draft async extension

doc/design/external_special_remote_protocol.mdwn

@@ -45,10 +45,10 @@ Recent versions of git-annex respond with a message indicating
 protocol extensions that it supports. Older versions of
 git-annex do not send this message.
 
-	EXTENSIONS INFO
+	EXTENSIONS INFO ASYNC
 
-The special remote can respond to that with its own EXTENSIONS message, which
+The special remote can respond to that with its own EXTENSIONS message, listing
-could have its own protocol extension details, but none are currently used.
+any extensions it wants to use. 
 (It's also fine to reply with UNSUPPORTED-REQUEST.)
 
 	EXTENSIONS 
@@ -132,8 +132,9 @@ The following requests *must* all be supported by the special remote.
   It's important that, while a Key is being stored, `CHECKPRESENT`
   not indicate it's present until all the data has been transferred.  
   While the transfer is running, the remote can send any number of
-  `PROGRESS` messages. Once the transfer is done, it finishes by sending
+  `PROGRESS` messages to indicate its progress. It can also send any of the
-  one of these replies:
+  other special remote messages. Once the transfer is done, it finishes by
+  sending one of these replies:
   * `TRANSFER-SUCCESS STORE|RETRIEVE Key`  
     Indicates the transfer completed successfully.
   * `TRANSFER-FAILURE STORE|RETRIEVE Key ErrorMsg`  
@@ -170,10 +171,10 @@ the special remote can reply with `UNSUPPORTED-REQUEST`.
   Sent to indicate protocol extensions which git-annex is capable
   of using. The list is a space-delimited list of protocol extension
   keywords. The remote can reply to this with its own EXTENSIONS list.
+  See the section on extensions below for details.
   * `EXTENSIONS List`  
-    Sent in response to a EXTENSIONS request, the List could be used to indicate
+    Sent in response to a EXTENSIONS request, to indicate the protocol
-    protocol extensions that the special remote uses, but there are currently
+    extensions that the special remote is using.
-    no such extensions.
 * `LISTCONFIGS`  
   Requests the remote to return a list of settings it uses (with
   `GETCONFIG` and `SETCONFIG`). Providing a list makes `git annex initremote`
@@ -381,14 +382,12 @@ handling a request.
 * `DEBUG message`  
   Tells git-annex to display the message if --debug is enabled.  
   (git-annex does not send a reply to this message.)
-
-These messages are protocol extensions; it's only safe to send them to
-git-annex after it sent a EXTENSIONS that included the name of the message.
-
 * `INFO message`
   Tells git-annex to display the message to the user.  
   When git-annex is in --json mode, the message will be emitted immediately
   in its own json object, with an "info" field.  
+  This message is a protocol extension; it's only safe to send it to
+  git-annex after it sent an EXTENSIONS that included INFO.  
   (git-annex does not send a reply to this message.)  
 
 ## general messages
@@ -403,6 +402,17 @@ remote.
   git-annex will not talk to it any further. If the program receives
   an ERROR from git-annex, it can exit with its own ERROR.
 
+## extensions
+
+These protocol extensions are currently supported.
+
+* `INFO`  
+  This makes the `INFO` message available to use.
+* `ASYNC`  
+  This lets multiple actions be performed at the same time by
+  a single external special remote program, rather than starting multiple
+  programs. See the [[async_appendix]] for details.
+
 ## signals
 
 The external special remote program should not block SIGINT, or SIGTERM.

