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The purpose is to store a head and a tail that are independent so that we can further improve the API to update them independently from each other. The struct was arranged like the original one so that as long as a ring has its head set to zero (i.e. no recycling) it will continue to work. The new format is already detectable thanks to the "rsvd" field which indicates the number of reserved bytes at the beginning. It's located where the buffer's area pointer previously was, so that older versions of haring can continue to open the ring in repair mode, and newer ones can use the fact that the upper bits of that variable are zero to guess that it's working with the new format instead of the old one. Also let's keep in mind that the layout will further change to place some alignment constraints. The haring tool will thus updated based on this and it detects that the rsvd field is smaller than a page and that the sum of it with the size equals the mapped size, in which case it uses the new dump_v2() function instead of dump_v1(). The new function also creates a buffer from the ring's area, size, head and tail and calls the generic one so that no other code had to be adapted.
This needs to be built from the top makefile, for example : make dev/haring/haring If HAProxy is built with special options such -DDEBUG_THREAD or with multi-threading support enabled (which changes the ring's header size), it can be worth reusing the same build options for haring, usually they will remain compatible, and will simplify the handling of different file layouts, at the expense of dragging more dependencies into the executable.