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?

    • pr06lefsOP
      link
      fedilink
      arrow-up
      2
      ·
      1 year ago

      I’ve been looking into it a bit - not pinning per se but self referential. There’s a library called ouroboros that looks helpful. There’s even an example on github where someone uses rusqlite and ouroboros together.

      So it seems like this should work:

      #[self_referencing]
      pub struct ZkNoteStream {
        conn: Connection,
        #[borrows(conn)]
        pstmt: rusqlite::Statement<'this>,
        #[borrows(mut pstmt)]
        #[covariant]
        rec_iter: rusqlite::Rows<'this>,
      }
      
      impl ZkNoteStream {
        pub fn init(conn: Connection, user: i64, search: &ZkNoteSearch) -> Result<Self, Box<dyn Error>> {
          let (sql, args) = build_sql(&conn, user, search.clone())?;
      
          Ok(
            ZkNoteStreamTryBuilder {
              conn: conn,
              pstmt_builder: |conn: &Connection| conn.prepare(sql.as_str()),
              rec_iter_builder: |pstmt: &mut rusqlite::Statement<'_>| {
                pstmt.query(rusqlite::params_from_iter(args.iter()))
              },
            }
            .try_build()?,
          )
        }
      }
      

      Unfortunately I get this:

      error[E0597]: `pstmt` does not live long enough
         --> server-lib/src/search.rs:880:1
          |
      880 | #[self_referencing]
          | ^^^^^^^^^^^^^^^^^^-
          | |                 |
          | |                 `pstmt` dropped here while still borrowed
          | |                 borrow might be used here, when `pstmt` is dropped and runs the `Drop` code for type `Statement`
          | borrowed value does not live long enough
          |
          = note: this error originates in the attribute macro `self_referencing` (in Nightly builds, run with -Z macro-backtrace for more info)
      

      So close! But no cigar so far. No idea why its complaining.

    • pr06lefsOP
      link
      fedilink
      arrow-up
      2
      ·
      edit-2
      11 months ago

      Got this solved over on the rust-lang discourse. The solution was to use the async_stream crate. Ends up being a small amount of code too.