adds fallback to posix-rename@openssh.com extension if possible and c…

src/main/java/net/schmizz/sshj/sftp/SFTPEngine.java

@@ -237,16 +237,71 @@ public void rename(String oldPath, String newPath, Set<RenameFlags> flags)
         if (operativeVersion < 1)
             throw new SFTPException("RENAME is not supported in SFTPv" + operativeVersion);
 
-        final Request request = newRequest(PacketType.RENAME).putString(oldPath, sub.getRemoteCharset()).putString(newPath, sub.getRemoteCharset());
-        // SFTP Version 5 introduced rename flags according to Section 6.5 of the specification
-        if (operativeVersion >= 5) {
-            long renameFlagMask = 0L;
-            for (RenameFlags flag : flags) {
-                renameFlagMask = renameFlagMask | flag.longValue();
+        // request variables to be determined
+        PacketType type = PacketType.RENAME; // Default
+        long renameFlagMask = 0L;
+        String serverExtension = null;
+
+        if (!flags.isEmpty()) {
+            // SFTP Version 5 introduced rename flags according to Section 6.5 of the specification
+            if (operativeVersion >= 5) {
+                for (RenameFlags flag : flags) {
+                    renameFlagMask = renameFlagMask | flag.longValue();
+                }
             }
-            request.putUInt32(renameFlagMask);
+            // Try to find a fallback solution if flags are not supported by the server.
+
+            // "posix-rename@openssh.com" provides ATOMIC and OVERWRITE behaviour.
+            //  From the SFTP-spec, Section 6.5:
+            // "If SSH_FXP_RENAME_OVERWRITE is specified, the server MAY perform an atomic rename even if it is
+            // not requested."
+            // So, if overwrite is allowed we can always use the posix-rename as a fallback.
+            else if (flags.contains(RenameFlags.OVERWRITE) &&
+                    supportsServerExtension("posix-rename","openssh.com")) {
+
+                type = PacketType.EXTENDED;
+                serverExtension = "posix-rename@openssh.com";
+            }
+
+            // Because the OVERWRITE flag changes the behaviour in a possibly unintended way, it has to be
+            // explicitly requested for the above fallback to be applicable.
+            // Tell this to the developer if ATOMIC is requested without OVERWRITE.
+            else if (flags.contains(RenameFlags.ATOMIC) &&
+                    !flags.contains(RenameFlags.OVERWRITE) &&
+                    !flags.contains(RenameFlags.NATIVE) && // see next case below
+                    supportsServerExtension("posix-rename","openssh.com")) {
+
+                    throw new SFTPException("RENAME-FLAGS are not supported in SFTPv" + operativeVersion + " but " +
+                            "the \"posix-rename@openssh.com\" extension could be used as fallback if OVERWRITE " +
+                            "behaviour is acceptable (needs to be activated via RenameFlags.OVERWRITE).");
+            }
+
+            // From the SFTP-spec, Section 6.5:
+            // "If flags includes SSH_FXP_RENAME_NATIVE, the server is free to do the rename operation in whatever
+            // fashion it deems appropriate. Other flag values are considered hints as to desired behavior, but not
+            // requirements."
+            else if (flags.contains(RenameFlags.NATIVE)) {
+                log.debug("Flags are not supported but NATIVE-flag allows to ignore other requested flags: " +
+                        flags.toString());
+            }
+
+            // finally: let the user know that the server does not support what was asked
+            else throw new SFTPException("RENAME-FLAGS are not supported in SFTPv" + operativeVersion + " and no " +
+                        "supported server extension could be found to achieve a similar result.");
         }
 
+        // build and send request
+        final Request request = newRequest(type);
+
+        if (serverExtension != null)
+            request.putString(serverExtension);
+
+        request.putString(oldPath, sub.getRemoteCharset())
+                .putString(newPath, sub.getRemoteCharset());
+
+        if (renameFlagMask != 0L)
+            request.putUInt32(renameFlagMask);
+
         doRequest(request).ensureStatusPacketIsOK();
     }