add support for conditional resuming of uploads

Signed-off-by: Nicola Murino <nicola.murino@gmail.com>

docs/full-configuration.md

@@ -62,13 +62,14 @@ The configuration file contains the following sections:
 
 - **"common"**, configuration parameters shared among all the supported protocols
   - `idle_timeout`, integer. Time in minutes after which an idle client will be disconnected. 0 means disabled. Default: 15
-  - `upload_mode` integer. 0 means standard: the files are uploaded directly to the requested path. 1 means atomic: files are uploaded to a temporary path and renamed to the requested path when the client ends the upload. Atomic mode avoids problems such as a web server that serves partial files when the files are being uploaded. In atomic mode, if there is an upload error, the temporary file is deleted and so the requested upload path will not contain a partial file. 2 means atomic with resume support: same as atomic but if there is an upload error, the temporary file is renamed to the requested path and not deleted. This way, a client can reconnect and resume the upload. Ignored for cloud-based storage backends (uploads are always atomic and resume is not supported for these backends) and for SFTP backend if buffering is enabled. Default: 0
+  - `upload_mode` integer. 0 means standard: the files are uploaded directly to the requested path. 1 means atomic: files are uploaded to a temporary path and renamed to the requested path when the client ends the upload. Atomic mode avoids problems such as a web server that serves partial files when the files are being uploaded. In atomic mode, if there is an upload error, the temporary file is deleted and so the requested upload path will not contain a partial file. 2 means atomic with resume support: same as atomic but if there is an upload error, the temporary file is renamed to the requested path and not deleted. This way, a client can reconnect and resume the upload. Ignored for cloud-based storage backends (uploads are always atomic and upload resume is not supported, by default, for these backends) and for SFTP backend if buffering is enabled. Default: `0`
   - `actions`, struct. It contains the command to execute and/or the HTTP URL to notify and the trigger conditions. See [Custom Actions](./custom-actions.md) for more details
     - `execute_on`, list of strings. Valid values are `pre-download`, `download`, `first-download`, `pre-upload`, `upload`, `first-upload`, `pre-delete`, `delete`, `rename`, `mkdir`, `rmdir`, `ssh_cmd`, `copy`. Leave empty to disable actions.
     - `execute_sync`, list of strings. Actions, defined in the `execute_on` list above, to be performed synchronously. The `pre-*` actions are always executed synchronously while the other ones are asynchronous. Executing an action synchronously means that SFTPGo will not return a result code to the client (which is waiting for it) until your hook have completed its execution. Leave empty to execute only the defined `pre-*` hook synchronously
     - `hook`, string. Absolute path to the command to execute or HTTP URL to notify.
   - `setstat_mode`, integer. 0 means "normal mode": requests for changing permissions, owner/group and access/modification times are executed. 1 means "ignore mode": requests for changing permissions, owner/group and access/modification times are silently ignored. 2 means "ignore mode if not supported": requests for changing permissions and owner/group are silently ignored for cloud filesystems and executed for local/SFTP filesystem. Requests for changing modification times are always executed for local/SFTP filesystems and are executed for cloud based filesystems if the target is a file and there is a metadata plugin available. A metadata plugin can be found [here](https://github.com/sftpgo/sftpgo-plugin-metadata).
   - `rename_mode`, integer. By default (`0`), renaming of non-empty directories is not allowed for cloud storage providers (S3, GCS, Azure Blob). Set to `1` to enable recursive renames for these providers, they may be slow, there is no atomic rename API like for local filesystem, so SFTPGo will recursively list the directory contents and do a rename for each entry (partial renaming and incorrect disk quota updates are possible in error cases). Default `0`.
+  - `resume_max_size`, integer. defines the maximum size allowed, in bytes, to resume uploads on storage backends with immutable objects. By default, resuming uploads is not allowed for cloud storage providers (S3, GCS, Azure Blob) because SFTPGo must rewrite the entire file. Set to a value greater than 0 to allow resuming uploads of files smaller than or equal to the defined size. Please note that uploads for these backends are still atomic, the client must intentionally upload a portion of the target file and then resume uploading.. Default `0`.
   - `temp_path`, string. Defines the path for temporary files such as those used for atomic uploads or file pipes. If you set this option you must make sure that the defined path exists, is accessible for writing by the user running SFTPGo, and is on the same filesystem as the users home directories otherwise the renaming for atomic uploads will become a copy and therefore may take a long time. The temporary files are not namespaced. The default is generally fine. Leave empty for the default.
   - `proxy_protocol`, integer. Support for [HAProxy PROXY protocol](https://www.haproxy.org/download/1.8/doc/proxy-protocol.txt). If you are running SFTPGo behind a proxy server such as HAProxy, AWS ELB or NGINX, you can enable the proxy protocol. It provides a convenient way to safely transport connection information such as a client's address across multiple layers of NAT or TCP proxies to get the real client IP address instead of the proxy IP. Both protocol versions 1 and 2 are supported. If the proxy protocol is enabled in SFTPGo then you have to enable the protocol in your proxy configuration too. For example, for HAProxy, add `send-proxy` or `send-proxy-v2` to each server configuration line. The PROXY protocol is supported for SSH/SFTP and FTP/S. The following modes are supported:
     - 0, disabled