New Session Manager

Important

"New Session Manager" is a community version of the "Non Session Manager" by Jonathan Moore Liles, who also wrote the majority of this API document, especially the API itself. The API is the same. Any technical changes or differences in behaviour are described in [API Versions and Behaviour Changes]. All other changes to this document can be reviewed by accessing the git log. This document is licensed under CC-By-Sa v2.5. See LICENSE

Important

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.

The "New Session Manager"-API is used by many music and audio programs in Linux distributions to allow any number of independent programs to be managed together as part of a logical session (i.e. a song). Thus, operations such as loading and saving are synchronized.

The API comprises a simple Open Sound Control (OSC) based protocol, along with some behavioral guidelines, which can easily be implemented by various applications.

This project contains a program called nsmd which is an implementation of the server side of the NSM API. nsmd can be controlled by direct OSC messages, or (more commonly) a GUI: Included in this package is the nsm-legacy-gui, which gets symlinked to "non-session-manager`. Another GUI is "Agordejo". Other applications exist that (partially) support the NSM API and are able to load clients, but they do not use the New-Session-Manager (or Non-Session-Manager) implementation and are therefore out of scope for this document.

However, the same server-side API can also be implemented by other programs (such as Carla), although consistency and robustness will likely suffer if non-NSM compliant clients are allowed to participate in a session.

There is no direct dependency for client implementations, as long as they can send and receive OSC. Some clients use liblo (the OSC library), which becomes a dependency if you choose to implement NSM-support with the provided header file nsm.h (extras/nsm.h/nsm.h in the git repository). Some clients use the provided single-file python library pynsm (extras/pynsm/nsmclient.py in the git repository) which has no dependencies outside the Python3 standard library.

The aim of this project is to thoroughly define the behavior required of clients. Often the difficulty with other session-management approaches has been not in implementing code-support for them, but in not defining rules and behaviour clearly enough.

As written above unambiguous rules are created by using RFC 2119 in this document. For the good of the user, these rules are meant to be followed and are non-negotiable. If an application does not conform to this specification it should be considered broken. Consistency across applications under session management is very important for a good user experience.

1. Client Behavior Under Session Management

Most graphical applications make available to the user a common set of file operations, typically presented under a File or Project menu.

These are: New, Open, Save, Save As, Close and Quit or Exit.

The following sub-sections describe how these options should behave when the application is part of an NSM session. These rules only apply when session management is active, that is, after the announce handshake described in the [NSM OSC Protocol] section. In order to provide a consistent and predictable user experience, it is critically important for applications to adhere to these guidelines.

1.1. File Menu

1.1.1. New

This option MAY empty/reset the current file or project (possibly after user confirmation). It MUST NOT allow the user to create a new project/file in another location.

1.1.2. Open

This option MUST be disabled.

The application MAY elect to implement an option called "Import into Session", which creates a copy of a file/project which is then saved at the session path provided by NSM.

1.1.3. Save

This option should behave as normal, saving the current file/project as established by the NSM open message.

This option MUST NOT present the user with a choice of where to save the file.

1.1.4. Save As

This option MUST be disabled.

The application MAY elect to implement an option called 'Export from Session', which creates a copy of the current file/project which is then saved in a user-specified location outside of the session path provided by NSM.

1.1.5. Close (as distinguished from Quit or Exit)

This option MUST be disabled unless its meaning is to disconnect the application from session management.

1.1.6. Quit or Exit

This option MAY behave as normal (possibly asking the user to confirm exiting), or MAY do nothing to only allow quit from the session-manager control. When the client supports :optional-gui: this option SHOULD be replaced with hiding the client’s GUI so a quit by window manager hides.

1.2. Data Storage

1.2.1. Internal Files

All project specific data created by a client MUST be stored in the per-client storage area provided by NSM. This includes all recorded audio and MIDI files, snapshots, etc. Only global configuration items, exports, and renders of the project may be stored elsewhere (wherever the user specifies).

1.2.2. External Files

Files required by the project but external to it (typically read-only data such as audio samples) SHOULD be referenced by creating a symbolic link within the assigned session area, and then referring to the symlink. This allows sessions to be archived and transported simply (e.g. with "tar -h") by tools that have no knowledge of the project formats of the various clients in the session. The symlinks thus created should, at the very least, be named after the files they refer to. Some unique component may be required to prevent collisions.

