michaeljmarshall commented on code in PR #19849:
URL: https://github.com/apache/pulsar/pull/19849#discussion_r1151457143


##########
pulsar-broker-auth-oidc/src/main/java/org/apache/pulsar/broker/authentication/oidc/OpenIDProviderMetadataCache.java:
##########
@@ -0,0 +1,124 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.pulsar.broker.authentication.oidc;
+
+import static 
org.apache.pulsar.broker.authentication.oidc.AuthenticationProviderOpenID.CACHE_EXPIRATION_SECONDS;
+import static 
org.apache.pulsar.broker.authentication.oidc.AuthenticationProviderOpenID.CACHE_EXPIRATION_SECONDS_DEFAULT;
+import static 
org.apache.pulsar.broker.authentication.oidc.AuthenticationProviderOpenID.CACHE_SIZE;
+import static 
org.apache.pulsar.broker.authentication.oidc.AuthenticationProviderOpenID.CACHE_SIZE_DEFAULT;
+import static 
org.apache.pulsar.broker.authentication.oidc.AuthenticationProviderOpenID.incrementFailureMetric;
+import static 
org.apache.pulsar.broker.authentication.oidc.ConfigUtils.getConfigValueAsInt;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.fasterxml.jackson.databind.ObjectReader;
+import com.github.benmanes.caffeine.cache.AsyncCacheLoader;
+import com.github.benmanes.caffeine.cache.AsyncLoadingCache;
+import com.github.benmanes.caffeine.cache.Caffeine;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.TimeUnit;
+import javax.annotation.Nonnull;
+import javax.naming.AuthenticationException;
+import org.apache.pulsar.broker.ServiceConfiguration;
+import org.asynchttpclient.AsyncHttpClient;
+
+/**
+ * Class used to cache metadata responses from OpenID Providers.
+ */
+class OpenIDProviderMetadataCache {
+
+    private final ObjectReader reader = new 
ObjectMapper().readerFor(OpenIDProviderMetadata.class);
+
+    /**
+     * A loader for the cache that retrieves the metadata from the issuer's 
/.well-known/openid-configuration endpoint.
+     * @return a connection to the issuer's /.well-known/openid-configuration 
endpoint
+     * @throws AuthenticationException if the URL is malformed or there is an 
exception while opening the connection
+     */
+    private AsyncCacheLoader<String, OpenIDProviderMetadata> 
getLoader(AsyncHttpClient client) {
+        return (issuer, executor) ->
+                // TODO OIDC spec 
https://openid.net/specs/openid-connect-discovery-1_0.html#NormalizationSteps
+                // calls for normalization according to RFC3986. Is that 
important to verify here?
+                client
+                    .prepareGet(issuer + "/.well-known/openid-configuration")
+                    .execute()
+                    .toCompletableFuture()
+                    .thenCompose(result -> {
+                        CompletableFuture<OpenIDProviderMetadata> future = new 
CompletableFuture<>();
+                        try {
+                            OpenIDProviderMetadata openIDProviderMetadata =
+                                    
reader.readValue(result.getResponseBodyAsBytes());
+                            verifyIssuer(issuer, openIDProviderMetadata);
+                            future.complete(openIDProviderMetadata);
+                        } catch (AuthenticationException e) {
+                            
incrementFailureMetric(AuthenticationExceptionCode.ERROR_RETRIEVING_PROVIDER_METADATA);
+                            future.completeExceptionally(e);
+                        } catch (Exception e) {
+                            
incrementFailureMetric(AuthenticationExceptionCode.ERROR_RETRIEVING_PROVIDER_METADATA);
+                            future.completeExceptionally(new 
AuthenticationException(
+                                    "Error retrieving OpenID Provider Metadata 
at " + issuer + ": " + e.getMessage()));
+                        }
+                        return future;
+                    });
+    }
+
+    private final AsyncLoadingCache<String, OpenIDProviderMetadata> cache;
+
+    OpenIDProviderMetadataCache(ServiceConfiguration config, AsyncHttpClient 
httpClient) {
+        int maxSize = getConfigValueAsInt(config, CACHE_SIZE, 
CACHE_SIZE_DEFAULT);
+        int expireAfterSeconds = getConfigValueAsInt(config, 
CACHE_EXPIRATION_SECONDS,
+                CACHE_EXPIRATION_SECONDS_DEFAULT);
+        this.cache = Caffeine.newBuilder()
+                .maximumSize(maxSize)
+                .expireAfterWrite(expireAfterSeconds, TimeUnit.SECONDS)
+                .buildAsync(getLoader(httpClient));
+    }
+
+    /**
+     * Retrieve the OpenID Provider Metadata for the provided issuer.
+     * <p>
+     * Note: this method does not do any validation on the parameterized 
issuer. The OpenID Connect discovery
+     * spec requires that the issuer use the HTTPS scheme: 
https://openid.net/specs/openid-connect-discovery-1_0.html#ProviderMetadata.
+     * The {@link AuthenticationProviderOpenID} class handles this 
verification.
+     *
+     * @param issuer - authority from which to retrieve the OpenID Provider 
Metadata
+     * @return the {@link OpenIDProviderMetadata} for the given issuer
+     * @throws AuthenticationException if any exceptions occur while 
retrieving the metadata.
+     */
+    CompletableFuture<OpenIDProviderMetadata> 
getOpenIDProviderMetadataForIssuer(@Nonnull String issuer) {
+        return cache.get(issuer);
+    }
+
+    /**
+     * Verify the issuer url, as required by the OpenID Connect spec:
+     *
+     * Per the OpenID Connect Discovery spec, the issuer value returned MUST 
be identical to the
+     * Issuer URL that was directly used to retrieve the configuration 
information. This MUST also
+     * be identical to the iss Claim value in ID Tokens issued from this 
Issuer.
+     * 
https://openid.net/specs/openid-connect-discovery-1_0.html#ProviderConfigurationValidation
+     *
+     * @param issuer - the issuer used to retrieve the metadata
+     * @param metadata - the OpenID Provider Metadata
+     * @throws AuthenticationException if the issuer does not exactly match 
the metadata issuer
+     */
+    private void verifyIssuer(@Nonnull String issuer, OpenIDProviderMetadata 
metadata) throws AuthenticationException {
+        if (!issuer.equals(metadata.getIssuer())) {
+            
incrementFailureMetric(AuthenticationExceptionCode.ISSUER_MISMATCH);
+            throw new AuthenticationException(String.format("Issuer URL 
mismatch: [%s] should match [%s]",
+                    issuer, metadata.getIssuer()));
+        }
+    }

Review Comment:
   I agree that using 
`https://kubernetes.default.svc/.well-known/openid-configuration` enhances 
simplicity. I decided to prototype it using the kubernetes `ApiClient` since 
that takes it a step further and discovers the URL, the relevant root CA, and 
the appropriate authentication.



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