build: Port use of shutdown() on mingw

Even though mingw's <unistd.h> has a definition for SHUT_WR, it is
guarded by _POSIX_SOURCE, and does not get enabled in our build setup.
We can instead utilize winsock2.h's non-standard names for
shutdown(how).

We also need to allow for a missing definition of ESHUTDOWN.

Fixes: bf8653b ("server: Give client EOF when we are done writing")
Fixes: 6a7def7 ("blocksize-policy: Add blocksize-write-disconnect=N parameter")

common/utils/windows-compat.h

@@ -1,5 +1,5 @@
 /* nbdkit
- * Copyright (C) 2020 Red Hat Inc.
+ * Copyright (C) 2020-2022 Red Hat Inc.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
@@ -85,6 +85,12 @@ struct sockaddr_un
 #define ESHUTDOWN ECONNABORTED
 #endif
 
+/* Windows uses non-standard names for shutdown(how). */
+#ifndef SHUT_WR
+#define SHUT_WR SD_SEND
+#define SHUT_RDWR SD_BOTH
+#endif
+
 /* This generated function translates Winsock errors into errno codes. */
 extern int translate_winsock_error (const char *fn, int err);
 

filters/blocksize-policy/policy.c

@@ -43,6 +43,7 @@
 
 #include "ispowerof2.h"
 #include "rounding.h"
+#include "windows-compat.h"
 
 /* Block size constraints configured on the command line (0 = unset). */
 static uint32_t config_minimum;