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

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

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.
 

input/ipc-unix.c

@@ -18,7 +18,7 @@
 #include <pthread.h>
 #include <errno.h>
 #include <unistd.h>
-
+#include <limits.h>
 #include <poll.h>
 #include <signal.h>
 #include <sys/types.h>
@@ -248,9 +248,10 @@ static void ipc_start_client_json(struct mp_ipc_ctx *ctx, int id, int fd)
 {
     struct client_arg *client = talloc_ptrtype(NULL, client);
     *client = (struct client_arg){
-        .client_name = talloc_asprintf(client, "ipc-%d", id),
-        .client_fd   = fd,
-        .close_client_fd = true,
+        .client_name =
+            id >= 0 ? talloc_asprintf(client, "ipc-%d", id) : "ipc",
+        .client_fd = fd,
+        .close_client_fd = id >= 0,
         .writable = true,
     };
 
@@ -384,6 +385,21 @@ struct mp_ipc_ctx *mp_init_ipc(struct mp_client_api *client_api,
         .death_pipe = {-1, -1},
     };
 
+    if (opts->ipc_client && opts->ipc_client[0]) {
+        int fd = -1;
+        if (strncmp(opts->ipc_client, "fd://", 5) == 0) {
+            char *end;
+            unsigned long l = strtoul(opts->ipc_client + 5, &end, 0);
+            if (!end[0] && l <= INT_MAX)
+                fd = l;
+        }
+        if (fd < 0) {
+            MP_ERR(arg, "Invalid IPC client argument: '%s'\n", opts->ipc_client);
+        } else {
+            ipc_start_client_json(arg, -1, fd);
+        }
+    }
+
     talloc_free(opts);
 
     if (!arg->path || !arg->path[0])

options/options.c

@@ -719,6 +719,9 @@ static const m_option_t mp_opts[] = {
     {"input-terminal", OPT_FLAG(consolecontrols), .flags = UPDATE_TERM},
 
     {"input-ipc-server", OPT_STRING(ipc_path), .flags = M_OPT_FILE},
+#if HAVE_POSIX
+    {"input-ipc-client", OPT_STRING(ipc_client)},
+#endif
 
     {"screenshot", OPT_SUBSTRUCT(screenshot_image_opts, screenshot_conf)},
     {"screenshot-template", OPT_STRING(screenshot_template)},

options/options.h

@@ -319,6 +319,7 @@ typedef struct MPOpts {
     struct encode_opts *encode_opts;
 
     char *ipc_path;
+    char *ipc_client;
 
     int wingl_dwm_flush;
 

player/command.c

@@ -6229,7 +6229,7 @@ void mp_option_change_callback(void *ctx, struct m_config_option *co, int flags,
     if (flags & UPDATE_INPUT)
         mp_input_update_opts(mpctx->input);
 
-    if (init || opt_ptr == &opts->ipc_path) {
+    if (init || opt_ptr == &opts->ipc_path || opt_ptr == &opts->ipc_client) {
         mp_uninit_ipc(mpctx->ipc_ctx);
         mpctx->ipc_ctx = mp_init_ipc(mpctx->clients, mpctx->global);
     }