GH-855: single SftpClient in SftpFileSystem

Drop the SftpClient pool and use a single thread-safe SftpClient per
SftpFileSystem. The SftpClient is closed when the file system is closed.
If the server side should close the channel, operations using this
client may fail, but a server should not close channels unless there is
no traffic. If a channel is closed by the server because it is idle, the
SftpFileSystem will re-create a new SftpClient, thus opening a new
channel, on the next operation.

It is possible that a server decides to close a previously idle channel
just in the moment the client-side wants to use that channel for a new
operation. The new operation would then fail. But this possibility has
always existed in all previous versions of the SftpFileSystem code.

The main advantage of using a single SftpClient is that all SFTP
operations are done in the same channel. This can be important with
servers that restrict the number of concurrently open channels per SSH
session.

Author: tomaswolf

CHANGES.md

@@ -34,6 +34,7 @@
 * [GH-809](https://github.com/apache/mina-sshd/pull/809) Fix server-side authentication for FIDO/U2F sk-* keys with flags in `authorized_keys`
 * [GH-827](https://github.com/apache/mina-sshd/issues/827) Don't fail on invalid `known_hosts` lines; log and skip them
 * [GH-830](https://github.com/apache/mina-sshd/issues/830) EC public keys: let Bouncy Castle generate X.509 encodings with the curve OID as algorithm parameter
+* [GH-855](https://github.com/apache/mina-sshd/issues/855) SFTP: use a single `SftpClient` per `SftpFileSystem`
 * [GH-856](https://github.com/apache/mina-sshd/issues/856) Fix using ed25519 with BC-FIPS
 * [GH-861](https://github.com/apache/mina-sshd/issues/861) SFTP client: prevent sending zero-length writes in `SftpOutputStreamAsync`
 
@@ -46,5 +47,9 @@ safe despite that CVE in the dependency.
 
 ## Potential Compatibility Issues
 
+[GH-855](https://github.com/apache/mina-sshd/issues/855) changes the way `SftpFileSystem` deals with multiple threads. It newly uses a single SSH channel via a single thread-safe `SftpClient`, serializing writes at the channel level. The properties relating to the previously used pool of `SftpClient`s have been deprecated and have no effect anymore. User applications using the library should not see any changes.
+
+A beneficial side-effect of this change is that an `SftpFileSystem` creates the SSH session and SFTP channel only when the first SFTP operation is performed. Previously the session and channel were opened right away when an `SftpFileSystem` was instantiated.
+
 ## Major Code Re-factoring
 

sshd-sftp/src/main/java/org/apache/sshd/sftp/SftpModuleProperties.java

@@ -63,8 +63,10 @@ public final class SftpModuleProperties {
      * The maximum size of the channel pool used by an {@link org.apache.sshd.sftp.client.fs.SftpFileSystem}; by default
      * 8. The value must be > zero.
      *
-     * @see org.apache.sshd.sftp.client.fs.SftpFileSystem
+     * @see        org.apache.sshd.sftp.client.fs.SftpFileSystem
+     * @deprecated since 2.17.0 this has no effect since there is no pool anymore
      */
+    @Deprecated
     public static final Property<Integer> POOL_SIZE
             = Property.integer("sftp-fs-pool-size", 8);
 
@@ -76,8 +78,10 @@ public final class SftpModuleProperties {
      * The duration should not be shorter than 1 millisecond. If it is, 1 millisecond will be assumed.
      * </p>
      *
-     * @see org.apache.sshd.sftp.client.fs.SftpFileSystem
+     * @see        org.apache.sshd.sftp.client.fs.SftpFileSystem
+     * @deprecated since 2.17.0 this has no effect since there is no pool anymore
      */
+    @Deprecated
     public static final Property<Duration> POOL_LIFE_TIME
             = Property.duration("sftp-fs-pool-life-time", Duration.ofSeconds(10));
 
@@ -87,8 +91,10 @@ public final class SftpModuleProperties {
      * {@link #POOL_SIZE}, channels will not expire and will be closed only then the file system is closed, or if the
      * server closes them.
      *
-     * @see org.apache.sshd.sftp.client.fs.SftpFileSystem
+     * @see        org.apache.sshd.sftp.client.fs.SftpFileSystem
+     * @deprecated since 2.17.0 this has no effect since there is no pool anymore
      */
+    @Deprecated
     public static final Property<Integer> POOL_CORE_SIZE
             = Property.integer("sftp-fs-pool-core-size", 1);