1.2.3. Session Root and Session Directories

Client programs MUST NOT handle the following themselves. This section is background-information.

NSM follows the XDG Base Directory Specifications

All existing and new sessions are directories below the session-root, which defaults to $XDG_DATA_HOME/nsm/, which usually results in $HOME/.local/share/nsm/.

Each session directory contains a file session.nsm with one client per line name:executable:UID\n For example:

JACKPatch:jackpatch:nBEIQ
jack_mixer:jack_mixer:nTXHV
Carla-Rack:carla-rack:nFAOD

nsmd loads and saves this file, client names are their self-reported names. The file format is final and frozen. Additions or changes SHALL NOT be made.

1.2.3.1. Subdirectories / Hierarchical Structure

Subdirectories MAY be made to organize sessions into meaningful structures, such as album/track or composer/genre/piece. For example: Johann Sebastian Bach/Kantaten/Wie schön leuchtet der Morgenstern. Which results in the same directory structure on disk. Session names can contain any characters that are supported by the underlying file system, usually UTF-8.

Subdirectories are created by either nsmd itself or by the users themselves, through their file manager or a GUI (while the session is not open).

The project_name from /nsm/server/new s:project_name accepts the format a/b/c/d.

Any session itself MUST be a "leaf" in this directory tree. A session MUST NOT contain further session subdirectories: any directory that contains a file session.nsm is the final element in the hierarchy.

1.2.3.2. Lockfiles

Because multiple nsmd can run at the same time we need to prevent accidental write-access to the same session by different nsm-daemons, and subsequently GUIs.

Therefore each currently open session creates a lockfile under $XDG_RUNTIME_DIR/nsm/ (usually /run/user/XXXX/nsm/) that tells nsmd to not open such a locked session. This directory gets cleaned by the operating system, preventing sessions to stay locked after e.g. a power failure.

The lockfile is named after the simple session name combined with a numeric ID for the session root. It is possible that two nsmd opened two different session roots, both with the same simple session name, e.g. "my song". Lockfiles are able to distinguish between those and will not prevent access in this scenario. The numeric ID is a djb2 hash modulo (%) 65521 of the session root directory (see src/file.cpp function simple_hash()).

The lockfile contains, on separate lines:

The absolute path to the session, including the root-dir, which could be overriden by nsmd --session-root, allowing two sessions of the same basic name in different roots.
the OSC URL of the server that runs this session, the same as $NSM_URL.

Example:

/home/johann/.local/share/nsm/cantatas/easter1751
osc.udp://myuser.localdomain:11287/


== NSM OSC Protocol

All message parameters are REQUIRED. All messages MUST be sent from the same socket as the `announce`
message, using the `lo_send_from` method of liblo or its equivalent, as the server uses the return
addresses to distinguish between clients.


Clients MUST create thier OSC servers using the same protocol (UDP,TCP) as found in `NSM_URL`.
`nsmd` itself is using UDP only.



=== Establishing a Connection

==== Announce

At launch, the client MUST check the environment for the value of `NSM_URL`. If present, the client
MUST send the following message to the provided address as soon as it is ready to respond to the
`/nsm/client/open` event:

[source%nowrap,OSC]
----
/nsm/server/announce s:application_name s:capabilities s:executable_name i:api_version_major i:api_version_minor i:pid
----

If `NSM_URL` is undefined, invalid, or unreachable, then the client should proceed assuming that
session management is unavailable.

`api_version_major` and `api_version_minor` must be the two parts of the version number of the NSM API
as defined by this document.

Note that if the application intends to register JACK clients, `application_name` MUST be the same as
the name that would normally be passed to `jack_client_open`. For example, Non-Mixer sends
"Non-Mixer" as its `application_name`. Applications MUST NOT register their JACK clients until
receiving an `open` message; the `open` message will provide a unique client name prefix suitable for
passing to JACK. This is probably the most complex requirement of the NSM API, but it isn't
difficult to implement, especially if the application simply wishes to delay its initialization
process briefly while awaiting the `announce` reply and subsequent `open` message.

