JAVA-5949 preserve connection pool on backpressure errors when establishing connections

driver-core/src/main/com/mongodb/MongoException.java

@@ -50,6 +50,22 @@ public class MongoException extends RuntimeException {
      */
     public static final String UNKNOWN_TRANSACTION_COMMIT_RESULT_LABEL = "UnknownTransactionCommitResult";
 
+    /**
+     * An error label indicating that the server is overloaded.
+     *
+     * @see #hasErrorLabel(String)
+     * @since 5.7
+     */
+    public static final String SYSTEM_OVERLOADED_ERROR_LABEL = "SystemOverloadedError";
+
+    /**
+     * An error label indicating that the operation is safely retryable.
+     *
+     * @see #hasErrorLabel(String)
+     * @since 5.7
+     */
+    public static final String RETRYABLE_ERROR_LABEL = "RetryableError";
+
     private static final long serialVersionUID = -4415279469780082174L;
 
     private final int code;

driver-core/src/main/com/mongodb/internal/connection/DefaultSdamServerDescriptionManager.java

@@ -16,6 +16,7 @@
 
 package com.mongodb.internal.connection;
 
+import com.mongodb.MongoException;
 import com.mongodb.annotations.ThreadSafe;
 import com.mongodb.connection.ClusterConnectionMode;
 import com.mongodb.connection.ServerDescription;
@@ -137,9 +138,28 @@ private void handleException(final SdamIssue sdamIssue, final boolean beforeHand
             serverMonitor.connect();
         } else if (sdamIssue.relatedToNetworkNotTimeout()
                 || (beforeHandshake && (sdamIssue.relatedToNetworkTimeout() || sdamIssue.relatedToAuth()))) {
-            updateDescription(sdamIssue.serverDescription());
-            connectionPool.invalidate(sdamIssue.exception().orElse(null));
-            serverMonitor.cancelCurrentCheck();
+            // Backpressure spec: Don't clear pool or mark server unknown for connection establishment failures
+            // (network errors or timeouts during handshake). Authentication errors after handshake should still
+            // clear the pool as they're not related to overload.
+            // TLS configuration errors (certificate validation, protocol mismatches) should also clear the pool
+            // as they indicate configuration issues, not server overload.
+            if (beforeHandshake && (sdamIssue.relatedToNetworkNotTimeout() || sdamIssue.relatedToNetworkTimeout())
+                    && !sdamIssue.relatedToAuth() && !sdamIssue.relatedToTlsConfigurationError()) {
+                // Don't update server description to Unknown
+                // Don't invalidate the connection pool
+                // Apply error labels for backpressure
+                sdamIssue.exception().ifPresent(exception -> {
+                    if (exception instanceof MongoException) {
+                        MongoException mongoException = (MongoException) exception;
+                        mongoException.addLabel(MongoException.SYSTEM_OVERLOADED_ERROR_LABEL);
+                        mongoException.addLabel(MongoException.RETRYABLE_ERROR_LABEL);
+                    }
+                });
+            } else {
+                updateDescription(sdamIssue.serverDescription());
+                connectionPool.invalidate(sdamIssue.exception().orElse(null));
+                serverMonitor.cancelCurrentCheck();
+            }
         } else if (sdamIssue.relatedToWriteConcern() || sdamIssue.relatedToStalePrimary()) {
             updateDescription(sdamIssue.serverDescription());
             serverMonitor.connect();

driver-core/src/main/com/mongodb/internal/connection/InternalStreamConnectionInitializer.java

@@ -198,6 +198,8 @@ private BsonDocument createHelloCommand(final Authenticator authenticator, final
                 helloCommandDocument.append("speculativeAuthenticate", speculativeAuthenticateDocument);
             }
         }
+        // Add backpressure support indication
+        helloCommandDocument.append("backpressure", BsonBoolean.TRUE);
         return helloCommandDocument;
     }
 

driver-core/src/main/com/mongodb/internal/connection/SdamServerDescriptionManager.java

@@ -30,6 +30,10 @@
 import com.mongodb.connection.TopologyVersion;
 import com.mongodb.lang.Nullable;
 
+import javax.net.ssl.SSLHandshakeException;
+import javax.net.ssl.SSLPeerUnverifiedException;
+import java.security.cert.CertPathValidatorException;
+import java.security.cert.CertificateException;
 import java.util.Optional;
 
 import static com.mongodb.assertions.Assertions.assertNotNull;
@@ -162,6 +166,53 @@ boolean relatedToWriteConcern() {
             return exception instanceof MongoWriteConcernWithResponseException;
         }
 
+        /**
+         * Checks if the exception is related to TLS configuration errors that are NOT due to server overload.
+         * These include certificate validation failures, protocol mismatches, etc.
+         *
+         * @return true if this is a TLS configuration error (not network-related)
+         */
+        boolean relatedToTlsConfigurationError() {
+            if (!(exception instanceof MongoSocketException)) {
+                return false;
+            }
+            Throwable cause = exception.getCause();
+            while (cause != null) {
+                // Check for various certificate validation and TLS configuration errors
+                if (cause instanceof CertificateException
+                        || cause instanceof CertPathValidatorException
+                        || cause instanceof SSLPeerUnverifiedException) {
+                    return true;  // Certificate/peer validation failure
+                }
+
+                // Check for SunCertPathBuilderException by class name to avoid compile-time dependency on internal classes
+                String className = cause.getClass().getName();
+                if (className.equals("sun.security.provider.certpath.SunCertPathBuilderException")) {
+                    return true;  // Certificate path building failure
+                }
+
+                // SSLHandshakeException can be either network or config, so we check the message
+                if (cause instanceof SSLHandshakeException) {
+                    String message = cause.getMessage();
+                    if (message != null) {
+                        String lowerMessage = message.toLowerCase();
+                        // These indicate configuration issues, not network issues
+                        if (lowerMessage.contains("certificate")
+                                || lowerMessage.contains("verify")
+                                || lowerMessage.contains("trust")
+                                || lowerMessage.contains("hostname")
+                                || lowerMessage.contains("protocol")
+                                || lowerMessage.contains("cipher")
+                                || lowerMessage.contains("handshake_failure")) {
+                            return true;
+                        }
+                    }
+                }
+                cause = cause.getCause();
+            }
+            return false;
+        }
+
         private static boolean stale(@Nullable final Throwable t, final ServerDescription currentServerDescription) {
             return TopologyVersionHelper.topologyVersion(t)
                     .map(candidateTopologyVersion -> TopologyVersionHelper.newerOrEqual(