Only override Context.PROVIDER_URL if the default isn't supported (#853)

JAVA-4423

Author: jyemin

driver-core/src/main/com/mongodb/internal/dns/DefaultDnsResolver.java

@@ -147,12 +147,18 @@ public String resolveAdditionalQueryParametersFromTxtRecords(final String host)
     private static InitialDirContext createDnsDirContext() {
         Hashtable<String, String> envProps = new Hashtable<String, String>();
         envProps.put(Context.INITIAL_CONTEXT_FACTORY, "com.sun.jndi.dns.DnsContextFactory");
-        envProps.put(Context.PROVIDER_URL, "dns:");
+
         try {
             return new InitialDirContext(envProps);
         } catch (NamingException e) {
-            throw new MongoClientException("Unable to support mongodb+srv// style connections as the 'com.sun.jndi.dns.DnsContextFactory' "
-                    + "class is not available in this JRE. A JNDI context is required for resolving SRV records.", e);
+            // Just in case the provider url default has been changed to a non-dns pseudo url, fallback to the JDK default
+            envProps.put(Context.PROVIDER_URL, "dns:");
+            try {
+                return new InitialDirContext(envProps);
+            } catch (NamingException ex) {
+                throw new MongoClientException("Unable to support mongodb+srv// style connections as the 'com.sun.jndi.dns.DnsContextFactory' "
+                        + "class is not available in this JRE. A JNDI context is required for resolving SRV records.", e);
+            }
         }
     }
 }

driver-core/src/test/unit/com/mongodb/internal/dns/DefaultDnsResolverTest.java

@@ -0,0 +1,50 @@
+/*
+ * Copyright 2008-present MongoDB, Inc.
+ *
+ * Licensed 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 com.mongodb.internal.dns;
+
+import com.mongodb.MongoConfigurationException;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.Test;
+
+import javax.naming.Context;
+
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+public class DefaultDnsResolverTest {
+    private static final String TEST_HOST = "test1.test.build.10gen.cc";
+    private static final String DEFAULT_PROVIDER_URL_VALUE = System.getProperty(Context.PROVIDER_URL);
+
+    @AfterEach
+    public void resetDefaultProviderUrl() {
+        if (DEFAULT_PROVIDER_URL_VALUE != null) {
+            System.setProperty(Context.PROVIDER_URL, DEFAULT_PROVIDER_URL_VALUE);
+        }
+    }
+
+    @Test
+    public void nonDnsProviderUrlShouldBeIgnored() {
+        System.setProperty(Context.PROVIDER_URL, "file:///tmp/provider.txt");
+        assertDoesNotThrow(() -> new DefaultDnsResolver().resolveHostFromSrvRecords(TEST_HOST, "mongodb"));
+    }
+
+    @Test
+    public void dnsProviderUrlShouldNotBeIgnored() {
+        System.setProperty(Context.PROVIDER_URL, "dns:///mongodb.unknown.server.com");
+        assertThrows(MongoConfigurationException.class, () -> new DefaultDnsResolver().resolveHostFromSrvRecords(TEST_HOST, "mongodb"));
+    }
+}