Today, every use of CBB, even CBB_init_fixed, requires a small, fallible
allocation to allocate the top-level CBB's cbb_buffer_st. We could embed
cbb_buffer_st directly in CBB, but then every child CBB wastes that
space, and needs an extra pointer to point back to the cbb_buffer_st.
But top-level and child CBBs have disjoint representations anyway. We
share a cbb_buffer_st pointer, but it's owning in one case and
borrowed in another. Child CBBs have length prefix information, but it's
never filed in for a top-level CBB.
Make this a sum type, with is_child as the discriminator and a union for
the two structures. (Elsewhere I've been trying to get rid of unions,
but this isn't using unions for type-punning, so it should valid even in
C++. We never access inactive arms.)
The implementation gains a few more branches, but now CBB_init_fixed is
infallible and allocation-less. I'm hoping this will let us more freely
convert functions like UTF8_putc into CBB because we don't need to worry
about cleanup or introducing allocations.
Change-Id: If0b28cd9e079418f35d5a614058c0aa73658822e
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/54645
Reviewed-by: Bob Beck <bbe@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: Adam Langley <agl@google.com>
David Benjamin