Switch handling of errno.

io.c

@@ -1355,11 +1355,13 @@ io_flush_buffer_sync(void *arg)
         fptr->wbuf.len = 0;
         return 0;
     }
+
     if (0 <= r) {
         fptr->wbuf.off += (int)r;
         fptr->wbuf.len -= (int)r;
         errno = EAGAIN;
     }
+
     return (VALUE)-1;
 }
 
@@ -12303,7 +12305,11 @@ nogvl_wait_for(VALUE th, rb_io_t *fptr, short events, struct timeval *timeout)
     }
 
     int fd = fptr->fd;
-    if (fd == -1) return -1;
+
+    if (fd == -1) {
+        errno = EBADF;
+        return -1;
+    }
 
     rb_fdset_t fds;
     int ret;

thread.c

@@ -1675,6 +1675,10 @@ rb_thread_io_blocking_region(rb_blocking_function_t *func, void *data1, int fd)
         .th = rb_ec_thread_ptr(ec)
     };
 
+    // `errno` is only valid when there is an actual error - but we can't
+    // extract that from the return value of `func` alone, so we clear any
+    // prior `errno` value here so that we can later check if it was set by
+    // `func` or not (as opposed to some previously set value).
     errno = 0;
 
     RB_VM_LOCK_ENTER();