GH-743: Use a singleton SftpFileSystemProvider for the ServiceLoader

When SFTP file systems are created through the standard Java mechanism
via java.nio.file.FileSystems.newFileSystem(URI, Map, Classloader), the
JDK uses the ServiceLoader to find registered file system providers.
It remembers any it can find via the system classloader or via the
platform classloader as "installed providers" and creates only exactly
one instance of them.

When it cannot find a provider matching the URI scheme in the
"installed providers" and a classloader is given, it then tries to
find a matching file system provider using that classloader. The
ServiceLoader instantiates each provider it finds to check its scheme
against the URI scheme. But it doesn't remember these file system
providers, so if FileSystems.newFileSystem() is called again, another
instance of SftpFileSystemProvider got created.

When the returned file system is used, the SftpFileSystemProvider
creates an SshClient, and an I/O thread pool is set up. If multiple
SftpFileSystemProviders get created, there will also be multiple
SshClients, and multiple thread pools.

Avoid this by registering via the FileSystemProvider SPI only a
stateless facade class. All instance of this facade class delegate
to a singleton SftpFileSystemProvider.

Author: tomaswolf

CHANGES.md

@@ -31,6 +31,7 @@
 
 ## Bug Fixes
 
+* [GH-743](https://github.com/apache/mina-sshd/issues/743) Ensure the Java `ServiceLoader` use a singleton `SftpFileSystemProvider`
 * [GH-879](https://github.com/apache/mina-sshd/issues/879) Close SSH channel gracefully on exception in port forwarding
 
 ## New Features

sshd-sftp/src/main/filtered-resources/META-INF/services/java.nio.file.spi.FileSystemProvider

@@ -17,4 +17,4 @@
 ## under the License.
 ##
 
-org.apache.sshd.sftp.client.fs.SftpFileSystemProvider
+org.apache.sshd.sftp.client.fs.SftpFileSystemProviderFacade

sshd-sftp/src/main/java/org/apache/sshd/sftp/client/fs/SftpFileSystemProviderFacade.java