`capabilities` MUST be a string containing a colon separated list of the special capabilities the
client possesses. e.g. `:dirty:switch:progress:`

`executable_name` MUST be the executable name that the program was launched with. For C programs,
this is simply the value of `argv[0]`. Note that hardcoding the name of the program here is not the
same as using, as the user may have launched the program from a script with a different name using
exec, or have created a symlink to the program. Getting the correct value in scripting languages
like Python can be more challenging.

.Available Client Capabilities
[options="header", stripes=even]
|===

|Name | Description

|switch         | client is capable of responding to multiple `open` messages without restarting
|dirty          | client knows when it has unsaved changes
|progress       | client can send progress updates during time-consuming operations
|message        | client can send textual status updates
|optional-gui   | client has an optional GUI

|===


==== Response

The server will respond to the client's announce message with the following message:

[source%nowrap,OSC]
----
/reply "/nsm/server/announce" s:message s:name_of_session_manager s:capabilities
----

`message` is a welcome message.

The value of `name_of_session_manager` will depend on the implementation of the NSM server. It
might say "New Session Manager", or it might say "Non Session Manager" etc. This is for display to
the user.

`capabilities` will be a string containing a colon separated list of special server capabilities.

Presently, the server `capabilities` are:

.Available Server Capabilities
[options="header", stripes=even]
|===

|Name | Description

|server-control | client-to-server control
|broadcast      | server responds to /nsm/server/broadcast message
|optional-gui   | server responds to optional-gui messages. This capability is always present and MUST be supported by any server implementation.

|===

A client should not consider itself to be under session management until it receives this response.
For example, the Non applications activate their "SM" blinkers at this time.

If there is an error, a reply of the following form will be sent to the client:


[source%nowrap,OSC]
----
/error "/nsm/server/announce" i:error_code s:error_message
----

The following table defines possible values of `error_code`:

.Response codes
[options="header", stripes=even]
|===

|Code | Meaning

|ERR_GENERAL            | General Error
|ERR_INCOMPATIBLE_API   | Incompatible API version
|ERR_BLACKLISTED        | Client has been blacklisted.

|===


=== Server to Client Control Messages

Compliant clients MUST accept the client control messages described in this section. All client
control messages REQUIRE a response. Responses MUST be delivered back to the sender (`nsmd`) from the
same socket used by the client in its `announce` message (by using `lo_send_from`) AFTER the action has
been completed or if an error is encountered. The required response is described in the subsection
for each message.

If there is an error and the action cannot be completed, then `error_code` MUST be set to a valid
error code (see <<Error Code Definitions>>) and `message` to a string describing the problem
(suitable for display to the user).

The reply can take one of the following two forms, where path MUST be the `path` of the message being
replied to (e.g. "nsm/client/save":

[source%nowrap,OSC]
----
/reply s:path s:message
----

[source%nowrap,OSC]
----
/error s:path i:error_code s:message
----


==== Quit

There is no message for this. Clients will receive the Unix SIGTERM signal and MUST close cleanly
IMMEDIATELY, without displaying any kind of dialog to the user and regardless of whether or not
unsaved changes would be lost. When a session is closed the application will receive this signal
soon after having responded to a `save` message.


[#server-to-client-control-messages-open]
==== Open

[source%nowrap,OSC]
----
/nsm/client/open s:path_to_instance_specific_project s:display_name s:client_id
----

`path_to_instance_specific_project` is a path name in the form client_name.ID, assigned to the
client for storing its project data. The client MUST choose one of the four strategies below to
save, so that every file in the session can be traced back to a client and, vice versa, a client
name.ID can be used to look up all its files. (For example to clean up the session dir)

* The client has no state and does not save at all
  ** and it MUST NOT misuse e.g. ~/.config to save session specific information e.g. synth-instrument settings
* The client may use the path client_name.ID directly, resulting in a file client_name.ID in the session directory
* The client may append its native file extension (e.g. `.json`) to the path client_name.ID
* The client may use the path as directory, creating arbitrary files below, for example recorded .wav.
 ** and it MUST NOT use the client ID below this point. This way the data stays transferable by hand to another client instance (in another session).
 ** best case practice is to always use the same file names, for example `client_name.ID/savefile.json`

If a project exists at the path, the client MUST immediately open it.

