ipc: add --input-ipc-client option

While --input-file was removed for justified reasons, wanting to pass
down socket FDs this way is legitimate, useful, and easy to implement.

One odd thing is that

Fixes: #7592

2 files changed, 32 insertions, 2 deletions

diff --git a/DOCS/interface-changes.rst b/DOCS/interface-changes.rst
index cd391aa034..666af5f55e 100644
--- a/DOCS/interface-changes.rst
+++ b/DOCS/interface-changes.rst
@@ -59,7 +59,8 @@ Interface changes
       "error" field for end-file will silently break at some point in the
       future.
     - deprecate encoding mode (lack of maintainer)
-    - remove deprecated --input-file option
+    - remove deprecated --input-file option, add --input-ipc-client, which is
+      vaguely a replacement of the removed option, but not the same
  --- mpv 0.32.0 ---
     - change behavior when using legacy option syntax with options that start
       with two dashes (``--`` instead of a ``-``). Now, using the recommended
@@ -81,7 +82,7 @@ Interface changes
       mpv 0.30.0 (related to the previous changelog entry). This affects video
       outputs like vo_x11 and vo_drm, and screenshots, but not much else.
     - deprecate --input-file (there are no plans to remove this short-term,
-      but it will probably eventually go away)
+      but it will probably eventually go away <- that was a lie)
     - deprecate --video-sync=display-adrop (might be removed if it's in the way;
       undeprecated or readded if it's not too much of a problem)
     - deprecate all input section commands (these will be changed/removed, as
diff --git a/DOCS/man/options.rst b/DOCS/man/options.rst
index 7fd01f0592..f5b59d291a 100644
--- a/DOCS/man/options.rst
+++ b/DOCS/man/options.rst
@@ -3704,6 +3704,35 @@ Input
 
     See `JSON IPC`_ for details.
 
+``--input-ipc-client=fd://<N>``
+    Connect a single IPC client to the given FD. This is somewhat similar to
+    ``--input-ipc-server``, except no socket is created, and instead the passed
+    FD is treated like a socket connection received from ``accept()``. In
+    practice, you could pass either a FD created by ``socketpair()``, or a pipe.
+    In both cases, you must sure the FD is actually inherited by mpv (do not
+    set the POSIX ``CLOEXEC`` flag).
+
+    This is somewhat similar to the removed ``--input-file`` option, except it
+    supports only integer FDs, and cannot open actual paths.
+
+    .. admonition:: Example
+
+        ``--input-ipc-client=fd://123``
+
+    .. note::
+
+        Does not and will not work on Windows.
+
+    .. warning::
+
+        Writing to the ``input-ipc-server`` option at runtime will start another
+        instance of an IPC client handler for the ``input-ipc-client`` option,
+        because initialization is bundled, and this thing is stupid. This is a
+        bug. Writing to ``input-ipc-client`` at runtime will start another IPC
+        client handler for the new value, without stopping the old one, even if
+        the FD value is the same (but the string is different e.g. due to
+        whitespace). This is not a bug.
+
 ``--input-gamepad=<yes|no>``
     Enable/disable SDL2 Gamepad support. Disabled by default.