@@ -0,0 +1,182 @@
+/*
+ * 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.sshd.sftp.client.fs;
+
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.OutputStream;
+import java.net.URI;
+import java.nio.channels.FileChannel;
+import java.nio.channels.SeekableByteChannel;
+import java.nio.file.AccessMode;
+import java.nio.file.CopyOption;
+import java.nio.file.DirectoryStream;
+import java.nio.file.DirectoryStream.Filter;
+import java.nio.file.FileStore;
+import java.nio.file.FileSystem;
+import java.nio.file.LinkOption;
+import java.nio.file.OpenOption;
+import java.nio.file.Path;
+import java.nio.file.attribute.BasicFileAttributes;
+import java.nio.file.attribute.FileAttribute;
+import java.nio.file.attribute.FileAttributeView;
+import java.nio.file.spi.FileSystemProvider;
+import java.util.Map;
+import java.util.Set;
+
+import org.apache.sshd.sftp.common.SftpConstants;
+
+/**
+ * SPI-registered SFTP file system provider using a singleton instance.
+ *
+ * @author <a href="mailto:dev@mina.apache.org">Apache MINA SSHD Project</a>
+ */
+public class SftpFileSystemProviderFacade extends FileSystemProvider {
+
+    private static final class DefaultSftpFileSystemSingleton {
+        static final SftpFileSystemProvider INSTANCE = new SftpFileSystemProvider();
+
+        private DefaultSftpFileSystemSingleton() {
+            // No instantiation
+        }
+    }
+
+    public SftpFileSystemProviderFacade() {
+        // Nothing
+    }
+
+    @Override
+    public String getScheme() {
+        return SftpConstants.SFTP_SUBSYSTEM_NAME;
+    }
+
+    @Override
+    public FileSystem newFileSystem(URI uri, Map<String, ?> env) throws IOException {
+        return DefaultSftpFileSystemSingleton.INSTANCE.newFileSystem(uri, env);
+    }
+
+    @Override
+    public FileSystem getFileSystem(URI uri) {
+        return DefaultSftpFileSystemSingleton.INSTANCE.getFileSystem(uri);
+    }
+
+    @Override
+    public Path getPath(URI uri) {
+        return DefaultSftpFileSystemSingleton.INSTANCE.getPath(uri);
+    }
+
+    @Override
+    public SeekableByteChannel newByteChannel(Path path, Set<? extends OpenOption> options, FileAttribute<?>... attrs)
+            throws IOException {
+        return DefaultSftpFileSystemSingleton.INSTANCE.newByteChannel(path, options, attrs);
+    }
+
+    @Override
+    public FileChannel newFileChannel(Path path, Set<? extends OpenOption> options, FileAttribute<?>... attrs)
+            throws IOException {
+        return DefaultSftpFileSystemSingleton.INSTANCE.newFileChannel(path, options, attrs);
+    }
+
+    @Override
+    public InputStream newInputStream(Path path, OpenOption... options) throws IOException {
+        return DefaultSftpFileSystemSingleton.INSTANCE.newInputStream(path, options);
+    }
+
+    @Override
+    public OutputStream newOutputStream(Path path, OpenOption... options) throws IOException {
+        return DefaultSftpFileSystemSingleton.INSTANCE.newOutputStream(path, options);
+    }
+
+    @Override
+    public DirectoryStream<Path> newDirectoryStream(Path dir, Filter<? super Path> filter) throws IOException {
+        return DefaultSftpFileSystemSingleton.INSTANCE.newDirectoryStream(dir, filter);
+    }
+
+    @Override
+    public void createSymbolicLink(Path link, Path target, FileAttribute<?>... attrs) throws IOException {
+        DefaultSftpFileSystemSingleton.INSTANCE.createSymbolicLink(link, target, attrs);
+    }
+
+    @Override
+    public Path readSymbolicLink(Path link) throws IOException {
+        return DefaultSftpFileSystemSingleton.INSTANCE.readSymbolicLink(link);
+    }
+
+    @Override
+    public void createDirectory(Path dir, FileAttribute<?>... attrs) throws IOException {
+        DefaultSftpFileSystemSingleton.INSTANCE.createDirectory(dir, attrs);
+    }
+
+    @Override
+    public void delete(Path path) throws IOException {
+        DefaultSftpFileSystemSingleton.INSTANCE.delete(path);
+    }
+
+    @Override
+    public void copy(Path source, Path target, CopyOption... options) throws IOException {
+        DefaultSftpFileSystemSingleton.INSTANCE.copy(source, target, options);
+    }
+
+    @Override
+    public void move(Path source, Path target, CopyOption... options) throws IOException {
+        DefaultSftpFileSystemSingleton.INSTANCE.move(source, target, options);
+    }
+
+    @Override
+    public boolean isSameFile(Path path, Path path2) throws IOException {
+        return DefaultSftpFileSystemSingleton.INSTANCE.isSameFile(path, path2);
+    }
+
+    @Override
+    public boolean isHidden(Path path) throws IOException {
+        return DefaultSftpFileSystemSingleton.INSTANCE.isHidden(path);
+    }
+
+    @Override
+    public FileStore getFileStore(Path path) throws IOException {
+        return DefaultSftpFileSystemSingleton.INSTANCE.getFileStore(path);
+    }
+
+    @Override
+    public void checkAccess(Path path, AccessMode... modes) throws IOException {
+        DefaultSftpFileSystemSingleton.INSTANCE.checkAccess(path, modes);
+    }
+
+    @Override
+    public <V extends FileAttributeView> V getFileAttributeView(Path path, Class<V> type, LinkOption... options) {
+        return DefaultSftpFileSystemSingleton.INSTANCE.getFileAttributeView(path, type, options);
+    }
+
+    @Override
+    public <A extends BasicFileAttributes> A readAttributes(Path path, Class<A> type, LinkOption... options)
+            throws IOException {
+        return DefaultSftpFileSystemSingleton.INSTANCE.readAttributes(path, type, options);
+    }
+
+    @Override
+    public Map<String, Object> readAttributes(Path path, String attributes, LinkOption... options) throws IOException {
+        return DefaultSftpFileSystemSingleton.INSTANCE.readAttributes(path, attributes, options);
+    }
+
+    @Override
+    public void setAttribute(Path path, String attribute, Object value, LinkOption... options) throws IOException {
+        DefaultSftpFileSystemSingleton.INSTANCE.setAttribute(path, attribute, value, options);
+    }
+
+}

sshd-sftp/src/test/java/org/apache/sshd/sftp/client/fs/SftpFileSystemTest.java

@@ -1081,7 +1081,7 @@ void fileSystemProviderServiceEntry() throws IOException {
                 }
 
                 assertFalse(found, "Multiple configurations: " + line);
-                assertEquals(SftpFileSystemProvider.class.getName(), line, "Mismatched configuration");
+                assertEquals(SftpFileSystemProviderFacade.class.getName(), line, "Mismatched configuration");
                 found = true;
             }
         }