If a project does not exist at the path, then the client MUST immediately create and open a new one
at the specified path or, for clients which hold all their state in memory, store the path for
later use when responding to the `save` message.

No file or directory will be created at the specified path by the server. It is up to the client to
create what it needs.

For clients which HAVE NOT specified the `:switch:` capability, the `open` message will only be
delivered once, immediately following the `announce` response.

For clients which HAVE specified the `:switch:` capability, the client MUST immediately switch to the
specified project or create a new one if it doesn't exist.

Clients which are incapable of switching projects or are prone to crashing upon switching MUST NOT
include `:switch:` in their capability string.

If the user the is allowed to run two or more instances of the application simultaneously
then such an application MUST PRE-PEND the provided `client_id` string, followed by "/", to any
names it registers with common subsystems (e.g. JACK client names). This ensures that multiple
instances of the same application can be restored in any order without scrambling the JACK
connections or causing other conflicts.

The provided `client_id` will be a concatenation of the value of `application_name` sent by the
client in its `announce` message and a unique identifier.

Therefore, applications which create single JACK clients can use the value of `client_id` directly
as their JACK client name.

Applications which register multiple JACK clients (e.g. Carla or Non-Mixer) MUST PRE-PEND
`client_id` value, followed by "/", to the client names they register with JACK and the application
determined part MUST be unique for that (JACK) client.

For example, Carla is a plugin-host that loads each plugin as JACK client.
Suitable JACK client names are: `carla-jack-multi.nBAF/ZynAddSubFx` or `carla-jack-multi.nBAF/Helm`
Please note that ZynAddSubFx and Helm are *not ports* but clients. Each of them can have any number
of audio and midi ports below them.

Note that this means that the application MUST NOT register with JACK (or any
other subsystem requiring unique names) until it receives an `open` message from NSM. Likewise,
applications with the `:switch:` capability should close their JACK clients and re-create them with
using the new `client_id` (renaming JACK-clients is not possible, only ports).

A response is REQUIRED as soon as the open operation has been completed. Ongoing progress MAY be
indicated by sending messages to `/nsm/client/progress`.


===== Response

The client MUST respond to the 'open' message with:

[source%nowrap,OSC]
----
/reply "/nsm/client/open" s:message
----

Or

[source%nowrap,OSC]
----
/error "/nsm/client/open" i:error_code s:message
----


.Response codes
[options="header", stripes=even]
|===

|Code | Meaning

|ERR                  | General Error
|ERR_BAD_PROJECT      | An existing project file was found to be corrupt
|ERR_CREATE_FAILED    | A new project could not be created
|ERR_UNSAVED_CHANGES  | Unsaved changes would be lost
|ERR_NOT_NOW          | Operation cannot be completed at this time

|===


==== Save

[source%nowrap,OSC]
----
/nsm/client/save
----

This message will only be delivered after a previous `open` message, and may be sent any number of
times within the course of a session (including zero, if the user aborts the session).

===== Response

[source%nowrap,OSC]
----
/reply "/nsm/client/save" s:message
----

Or

[source%nowrap,OSC]
----
/error "/nsm/client/save" i:error_code s:message
----


.Response codes
[options="header", stripes=even]
|===

|Code | Meaning

|ERR                 | General Error
|ERR_SAVE_FAILED     | Project could not be saved
|ERR_NOT_NOW         | Operation cannot be completed at this time

|===


=== Server to Client Informational Messages

==== Session is Loaded

Accepting this message is optional. The intent is to signal to clients which may have some
interdependence (say, peer to peer OSC connections) that the session is fully loaded and all their
peers are available. Most clients will not need to act on this message. This message has no meaning
when a session is being built or run; only when it is initially loaded. Clients who intend to act
on this message MUST NOT do so by delaying initialization waiting for it.

[source%nowrap,OSC]
----
/nsm/client/session_is_loaded
----

This message does not require a response.


==== Show Optional Gui

If the client has specified the `optional-gui` capability, then it may receive this message from the
server when the user wishes to change the visibility state of the GUI. It doesn't matter if the
optional GUI is integrated with the program or if it is a separate program \(as is the case with
SooperLooper\). When the GUI is hidden, there should be no window mapped and if the GUI is a
separate program, it should be killed.

