Establish a savepoint and rollback to it in a trigger - oracle

Question

I have a PL/SQL package that provides a transaction API for creating an instance of an entity (say a new customer). The API involves several DML steps.
There is a view that exposes instances of this (customer) entity and there is an INSTEAD OF trigger on the view that calls the transaction API whenever someone inserts into the view.
Normally, I would like my transaction API to not know or care about whether it is being called from a trigger. I want it to work like a typical API (typical around here, anyway):
Establish savepoint
Do steps 1-3 of DML
Do NOT commit (leave that to caller / client)
On others rollback to savepoint
The problem is that an API like this fails if called from a trigger.
I understand why Oracle cannot allow us to commit or rollback in trigger. But why does Oracle not allow us to rollback to savepoint that the trigger established?
How can I write my API so that:
It cannot have side-effects if any DML step fails halfway through
It's successful work is commited when the caller / client commits (i.e., autonomous transaction is a no-go)
It does not rely on the caller to raise_application_error if it fails. (Obviously, if I could rely on trigger callers to do this and on client code to then rollback, I won't need to worry about side-effects).

Answer 1

I'm a bit confused by your statement. PL/SQL programs automatically have a "natural" savepoint at their commencement, so that if it fails, it will rollback any changes it made, but leave existing changes in the current transaction untouched, eg
SQL> create table t ( x int );
Table created.
SQL>
SQL> create or replace
2 procedure my_api(p_fail boolean) is
3 v int;
4 begin
5 insert into t values (4);
6
7 insert into t values (5);
8
9 if p_fail then
10 v := 1/0;
11 end if;
12
13 insert into t values (6);
14 end;
15 /
Procedure created.
SQL>
SQL> insert into t values (1);
1 row created.
SQL> insert into t values (2);
1 row created.
SQL> insert into t values (3);
1 row created.
SQL> exec my_api(false);
PL/SQL procedure successfully completed.
SQL>
SQL> select * from t;
X
----------
1
2
3
4
5
6
6 rows selected.
API worked, so pre-API changes PLUS the API changes are all there. I'll roll back now to an empty state and let the API fail during execution
SQL>
SQL> rollback;
Rollback complete.
SQL> select * from t;
no rows selected
SQL>
SQL> insert into t values (1);
1 row created.
SQL> insert into t values (2);
1 row created.
SQL> insert into t values (3);
1 row created.
SQL> exec my_api(true);
BEGIN my_api(true); END;
*
ERROR at line 1:
ORA-01476: divisor is equal to zero
ORA-06512: at "MCDONAC.MY_API", line 9
ORA-06512: at line 1
SQL>
SQL> select * from t;
X
----------
1
2
3
My API inserts are rolled back but the pre-API inserts are still there.