SQL Server: Pulling updated data from a function during a CROSS APPLY - sql-server

Question

Quick Summary: I have a function that pulls data from table X. I'm running an UPDATE on table X, and using a CROSS APPLY on the function that is pulling data from X (during the update) and the function doesn't look to be returning updated data.
The real-world scenario is much more complicated, but here's a sample of what I'm seeing.
Table
create table BO.sampleData (id int primary key, data1 int, val int)
Function
create function BO.getPrevious(
#id int
)
returns #info table (
id int, val int
)
as
begin
declare #val int
declare #prevRow int = #id - 1
-- grab data from previous row
insert into #info
select #id, val
from BO.sampleData where id = #prevRow
-- if previous row doesn't exist, return 3*prev row id
if ##rowcount = 0
insert into #info values (#id, #prevRow * 3)
return
end
Issue
Populate some sample data:
delete BO.sampleData
insert into BO.sampleData values (10, 20, 0)
insert into BO.sampleData values (11, 22, 0)
insert into BO.sampleData values (12, 24, 0)
insert into BO.sampleData values (13, 26, 0)
insert into BO.sampleData values (14, 28, 0)
select * from BO.sampleData
id data1 val
----------- ----------- -----------
10 20 0
11 22 0
12 24 0
13 26 0
14 28 0
Update BO.sampleData using a CROSS APPLY on BO.getPrevious (which accesses data from BO.sampleData):
update t
set t.val = ca.val
from bo.sampleData t
cross apply BO.getPrevious(t.id) ca
where t.id = ca.id
Problem
I'm expecting the row with id 10 to have the value 27 (since there is no row 9, the function will return 9*3). For id 11, I assumed it would look in 10 (which just got updated with 27) and set it's val to 27 -- and this would cascade down the rest of the table. But what I get is:
id data1 val
----------- ----------- -----------
10 20 27
11 22 0
12 24 0
13 26 0
14 28 0
I'm guessing this isn't allowed/supported -- the function doesn't have access to the updated data yet? Or I've got something wrong with the syntax? In the real scenario I'm researching, the function is much more complex, does some child table look ups, aggregates, etc.. before returning a result. But this represents the basics of what I'm seeing -- the function that queries BO.sampleData doesn't seem to have access to the updated values of BO.sampleData within the CROSS APPLY during the UPDATE.
Any ideas welcomed.

Answer 1

Thanks to #Martin Smith for identifying the issue -- i.e. "Halloween Protection". Now that my issue has a name, I did some research and found the following article which mentions this specific scenario in SQL Server:
... update plans consist of two parts: a read cursor that identifies
the rows to be updated and a write cursor that actually performs the
updates. Logically speaking, SQL Server must execute the read cursor
and write cursor of an update plan in two separate steps or phases.
To put it another way, the actual update of rows must not affect the
selection of which rows to update.
Emphasis mine. It makes sense now. The CROSS APPLY is happening over the read cursor where all of the values are still zero.

Answer 2

The data is always coming from #info
For input id = 11, it will execute:
insert into #info
select #id, val --which #id = 10, val = 0
from BO.sampleData where id = 10
so from the #info, the val for id=10 is 0(which comes from BO.sampleData where id = 11), then cross apply, it deal with id = 10 from #info, which is val = 10.
everything is what it is in your UDF. And there is no update val to 27 when id = 10 from #info in your UDF, be careful that #info is the table get returned.