Child and root CBBs share a type, but are different kinds of things. C++
programmers sometimes mistakenly believe they should use ScopedCBB for
everything. This mostly works because we NULL cbb->child->base on flush,
making CBB_cleanup a no-op. This zeroing also skips the assert in
CBB_cleanup. (If we ran it unconditionally, CBB_zero + CBB_cleanup would
not work.)
However, if a CBB operation fails and a function returns early, the
child CBB is not cleared. ScopedCBB will then call CBB_cleanup which
trips the assert but, in release build, misbehaves.
Run the assert unconditionally and, when the assert fails, still behave
well. To make this work with CBB_zero, negate is_top_level to is_child,
so a flushed child CBB and a (presumably) root CBB in the zero state are
distinguishable.
Update-Note: Code that was using CBB wrong may trip an assert in debug builds.
Change-Id: Ifea7759e1d0331f2e727c59bbafa355d70fb9dba
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/35524
Reviewed-by: Adam Langley <agl@google.com>
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