input/ipc-win: allow Windows handles for --input-ipc-client

Previously, you could only provide a CRT FD, which works great for the
most part, but when using CreateProcess on Windows, it requires the use
of the reserved fields of STARTUPINFO to inherit the FD to a child mpv
process. This becomes a problem for example if you use Go's os/exec API,
which does not expose a way to set those reserved fields. Exposing a way
to allow passing handles directly fixes the mentioned scenario, and
makes --input-ipc-client more flexible at no cost.

DOCS/man/options.rst

@@ -4555,14 +4555,16 @@ Input
     .. admonition:: Example
 
         ``--input-ipc-client=fd://123``
+        ``--input-ipc-client=handle://123``
 
     .. note::
 
         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.
+        already connected. Alternatively, the Windows HANDLE can be passed by
+        prefixing it with handle:// if mpv inherited it from the parent. 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

@@ -17,6 +17,7 @@
 
 #include <windows.h>
 #include <sddl.h>
+#include <stdint.h>
 
 #include "osdep/io.h"
 #include "osdep/threads.h"
@@ -471,21 +472,23 @@ struct mp_ipc_ctx *mp_init_ipc(struct mp_client_api *client_api,
     };
 
     if (opts->ipc_client && opts->ipc_client[0]) {
-        int fd = -1;
+        HANDLE h = INVALID_HANDLE_VALUE;
+
         bstr str = bstr0(opts->ipc_client);
         if (bstr_eatstart0(&str, "fd://") && str.len) {
-            long long ll = bstrtoll(str, &str, 0);
-            if (!str.len && ll >= 0 && ll <= INT_MAX)
-                fd = ll;
+            long long fd = bstrtoll(str, &str, 0);
+            if (!str.len && fd >= 0 && fd <= INT_MAX)
+                h = (HANDLE)_get_osfhandle(fd);
+        } else if (bstr_eatstart0(&str, "handle://") && str.len) {
+            long long handle = bstrtoll(str, &str, 0);
+            if (!str.len && handle >= 0 && handle <= UINT32_MAX)
+                h = (HANDLE)(uintptr_t)handle;
         }
-        if (fd < 0) {
-            MP_ERR(arg, "Invalid IPC client argument: '%s'\n", opts->ipc_client);
+
+        if (h && h != INVALID_HANDLE_VALUE && (intptr_t)h != -2) {
+            ipc_start_client_json(arg, -1, h);
         } 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);
+            MP_ERR(arg, "Invalid IPC client argument: '%s'\n", opts->ipc_client);
         }
     }