Use of Variable Length Arrays in generated code

[mdamle]

Problem

variable length arrays are used in generated code: CHIPClientCallbacks.cpp. For example: https://github.com/project-chip/connectedhomeip/blob/master/src/controller/data_model/gen/CHIPClientCallbacks.cpp#L854
uint16_t count = chip::Encoding::LittleEndian::Read16(message);
_AudioOutputInfodata[count];
There are about 24 such instances. There are 2 issues with this usage:

1. Variable Length Arrays are not universally supported by all compilers since they are not part of c++ standard
2. The generated code uses over the wire value to allocate data on stack which is dangerous.

• expected behavior

1. Check size before allocation
2. Allocate on heap instead of stack

• actual behavior
  As explained in the problem section.
• steps to reproduce
  Compile code on top of tree and inspect generated CHIPClientCallbacks.cpp file.

[mdamle]

cc @tcarmelveilleux for visibility

[bzbarsky-apple]

@vivien-apple @mrjerryjohns

[tcarmelveilleux]

The root cause of this issue is wanted to pass "flat arrays" to callbacks when lists are used. I think these lists should be handled by iteration over the payload with an abstracted list iterator backed by a TLV reader, rather than copy to flat arrays. That would avoid alloc entirely, and avoid requiring to trust any sizes from the wire for an alloc. A temporary reader could be used to schema-validate, but access to the list elements should be abstracted with an iterator.

[mlepage-google]

+1 for iterator access.

[bzbarsky-apple]

Fixed in #10681