[source%nowrap,OSC]
----
/nsm/client/show_optional_gui
----

[source%nowrap,OSC]
----
/nsm/client/hide_optional_gui
----

This message does not require a response.



=== Client to Server Informational Messages

==== Optional GUI

If the client has specified the `optional-gui` capability, then it MUST send this message whenever
the state of visibility of the optional GUI has changed. It also MUST send this message after its
announce message to indicate the initial visibility state of the optional GUI.

The client SHOULD always start hidden, if not saved as visible. That implies the first load, after
adding to the session, SHOULD always be hidden.

It is the responsibility of the client to remember the visibility state of its GUI across session
loads.

[source%nowrap,OSC]
----
/nsm/client/gui_is_hidden
----

[source%nowrap,OSC]
----
/nsm/client/gui_is_shown
----

No response will be delivered.


==== Progress

[source%nowrap,OSC]
----
/nsm/client/progress f:progress
----

For potentially time-consuming operations, such as `save` and `open`, progress updates may be
indicated throughout the duration by sending a floating point value between 0.0 and 1.0, 1.0
indicating completion, to the NSM server.

The server will not send a response to these messages, but will relay the information to the user.

Note that even when using the `progress` feature, the final response to the `save` or `open`
message is still REQUIRED.

Clients which intend to send progress messages MUST include `:progress:` in their `announce`
capability string.


==== Dirtiness

[source%nowrap,OSC]
----
/nsm/client/is_dirty
----

[source%nowrap,OSC]
----
/nsm/client/is_clean
----

Some clients may be able to inform the server when they have unsaved changes pending. Such clients
may optionally send `is_dirty` and `is_clean` messages.

Clients which have and use this capability MUST include `:dirty:` in their `announce` capability string.

==== Status Messsages

[source%nowrap,OSC]
----
/nsm/client/message i:priority s:message
----

Clients may send miscellaneous status updates to the server for possible display to the user. This
may simply be chatter that is normally written to the console. `priority` MUST be a number from 0
to 3, 3 being the most important.

Clients which have and use this capability MUST include `:message:` in their `announce` capability
string.


=== Error Code Definitions

.Error Code Definitions
[options="header", stripes=even]
|===

|Symbolic Name   | Integer Value

|ERR_GENERAL            | -1
|ERR_INCOMPATIBLE_API   | -2
|ERR_BLACKLISTED        | -3
|ERR_LAUNCH_FAILED      | -4
|ERR_NO_SUCH_FILE       | -5
|ERR_NO_SESSION_OPEN    | -6
|ERR_UNSAVED_CHANGES    | -7
|ERR_NOT_NOW            | -8
|ERR_BAD_PROJECT        | -9
|ERR_CREATE_FAILED      | -10

|===

=== Client to Server Control

If the server publishes the `:server-control:` capability, then clients can also initiate action by
the server. For example, a client might implement a 'Save All' option which sends a
`/nsm/server/save` message to the server, rather than requiring the user to switch to the session
management interface to effect the save.


=== Server Control API

The session manager not only manages clients via OSC, but it is itself controlled via OSC messages.
The server responds to the following messages.

All of the following messages will be responded to, at the sender's address, with one of the two
following messages:

[source%nowrap,OSC]
----
/reply s:path s:message
----

[source%nowrap,OSC]
----
/error s:path i:error_code s:message
----

The first parameter of the reply is the path to the message being replied to. The `/error` reply
includes an integer error code (non-zero indicates error). `message` will be a description of the
error.

The possible errors are:

.Responses
[options="header", stripes=even]
|===

|Code   |Meaning

|ERR_GENERAL         | General Error
|ERR_LAUNCH_FAILED   | Launch failed
|ERR_NO_SUCH_FILE    | No such file
|ERR_NO_SESSION      | No session is open
|ERR_UNSAVED_CHANGES | Unsaved changes would be lost

|===


* `/nsm/server/add s:executable_name`
  ** Adds a client to the current session.

* `/nsm/server/save`
  ** Saves the current session.

* `/nsm/server/open s:project_name`
  ** Saves the current session and loads a new session.

