openldap_read_function needs to set EWOULDBLOCK if the buffer is empty

If the openldap_read_function has returned all of the buffered data,
it needs to set errno to EWOULDBLOCK to let the code know that it needs
to read more data into the buffer.

ldap/servers/slapd/connection.c

@@ -1569,6 +1569,14 @@ openldap_read_function(Sockbuf_IO_Desc *sbiod, void *buf, ber_len_t len)
 
 	if (bytes_to_copy <= 0) {
 		bytes_to_copy = 0;	/* never return a negative result */
+		/* in this case, we don't have enough data to satisfy the
+		   caller, so we have to let it know we need more */
+#if defined(EWOULDBLOCK)
+		errno = EWOULDBLOCK;
+#elif defined(EAGAIN)
+		errno = EAGAIN;
+#endif			
+		PR_SetError(PR_WOULD_BLOCK_ERROR, 0);
 	} else {
 		/* copy buffered data into output buf */
 		SAFEMEMCPY(buf, readbuf + offset, bytes_to_copy);