Fix gai_strerror() thread-safety on Windows.

Commit 5579388d removed code that supplied a fallback implementation of
getaddrinfo(), which was dead code on modern systems.  One tiny piece of
the removed code was still doing something useful on Windows, though:
that OS's own gai_strerror()/gai_strerrorA() function returns a pointer
to a static buffer that it overwrites each time, so it's not
thread-safe.  In rare circumstances, a multi-threaded client program
could get an incorrect or corrupted error message.

Restore the replacement gai_strerror() function, though now that it's
only for Windows we can put it into a win32-specific file and cut it
down to the errors that Windows documents.  The error messages here are
taken from FreeBSD, because Windows' own messages seemed too verbose.

Back-patch to 16.

Reviewed-by: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Discussion: https://postgr.es/m/CA%2BhUKGKz%2BF9d2PTiXwfYV7qJw%2BWg2jzACgSDgPizUw7UG%3Di58A%40mail.gmail.com

configure

@@ -16388,6 +16388,12 @@ esac
  ;;
 esac
 
+  case " $LIBOBJS " in
+  *" win32gai_strerror.$ac_objext "* ) ;;
+  *) LIBOBJS="$LIBOBJS win32gai_strerror.$ac_objext"
+ ;;
+esac
+
   case " $LIBOBJS " in
   *" win32getrusage.$ac_objext "* ) ;;
   *) LIBOBJS="$LIBOBJS win32getrusage.$ac_objext"

configure.ac

@@ -1885,6 +1885,7 @@ if test "$PORTNAME" = "win32"; then
   AC_LIBOBJ(win32env)
   AC_LIBOBJ(win32error)
   AC_LIBOBJ(win32fdatasync)
+  AC_LIBOBJ(win32gai_strerror)
   AC_LIBOBJ(win32getrusage)
   AC_LIBOBJ(win32link)
   AC_LIBOBJ(win32ntdll)

src/include/port/win32/sys/socket.h

@@ -23,4 +23,12 @@
 #define ERROR PGERROR
 #endif
 
+/*
+ * We don't use the Windows gai_strerror[A] function because it is not
+ * thread-safe.  We define our own in src/port/win32gai_strerror.c.
+ */
+#undef gai_strerror
+
+extern const char *gai_strerror(int ecode);
+
 #endif							/* WIN32_SYS_SOCKET_H */

src/port/meson.build

@@ -35,6 +35,7 @@ if host_system == 'windows'
     'win32error.c',
     'win32fdatasync.c',
     'win32fseek.c',
+    'win32gai_strerror.c',
     'win32getrusage.c',
     'win32link.c',
     'win32ntdll.c',

src/port/win32gai_strerror.c

@@ -0,0 +1,45 @@
+/*-------------------------------------------------------------------------
+ *
+ * win32gai_strerror.c
+ *	  Thread-safe gai_strerror() for Windows.
+ *
+ * Portions Copyright (c) 2024, PostgreSQL Global Development Group
+ *
+ * IDENTIFICATION
+ *    src/port/win32gai_strerror.c
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#include <sys/socket.h>
+
+/*
+ * Windows has gai_strerrorA(), but it is not thread-safe so we avoid it.
+ *
+ * https://learn.microsoft.com/en-us/windows/win32/api/ws2tcpip/nf-ws2tcpip-gai_strerrora
+ */
+const char *
+gai_strerror(int errcode)
+{
+	switch (errcode)
+	{
+		case EAI_AGAIN:
+			return "Temporary failure in name resolution";
+		case EAI_BADFLAGS:
+			return "Bad value for ai_flags";
+		case EAI_FAIL:
+			return "Non-recoverable failure in name resolution";
+		case EAI_FAMILY:
+			return "ai_family not supported";
+		case EAI_MEMORY:
+			return "Memory allocation failure";
+		case EAI_NONAME:
+			return "Name or service not known";
+		case EAI_SERVICE:
+			return "Servname not supported for ai_socktype";
+		case EAI_SOCKTYPE:
+			return "ai_socktype not supported";
+		default:
+			return "Unknown server error";
+	}
+}