Ed: solved with the help of the async_stream crate.
I'm struggling with the borrow checker!
My problem: I'm using actix-web and rusqlite. I want to return an unlimited number of records from an rusqlite query, and actix provides a Stream trait for that kind of thing. You just impl the trait and return your records from a poll_next() fn.
On the rusqlite side, there's this query_map that returns an iterator of records from a query. All I have to do is smush these two features together.
So the plan is to put the iterator returned by query_map into a struct that impls Stream. Problem is the lifetime of a var used by query_map. How to make the var have the same lifetime as the iterator??
So here's the code:
pub struct ZkNoteStream<'a, T> {
rec_iter: Box<dyn Iterator<Item = T> + 'a>,
}
// impl of Stream just calls next() on the iterator. This compiles fine.
impl<'a> Stream for ZkNoteStream<'a, serde_json::Value> {
type Item = serde_json::Value;
fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
Poll::Ready(self.rec_iter.next())
}
}
// init function to set up the ZkNoteStream.
impl<'a> ZkNoteStream<'a, Result<ZkListNote, rusqlite::Error>> {
pub fn init(
conn: &'a Connection,
user: i64,
search: &ZkNoteSearch,
) -> Result<Self, Box<dyn Error>> {
let (sql, args) = build_sql(&conn, user, search.clone())?;
let sysid = user_id(&conn, "system")?;
let mut pstmt = conn.prepare(sql.as_str())?;
// Here's the problem! Borrowing pstmt.
let rec_iter = pstmt.query_map(rusqlite::params_from_iter(args.iter()), move |row| {
let id = row.get(0)?;
let sysids = get_sysids(&conn, sysid, id)?;
Ok(ZkListNote {
id: id,
title: row.get(1)?,
is_file: {
let wat: Option<i64> = row.get(2)?;
wat.is_some()
},
user: row.get(3)?,
createdate: row.get(4)?,
changeddate: row.get(5)?,
sysids: sysids,
})
})?;
Ok(ZkNoteStream::<Result<ZkListNote, rusqlite::Error>> {
rec_iter: Box::new(rec_iter),
})
}
}
And here's the error:
error[E0515]: cannot return value referencing local variable `pstmt`
--> server-lib/src/search.rs:170:5
|
153 | let rec_iter = pstmt.query_map(rusqlite::params_from_iter(args.iter()), move |row| {
| ----- `pstmt` is borrowed here
...
170 | / Ok(ZkNoteStream::<Result<ZkListNote, rusqlite::Error>> {
171 | | rec_iter: Box::new(rec_iter),
172 | | })
| |______^ returns a value referencing data owned by the current function
So basically it boils down to pstmt getting borrowed in the query_map call. It needs to have the same lifetime as the closure. How do I ensure that?