GH-9909: Fix SftpSession for shared client

Fixes: #9909
Issue link: https://github.com/spring-projects/spring-integration/issues/9909

The `SftpSession.close()` closes `sftpClient` and its `clientSession` unconditionally.
At the same time the `DefaultSftpSessionFactory.isSharedSession` is expected to expose only a single shared client.
When this `DefaultSftpSessionFactory` is used concurrently, there is a chance that one thread would close
that shared client and another won't be able to interact due to `clientSession` is closed.

* Fix `SftpSession` accepting an `isSharedClient` flag and doing nothing in the `close()` if client is shared.
* Propagate `isSharedSession` state down to the `SftpSession` from the `DefaultSftpSessionFactory`
* Closed shared client and its `clientSession` in the `DefaultSftpSessionFactory.destroy()`

(cherry picked from commit 2df9611dde)

spring-integration-sftp/src/main/java/org/springframework/integration/sftp/session/DefaultSftpSessionFactory.java

@@ -18,6 +18,7 @@ package org.springframework.integration.sftp.session;
 
 import java.io.IOException;
 import java.io.InputStream;
+import java.io.UncheckedIOException;
 import java.security.GeneralSecurityException;
 import java.security.KeyPair;
 import java.time.Duration;
@@ -316,7 +317,7 @@ public class DefaultSftpSessionFactory
 				sftpClient = createSftpClient(initClientSession(), this.sftpVersionSelector, SftpErrorDataHandler.EMPTY);
 				freshSftpClient = true;
 			}
-			sftpSession = new SftpSession(sftpClient);
+			sftpSession = new SftpSession(sftpClient, this.isSharedSession);
 			sftpSession.connect();
 			if (this.isSharedSession && freshSftpClient) {
 				this.sharedSftpClient = sftpClient;
@@ -449,6 +450,26 @@ public class DefaultSftpSessionFactory
 		if (this.isInnerClient && this.sshClient != null && this.sshClient.isStarted()) {
 			this.sshClient.stop();
 		}
+
+		SftpClient sharedSftpClientToClose = this.sharedSftpClient;
+		if (sharedSftpClientToClose != null) {
+			try {
+				sharedSftpClientToClose.close();
+			}
+			catch (IOException ex) {
+				throw new UncheckedIOException("failed to close an SFTP client", ex);
+			}
+
+			try {
+				ClientSession session = sharedSftpClientToClose.getSession();
+				if (session != null && session.isOpen()) {
+					session.close();
+				}
+			}
+			catch (IOException ex) {
+				throw new UncheckedIOException("failed to close an SFTP client (session)", ex);
+			}
+		}
 	}
 
 	/**

spring-integration-sftp/src/main/java/org/springframework/integration/sftp/session/SftpSession.java

@@ -55,9 +55,23 @@ public class SftpSession implements Session<SftpClient.DirEntry> {
 
 	private final SftpClient sftpClient;
 
+	private final boolean isSharedClient;
+
 	public SftpSession(SftpClient sftpClient) {
+		this(sftpClient, false);
+	}
+
+	/**
+	 * Construct an instance based on a {@link SftpClient} and its {@code shared} status.
+	 * When {@code isSharedClient == true}, the {@link #close()} is void.
+	 * @param sftpClient the {@link SftpClient} to use.
+	 * @param isSharedClient whether the {@link SftpClient} is shared.
+	 * @since 6.3.9
+	 */
+	public SftpSession(SftpClient sftpClient, boolean isSharedClient) {
 		Assert.notNull(sftpClient, "'sftpClient' must not be null");
 		this.sftpClient = sftpClient;
+		this.isSharedClient = isSharedClient;
 	}
 
 	@Override
@@ -152,6 +166,10 @@ public class SftpSession implements Session<SftpClient.DirEntry> {
 
 	@Override
 	public void close() {
+		if (this.isSharedClient) {
+			return;
+		}
+
 		try {
 			this.sftpClient.close();
 		}

spring-integration-sftp/src/test/java/org/springframework/integration/sftp/session/SftpSessionFactoryTests.java

@@ -24,6 +24,11 @@ import java.time.Duration;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicInteger;
 import java.util.stream.IntStream;
 
 import org.apache.sshd.client.SshClient;
@@ -37,6 +42,8 @@ import org.apache.sshd.core.CoreModuleProperties;
 import org.apache.sshd.server.SshServer;
 import org.apache.sshd.server.keyprovider.SimpleGeneratorHostKeyProvider;
 import org.apache.sshd.sftp.client.SftpClient;
+import org.apache.sshd.sftp.client.SftpErrorDataHandler;
+import org.apache.sshd.sftp.client.SftpVersionSelector;
 import org.apache.sshd.sftp.client.impl.AbstractSftpClient;
 import org.apache.sshd.sftp.server.SftpSubsystemFactory;
 import org.junit.jupiter.api.Test;
@@ -268,4 +275,64 @@ public class SftpSessionFactoryTests {
 		}
 	}
 
+	@Test
+	void sharedSessionConcurrentAccess() throws Exception {
+		try (SshServer server = SshServer.setUpDefaultServer()) {
+			server.setPasswordAuthenticator((arg0, arg1, arg2) -> true);
+			server.setPort(0);
+			server.setKeyPairProvider(new SimpleGeneratorHostKeyProvider(new File("hostkey.ser").toPath()));
+			server.setSubsystemFactories(Collections.singletonList(new SftpSubsystemFactory()));
+			server.start();
+
+			AtomicInteger clientInstances = new AtomicInteger();
+
+			DefaultSftpSessionFactory sftpSessionFactory = new DefaultSftpSessionFactory(true) {
+
+				@Override
+				protected SftpClient createSftpClient(ClientSession clientSession,
+						SftpVersionSelector initialVersionSelector, SftpErrorDataHandler errorDataHandler)
+						throws IOException {
+
+					clientInstances.incrementAndGet();
+					return super.createSftpClient(clientSession, initialVersionSelector, errorDataHandler);
+				}
+
+			};
+			sftpSessionFactory.setHost("localhost");
+			sftpSessionFactory.setPort(server.getPort());
+			sftpSessionFactory.setUser("user");
+			sftpSessionFactory.setPassword("pass");
+			sftpSessionFactory.setAllowUnknownKeys(true);
+
+			ExecutorService executorService = Executors.newFixedThreadPool(10);
+
+			CountDownLatch executionLatch = new CountDownLatch(20);
+			List<Exception> errors = Collections.synchronizedList(new ArrayList<>());
+
+			for (int i = 0; i < 20; i++) {
+				executorService.execute(() -> {
+					try (SftpSession session = sftpSessionFactory.getSession()) {
+						session.list(".");
+					}
+					catch (Exception e) {
+						errors.add(e);
+					}
+					executionLatch.countDown();
+				});
+			}
+
+			assertThat(executionLatch.await(10, TimeUnit.SECONDS)).isTrue();
+			synchronized (errors) {
+				assertThat(errors).isEmpty();
+			}
+
+			assertThat(clientInstances).hasValue(1);
+
+			executorService.shutdown();
+			assertThat(executorService.awaitTermination(10, TimeUnit.SECONDS)).isTrue();
+
+			sftpSessionFactory.destroy();
+		}
+	}
+
 }