osdep/io: add mp_dup_cloexec()

will be used in the next commit

osdep/io.c

@@ -63,6 +63,11 @@ bool mp_set_cloexec(int fd)
 }
 
 #ifndef _WIN32
+int mp_dup_cloexec(int fd)
+{
+    return fcntl(fd, F_DUPFD_CLOEXEC);
+}
+
 int mp_make_cloexec_pipe(int pipes[2])
 {
     if (pipe(pipes) != 0) {
@@ -263,6 +268,24 @@ static int hstat(HANDLE h, struct mp_stat *buf)
     return 0;
 }
 
+int mp_dup_cloexec(int fd)
+{
+    HANDLE proc = GetCurrentProcess();
+    HANDLE src = (HANDLE)_get_osfhandle(fd), dup = INVALID_HANDLE_VALUE;
+    if (src == INVALID_HANDLE_VALUE)
+        return -1;
+    BOOL ok = DuplicateHandle(proc, src, proc, &dup, 0, FALSE, DUPLICATE_SAME_ACCESS);
+    if (!ok) {
+        set_errno_from_lasterror();
+        return -1;
+    }
+    int oflag = _O_RDWR | _O_NOINHERIT; // FIXME: find out the proper flags from the HANDLE
+    int dupfd = _open_osfhandle((intptr_t)dup, oflag);
+    if (dupfd < 0)
+        CloseHandle(dup);
+    return dupfd;
+}
+
 int mp_stat(const char *path, struct mp_stat *buf)
 {
     wchar_t *wpath = mp_from_utf8(NULL, path);

osdep/io.h

@@ -75,6 +75,7 @@ static inline int mp_fseeko(FILE* fp, off64_t offset, int whence) {
 #endif
 
 bool mp_set_cloexec(int fd);
+int mp_dup_cloexec(int fd);
 int mp_make_cloexec_pipe(int pipes[2]);
 int mp_make_wakeup_pipe(int pipes[2]);
 void mp_flush_wakeup_pipe(int pipe_end);