerprint is read from scripts/cluster-ca.sha256 next to
# this script.  It can be overridden with the first argument.
#
# Usage:
#   kube-ca-refresh.sh [expected-fingerprint]
#
# Exit codes:
#   0  the cluster is reachable
#   1  the cluster is not reachable and could not be fixed safely

set -euo pipefail

SCRIPT_DIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
EXPECTED="${1:-}"
if [ -z "$EXPECTED" ] && [ -f "$SCRIPT_DIR/cluster-ca.sha256" ]; then
    # The file may carry comments; take the last non-comment line.
    EXPECTED=$(grep -vE '^[[:space:]]*(#|$)' "$SCRIPT_DIR/cluster-ca.sha256" | tail -1 | tr -d '[:space:]')
fi

if kubectl version --request-timeout=15s >/dev/null 2>&1; then
    echo "cluster reachable"
    exit 0
fi

if [ -z "$EXPECTED" ]; then
    echo "ERROR: the kubeconfig cannot verify the API server, and no expected CA" >&2
    echo "  fingerprint was supplied, so the CA cannot be refreshed safely." >&2
    echo "  and no pinned CA fingerprint could be found." >&2
    echo "  Set one in scripts/cluster-ca.sha256:" >&2
    echo "    openssl x509 -in server-ca.crt -noout -fingerprint -sha256" >&2
    exit 1
fi

SERVER=$(kubectl config view --raw -o jsonpath='{.clusters[0].cluster.server}')
# The cluster entry's NAME - patching the CA into a *new* entry named
# "default" would leave the original entry (and its stale CA) in use.
CLUSTER_NAME=$(kubectl config view --raw -o jsonpath='{.clusters[0].name}')
echo "server: $SERVER  cluster entry: $CLUSTER_NAME"

CA_FILE=$(mktemp)
trap 'rm -f "$CA_FILE"' EXIT

curl -sk "$SERVER/cacerts" -o "$CA_FILE"
if [ ! -s "$CA_FILE" ]; then
    echo "ERROR: could not fetch the cluster CA from $SERVER/cacerts" >&2
    exit 1
fi

# Normalise both to lowercase hex without separators, so the comparison does
# not depend on the colon-separated format openssl prints.
normalise() {
    tr -d ':' | tr 'A-Z' 'a-z'
}

ACTUAL=$(openssl x509 -in "$CA_FILE" -noout -fingerprint -sha256 | cut -d= -f2 | normalise)
WANTED=$(printf '%s' "$EXPECTED" | normalise)

echo "expected CA fingerprint: $WANTED"
echo "actual   CA fingerprint: $ACTUAL"

if [ "$ACTUAL" != "$WANTED" ]; then
    echo "ERROR: the cluster CA does not match the pinned fingerprint." >&2
    echo "  Refusing to use it.  Either the CA has been rotated and the" >&2
    echo "  pinned fingerprint needs updating, or the connection was" >&2
    echo "  intercepted." >&2
    exit 1
fi

# The fingerprint matches the pinned value, so this is the CA we expect.
kubectl config set-cluster "$CLUSTER_NAME" \
    --embed-certs=true \
    --certificate-authority="$CA_FILE" >/dev/null

if kubectl version --request-timeout=15s >/dev/null 2>&1; then
    echo "cluster reachable after refreshing the CA"
    exit 0
fi

echo "ERROR: cannot reach the cluster API server even with a refreshed CA." >&2
echo "  The kubeconfig secret may be stale or invalid." >&2
exit 1