* `/nsm/server/new s:project_name`
  ** Saves the current session and creates a new session.

* `/nsm/server/duplicate s:new_project`
  ** Saves and closes the current session, makes a copy, and opens it.

* `/nsm/server/close`
  ** Saves and closes the current session.

* `/nsm/server/abort`
  ** Closes the current session WITHOUT SAVING

* `/nsm/server/quit`
  ** Saves and closes the current session and terminates the server.

* `/nsm/server/list`
  ** Lists available projects. One `/reply` message will be sent for each existing project.
  ** Afer listing the last session one final `/reply` with `/nsm/server/list, ""` will be send. That is an empty string.


==== Client to Client Communication

If the server includes `:broadcast:` in its capability string, then clients may send broadcast
messages to each other through the NSM server. Clients may send messages to the server at the path
`/nsm/server/broadcast`.

The format of this message is as follows:

[source%nowrap,OSC]
----
/nsm/server/broadcast s:path [arguments...]
----

The message will then be relayed to all clients in the session at the path `path` (with the
arguments shifted by one).

For example the message:


[source%nowrap,OSC]
----
/nsm/server/broadcast /tempomap/update "0,120,4/4:12351234,240,4/4"
----

Would broadcast the following message to all clients in the session (except for the sender), some
of which might respond to the message by updating their own tempo maps.


[source%nowrap,OSC]
----
/tempomap/update "0,120,4/4:12351234,240,4/4"
----

The Non programs use this feature to establish peer to peer OSC communication by symbolic names
(client IDs) without having to remember the OSC URLs of peers across sessions.


== API Versions and Behaviour Changes

Here we will document all technical changes or differences in behaviour together with their API and
project version numbers. The term "original" refers to Non Session Manager and "new" refers to New
Session Manager.

Version numbers follow link:https://semver.org/spec/v2.0.0.html[Semantic Versioning 2.0.0]

.Semantic Versioning Scheme

Given a version number MAJOR.MINOR.PATCH, increment the:

MAJOR version when you make incompatible API changes, MINOR version when you add functionality in a backwards compatible manner, and PATCH version when you make backwards compatible bug fixes.

.NSM Version Numbers
[options="header", stripes=even]
|===

|Subject     | Version

|Non Session Manager at moment of fork          | 1.2 (June 2020)
|Non Session Manager API    | 1.0 link:https://github.com/original-male/non/blob/master/session-manager/src/nsmd.C[NON nsmd.C]
|Original API Document      | 1.0 link:http://non.tuxfamily.org/nsm/API.html[non.tuxfamily.org/nsm/API.html]
|New Session Manager        | 1.6.0
|New Session Manager API    | 1.1.2 link:https://github.com/jackaudio/new-session-manager/blob/master/src/nsmd.cpp[NEW nsmd.cpp]
|New API Document   | 1.5.0 link:#[Here]

|===


=== Guidelines

The most important factor in decision making is to keep client compatibility at 100%.
No client will ever receive an unrequested OSC message except those in API 1.0.0.

Messages that drastically change existing `/nsm/client/` or `/nsm/server` behaviour require an
inrecement to `API_VERSION_MAJOR`, which we want to avoid.

`nsmd` checks if the clients `API_VERSION_MAJOR` is greater than its own and refuses the client
with `ERR_INCOMPATIBLE_API`.

All changes (that concern client/server behaviour) that increment `API_VERSION_MINOR` will be
request-only or gated by new capabilities (e.g. `:optional-gui:`). `nsmd` will not send any
messages if a capability was not sent by the client in <<Announce,`announce`>>. This includes
mostly optional features about requesting extra information.

New actions for server-control, for example a hypothetical `/nsm/server/save_as`, which would be
triggered by the client and would only be *answered* by the server ("no unrequested message") will
increment `API_VERSION_MINOR`.

All changes that increment `API_VERSION_PATCH` will not have any effect on behaviour, except to
fix clear problems, where "problem" is defined by having a different effect than described in this
document, which includes technical problems such as crashes.

