feat: implement ets:lookup_elements/3

[MarkoMin]

If positions is an empty tuple, an empty tuple is returned without going further into the process.

I tried to replicate existing code patterns from lookup_element/3 for all versions of ets tables. Indexes and Positions are synonyms in code, but again - I tried to match existing naming per-file.

My BEAM-memory-management skills are pretty low so please check that part twice.

Testing is a bit scarce, but lookup_element/3 is also not tested that much directly.

I could also implement lookup_elements/4 if needed, but just wanted this to get some review before going into that.

Ping @RobinMorisset , @ansd and @jhogberg

Closes #10211

[github-actions]

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// Erlang/OTP Github Action Bot

[jhogberg]

Thanks for the PR! I've taken a brief look and have a few comments.

[jhogberg]

Checking for errors before locking the table is a bit less error prone (cf. if(index_cnt==0) { return TUPLE0; } not unlocking):

Suggested change

	CHECK_TABLES();
	DB_BIF_GET_TABLE(tb, DB_READ, LCK_READ, BIF_ets_lookup_elements_3);
	if (!is_tuple(BIF_ARG_3) || (tp = tuple_val(BIF_ARG_3),
	index_cnt = arityval(tp[0]),
	index_cnt < 0)) {
	db_unlock(tb, LCK_READ);
	BIF_ERROR(BIF_P, BADARG);
	}
	if(index_cnt==0) { return TUPLE0; }
	for(int i=0; i<index_cnt; i++) {
	if (is_not_small(tp[i+1]) || (signed_val(tp[i+1]) < 1)) {
	db_unlock(tb, LCK_READ);
	BIF_ERROR(BIF_P, BADARG);
	}
	}
	CHECK_TABLES();
	if (!is_tuple(BIF_ARG_3)) {
	BIF_ERROR(BIF_P, BADARG);
	}
	tp = tuple_val(BIF_ARG_3);
	index_cnt = arityval(tp[0]);
	if (index_cnt < 0) {
	BIF_RET(TUPLE0);
	}
	for (int i = 1; i <= index_cnt; i++) {
	if (is_not_small(tp[i]) || (signed_val(tp[i]) < 1)) {
	BIF_ERROR(BIF_P, BADARG);
	}
	}
	DB_BIF_GET_TABLE(tb, DB_READ, LCK_READ, BIF_ets_lookup_elements_3);

[jhogberg]

This will result in an allocation for each non-trivial element we've referenced. It would be better to follow what db_copy_element_from_ets does here and use a single allocation for everything (including the top-level tuple).