input/ipc-win: support fd:// for --input-ipc-client

This makes --input-ipc-client work on Windows.

To use this feature, a parent process needs to create a connected named pipe,
wrap the server handle in a CRT fd, and then spawn mpv as a child process
with the fd as the --input-ipc-client parameter.
The process can then communicate through the client handle.

The named pipe must be created duplex with overlapped IO and inheritable
handles.

DOCS/man/options.rst

@@ -4243,7 +4243,11 @@ Input
 
     .. note::
 
-        Does not and will not work on Windows.
+        To use this option on Windows, the fd must refer to a wrapped
+        (created by ``_open_osfhandle``) named pipe server handle with a client
+        already connected. The named pipe must be created duplex with overlapped
+        IO and inheritable handles. The program communicates with mpv through
+        the client handle.
 
     .. warning::
 

input/ipc-win.c

@@ -464,6 +464,25 @@ struct mp_ipc_ctx *mp_init_ipc(struct mp_client_api *client_api,
         .client_api = client_api,
     };
 
+    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 {
+            HANDLE h = (HANDLE)_get_osfhandle(fd);
+            if (h && h != INVALID_HANDLE_VALUE && (intptr_t)h != -2)
+                ipc_start_client_json(arg, -1, h);
+            else
+                MP_ERR(arg, "Invalid IPC client fd: '%d'\n", fd);
+        }
+    }
+
     if (!opts->ipc_path || !*opts->ipc_path)
         goto out;
 

options/options.c

@@ -881,9 +881,7 @@ static const m_option_t mp_opts[] = {
     {"input-terminal", OPT_BOOL(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)},