All messages regarding GUI-communication that start with `/nsm/gui/...` were undocumented in API
1.0.0 and only used by `non-session-manager` / `nsm-legacy-gui`. Until properly documented in this
document this part of the API is considered unstable and may change at any time without notice.
However, when changing already existing messages and behaviour it MAY increment `API_VERSION_MINOR`
or `API_VERSION_PATCH`. In that case it will appear in the list below.

Last factor of compatibility is that any unknown message sent to `nsmd` will just print a warning
message to stdout, but will otherwise be ignored. This secures a stable server, even when a client
misbehaves and sends too-new messages outside of announced :capabilites:

=== Changes in API Version 1.1.0

Rewritten API document without code changes to adapt to existing code or existing client behaviour:

* Changed versioning scheme to Semantic Versioning with three positions Major.Minor.Patch
* <<Quit or Exit>> SHOULD hide instead of exiting when :optional-gui: is supported and MAY not
    act on the quit through menu otherwise.
* <<#server-to-client-control-messages-open,Open>>: Make clear that there are only certain
    possibilities for save paths. We added MUST because the rule was just implied before.
* <<#server-to-client-control-messages-open,Open>>: Make clear that the delimiter for
    multi-jack clients is "/".
* <<Optional GUI>> SHOULD start hidden, always after a fresh add to the session. After that saving
    the visibility state may override it for next time.
* <<Progress>> MUST be announced in :capabilities: . Before there was a lower case "should",
    which means nothing. Parallel-examples in the specs cleary say that supporting optional features must be announced first.
  ** Same for <<Dirtiness>> and <<Status Messsages>>.
* <<Status Messsages>> have priority numbers between 0 and 3, so they MUST send that.
    It was never an arbitrary value.

Code changes:

* <<Server Control API>>: `/nsm/server/list` chain of single OSC messages, one for each session,
    is now finalized with sending and empty string "" as session name. Previously this was just
    a symbolically irrelevant console message `"Done."`
* Replies to `/nsm/server/save` etc. will now be sent back to the sender and not falsely to the last
   client who replied to `/nsm/client/save`.  This alone would only require API_VERSION_PATCH
   increment, but we are already incrementing minor.
* <<Server Control API>>: `/nsm/server/add` was replying with an undocumented error code on success.
   Instead, as this document always specificed, it now sends `"/reply", path, "Launched."`.
   Again, this would have been just API_VERSION_PATCH on its own.

Undocumented (Unstable) `/nsm/gui` protocol

* Send client status after a GUI attaches to running server. This
    was not happening before, but it was the intention. It was just broken in nsmd.cpp. This alone
    would only require API_VERSION_PATCH increment, but we are already incrementing minor.
* Send label "launch error!" when a program is added (or loaded) that
    does not exist in $PATH. This requires no adaptation of any client, server or GUI because labels
    are arbitrary already and this is not meant for automatic parsing, but as user information.
* `/nsm/gui/session/name` will now always send the same parameter format, regardless of how the session was opened:
   simple-session-name, relative session path with subdirs below session-root.
* When a GUI announces itself to nsmd it will receive the absolute path to the session directory
    through the message `/nsm/gui/session/root`. This is not a new addition but was already in
    non-session-manager git.

=== Changes in API Version 1.1.1

* Server-capability :optional-gui: is now mandatory for SERVER implementations. Reasoning:
This is an  important core feature of NSM and thus will be treated as such by guaranteeing it to exist.
After looking at all currently known clients and server-implementations it turns out that all servers
support :optional-gui: and the vast majority of clients not only support it, but actually assume it
and do _not_ test for the server capability, as it was written in this document.
There are now two choices: Adjust this document to the (good) reality or consider all clients broken.
Summary: We consider this API document wrong and therefore fix it, thus increasing API version
patch-level from 1.1.0 to 1.1.1

* Add API-section "Subdirectories / Hierarchical Structure" that explains the session directory.
This behaviour was already the case for nsm-legacy-gui and nsmd 1.5.0 was patched to adhere to this
behaviour more strictly as well, removing false session entries in 3rd party clients such as Agordejo.

=== Changes in API Version 1.1.2

* nsmd now follows the XDG Base Directory Specifications for it's session root and lock files. This
if of no consequence to clients but required documentation nevertheless, which was described as
"background information" in the new section.

New Session Manager - API