sys/suit: write size in suit_worker_try_prepare()

[benpicco]

Contribution description

suit_worker_try_prepare() is a bit awkward to use.
The idea is that we can request the buffer into which the SUIT manifest shall be written.
However it only tells us the size of the destination buffer if we call it with a size larger than the destination buffer.

Otherwise the caller has to either guess the size or call suit_worker_try_prepare() twice.

uint8_t *mani_buf;
size_t mani_size_max = UINT32_MAX;

suit_worker_try_prepare(&mani_buf, &mani_size_max); // get size
suit_worker_try_prepare(&mani_buf, &mani_size_max); // get buffer

Just populate size at all times to avoid such nonsense.

Testing procedure

Issues/PRs references

[maribu]

I wonder if existing users might rely on the value of *size not being changed on success. E.g. when the manifest size is known already at the point in time suit_worker_try_prepare() is called and the caller just passes that value first to suit_worker_try_prepare(), and then to some copy operation under the assumption size has not been increased?

Or can that be ruled out?

[benpicco]

I can't find any existing users in-tree.
@chrysn added this, do you still know the rationale behind this?

We can of course make this conditional to size being passed as 0.

[maribu]

If *size is only overwritten when it was 0 before, I don't see a way this can break existing valid users.

[riot-ci]

Murdock results

✔️ PASSED

8aa816a sys/suit: always write size in suit_worker_try_prepare()

Success	Failures	Total	Runtime
10932	0	10932	12m:32s

Artifacts

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