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178 lines
7.5 KiB
ReStructuredText
178 lines
7.5 KiB
ReStructuredText
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CLI and API compatibility policy
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================================
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Human users and API users rely on the mpv/libmpv/scripting/IPC interface not
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breaking their use case. On the other hand, active development occasionally
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requires breaking things, such as removing old options or changing options in
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a way that may break.
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This document lists rules when, what, and how incompatible changes can be made.
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It's interesting both for mpv developers, who want to change user-visible parts,
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and mpv users, who want to know what is guaranteed to remain stable.
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Any of the rules below may be overridden by statements in more specific
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documentation (for example, if the manpage says that a particular option may be
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removed any time, it means that, and the option probably won't even go through
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deprecation).
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Additions
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---------
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Additions are basically always allowed. API users etc. are supposed to deal with
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the possibility that for example new API functions are added. Some parts of the
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API may document how they are extended.
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Options, commands, properties, events, hooks (command interface)
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----------------------------------------------------------------
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All of these are important for interfacing both with end users and API users
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(which include Lua scripts, libmpv, and the JSON IPC). As such, they constitute
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a large part of the user interface and APIs.
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All incompatible changes to this must be declared in interface-changes.rst.
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(This may also list compatible additions, but it's not a requirement.)
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Degrees of importance and compatibility preservation
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Critical and central parts of the command interface have the strictest
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requirements. It may not be reasonable to break them, and other means to achieve
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some change have to be found. For example, the "seek" command is a bit of a
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mess, but since changing it would likely affect almost every user, it may be
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impossible to break at least the commonly used syntax. If changed anyway, there
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should be a deprecation period of at least 1 year, during which the command
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still works, and possibly a warning should remain even after this.
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Important/often used parts must be deprecated for at least 2 releases before
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they can be broken. There must be at least 1 release where the deprecated
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functionality still works, and a replacement is available (if technically
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reasonable). For example, a feature deprecated in mpv 0.30.0 may be removed in
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mpv 0.32.0. Minor releases do not count towards this.
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Less useful parts can be broken immediately, but must come with some sort of
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removal warning-
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Parts for debugging and testing may be removed any time, potentially even
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without any sort of documentation.
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Currently, the importance of a part is not documented and not even well-defined,
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which is probably a mistake.
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Renaming or removing options
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Typically, renaming an option can be done in a compatible way with OPT_REPLACED.
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You may need to check whether the corresponding properties still work (including
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messy details like observing properties for changes).
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OPT_REMOVED can be used to inform the user of alternatives or reasons for the
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removal, which is better than an option not found error. Likewise,
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m_option.deprecation_message should be set to something helpful.
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Both OPT_REPLACED and OPT_REMOVED can remain in the code for a long time, since
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they're unintrusive and hopefully make incompatible changes less painful for
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users.
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Scripting APIs
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--------------
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This affects internal scripting APIs (currently Lua and JavaScript).
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Vaguely the same rules as with the command interface apply. Since there is a
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large number of scripts, an effort should be made to provide compatibility
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for old scripts, but it does not need to be stronger than that of the command
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interface.
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Undocumented parts of the scripting APIs are _not_ guaranteed for compatibility.
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This applies especially for internals. Languages like Lua do not have strict
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access control (nor does the mpv code try to emulate any), so if a script
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accesses private parts, and breaks on the next mpv release, it's not mpv's
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problem.
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JSON IPC
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--------
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The JSON IPC is a thin protocol wrapping the libmpv API and the command
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interface. Compatibility-wise, it's about the same as the scripting APIs.
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The JSON protocol commands should remain as compatible as possible, and it
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should probably accept the current way commands are delimited (line breaks)
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forever.
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The protocol may accept non-standard JSON extensions, but only standard JSON
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(possibly with restrictions) is guaranteed for compatibility. Clients which want
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to remain compatible should not use any extensions.
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CLI
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---
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Things such as default key bindings do not necessarily require compatibility.
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However, the release notes should be extremely clear on changes to "important"
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key bindings. Bindings which restore the old behavior should be added to
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restore-old-bindings.conf.
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Some option parsing is CLI-only and not available from libmpv or scripting. No
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compatibility guarantees come with them. However, the rules which mpv uses to
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distinguish between options and filenames must remain consistent (if the
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non-deprecated options syntax is used).
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Terminal and log output
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-----------------------
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There are no compatibility guarantees for the terminal output, or the text
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logged via ``MPV_EVENT_LOG_MESSAGE`` and similar APIs. In particular, scripts
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invoking mpv CLI are extremely discouraged from trying to parse text output,
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and should use other mechanisms such as the JSON IPC.
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Protocols, filters, demuxers, etc.
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----------------------------------
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Which of these are present is generally not guaranteed, and can even depend
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on compile time settings.
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The filter list and their sub-options are considered part of the
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command-interface.
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libmpv C API
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------------
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The libmpv client API (such as ``<libmpv/client.h>``) mostly gives access to
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the command interface. The API itself (if looked at as a component separate
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from the command interface) is intended to be extremely stable.
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All API changes are documented in client-api-changes.rst.
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API compatibility
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^^^^^^^^^^^^^^^^^
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The API is *always* compatible. Incompatible changes are only allowed on major
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API version changes (see ``MPV_CLIENT_API_VERSION``). A major version change is
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an extremely rare event, which means usually no API symbols are ever removed.
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Essentially removing API functions by making them always return an error, or
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making it do nothing is allowed in cases where it is unlikely to break most
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clients, but requires a deprecation period of 2 releases. (This has happened to
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``mpv_suspend()`` for example.)
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API symbols can be deprecated. This should be clearly marked in the doxygen
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with ``@deprecated``, and if possible, the affected API symbols should not be
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visible if the API user defines ``MPV_ENABLE_DEPRECATED`` to 0.
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ABI compatibility
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^^^^^^^^^^^^^^^^^
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The ABI must never be broken, except on major API version changes. For example,
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constants don't change their values.
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Structs are tricky. If a struct can be allocated by a user (such as ``mpv_node``),
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no fields can be added. (Unless it's an union, and the addition does not change
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the offset or alignment of any of the fields or the struct itself. This has
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happened to ``mpv_node`` in the past.) If a struct is allocated by libmpv only,
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new fields can be appended to the end (for example ``mpv_event``).
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The ABI is only backward compatible. This means if a host application is linked
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to an older libmpv, and libmpv is updated to a newer version, it will still
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work (as in not causing any undefined behavior).
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Forward compatibility (an application would work with an older libmpv than it
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was linked to) is not required.
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