doc/design/external_special_remote_protocol/async_appendix.mdwn

@@ -0,0 +1,117 @@
+(This is a draft and not implemented yet.)
+
+This is an appendix to the [[external_special_remote_protocol]].
+
+[[!toc]]
+
+## introduction
+
+Normally, an external special remote can only be used to do one thing at a
+time. When git-annex has concurrency enabled, it will start up multiple
+processes for the same external special remote. 
+
+This extension lets a single external special remote process handle
+multiple concurrent requests, which can be useful if multiple processes
+would use too many resources, or if it can be better coordinated using a
+single process.
+
+## protocol overview
+
+This extension is negotiated by git-annex sending an `EXTENSIONS` message
+that includes `ASYNC`, and the external special remote responding in kind.
+The rest of the protocol startup is as usual.
+
+	VERSION 1
+	EXTENSIONS INFO ASYNC
+	EXTENSIONS ASYNC
+	PREPARE
+	PREPARE-SUCCESS
+
+Suppose git-annex wants to make some transfers. So it sends:
+
+	TRANSFER RETRIEVE Key1 file1
+
+The special remote can at this point send any of the 
+[special remote messages](https://git-annex.branchable.com/design/external_special_remote_protocol/#index5h2)
+it needs as usual, like `GETCONFIG` and `DIRHASH`, getting responses back from
+git-annex. git-annex will not send any other requests yet.
+(This is the only time it can send those messages, because git-annex
+is waiting on its reply here.)
+
+Once it's ready to start the async transfer, the special remote sends
+`START-ASYNC`, with an identifier for this async job. (The identifier can
+be anything you want to use, but the key is generally a good choice.)
+
+	START-ASYNC Key1
+
+Once that's sent, git-annex can send its next request immediately,
+while that transfer is still running. For example, it might request a
+second transfer, and the special remote can reply when it's started that
+transfer too:
+
+	TRANSFER RETRIEVE Key2 file2
+	START-ASYNC Key2
+
+To indicate progress of transfers, the special remote can send
+`UPDATE-ASYNC` messages, followed by usual PROGRESS messages:
+
+	UPDATE-ASYNC Key1
+	PROGRESS 10
+	UPDATE-ASYNC Key2
+	PROGRESS 500
+	UPDATE-ASYNC Key1
+	PROGRESS 20
+
+Once a transfer is done, the special remote indicates this with an
+`END-ASYNC` message, followed by the usual `TRANSFER-SUCCESS` or
+`TRANSFER-FAILURE`:
+
+	END-ASYNC Key2
+	TRANSFER-SUCCESS RETRIEVE Key2
+	UPDATE-ASYNC Key1
+	PROGRESS 100
+	END-ASYNC Key1
+	TRANSFER-SUCCESS RETRIEVE Key1
+
+This is not limited to transfers. Any and all requests that git-annex
+makes can be handled async if the special remote wants to. For example:
+
+	CHECKPRESENT Key3
+	START-ASYNC Key3
+	CHECKPRESENT Key4
+	START-ASYNC Key4
+	REMOVE Key5
+	START_ASYNC Key5
+	END-ASYNC Key3
+	CHECKPRESENT-SUCCESS Key3
+	END-ASYNC Key4
+	CHECKPRESENT-FAILURE Key4
+	END-ASYNC Key5
+	REMOVE-SUCCESS Key5
+
+## non-async replies
+
+It's also fine to not use `START-ASYNC` for a request, and instead
+use the usual protocol for the reply. This will prevent git-annex from
+sending any other requests until it sees the reply. 
+
+Since git-annex only runs one external special remote process for
+async-capable remotes, anything not processed async may result in
+suboptimal performance, when the user has requested concurrency.
+
+## added messages
+
+Here's the details about the additions to the protocol.
+
+* `START-ASYNC JobId`  
+  Can be sent in response to any request git-annex sends. Indicates that
+  the request will be performed async. This lets git-annex immediately
+  send its next request, without waiting for this one to finish.
+  The JobId is an arbitrary string, typically a number or key etc.
+* `END-ASYNC JobId`  
+  Indicates that an async job is complete. Must be followed by
+  a protocol reply, indicating the result of the job.
+* `UPDATE-ASYNC JobId`  
+  Used to send additional information about an async job. Must be followed
+  by a protocol message giving the information. git-annex does not send any
+  reply. Used only for PROGRESS so far.

doc/todo/idea__58___external_special_remote___34__async__34___protocol_for_transfers/comment_6_877ae38938a279fa100e58c03990e81a._comment

@@ -0,0 +1,17 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 6"""
+ date="2020-08-11T18:34:03Z"
+ content="""
+[[design/external_special_remote_protocol/async_appendix]] has a draft
+protocol extension.
+
+I improved on the design, so any and all requests can be handled async,
+or  sequentially, as the external special remote prefers. Had to add async
+job ids, but the protocol simplicity was worth it.
+
+(Implementation will be something like, a thread relaying to and from the
+special remote, with requests sent to it when it's not blocked, and with
+its async replies sent back to the corresponding requester based on the
+JobId.)
+"""]]