quickwit-oss/quickwit · error

The right iterator should not be empty.

Error message

The right iterator should not be empty.

What it means

When both iterators peek Some and left key > right key, right.next() must return the peeked element. The expect asserts the right iterator is not empty; failure means the right iterator breaks the Iterator contract or was mutated concurrently.

Solutions

1. Make the right iterator consistent between peek and next
2. Snapshot the underlying collection before diffing instead of iterating live data
3. Do not share the iterator across threads without synchronization
4. Fix any local modifications to quickwit-common::sorted_iter

Example fix

// before
let right = self.right.next().expect("The right iterator should not be empty.");
// after
let right = match self.right.next() { Some(r) => r, None => return None };

// Materialize the right sequence before diffing
let right: Vec<_> = right_iter.collect();

fn right_stable<I: Iterator + Clone>(it: &I) -> bool { let mut p = it.clone().peekable(); p.peek().is_some() == p.next().is_some() }

std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| diff_loop())).unwrap_or_else(|_| eprintln!("diff panicked on right iterator"));

When it happens

Trigger: SortedIterator diff where the right iterator's next() returns None right after peek() returned Some — caused by a misimplemented iterator or concurrent advancement of the right source.

Common situations: Custom iterators over live data (e.g. shard lists refreshed mid-iteration); shared iterators advanced from multiple threads; local patches to sorted_iter.rs.

Understand the failure class

Background: "This is a bug, please report it": internal invariant violations, unreachable panics, and SNH errors explained — this error's family across 47 libraries.

Related errors

• The left iterator should not be empty.
• More memory released than allocated, should never happen.
• storage factory and config backends should match
• subscription map should exist
• 1 is always non-zero.

AI-assisted analysis of quickwit-oss/quickwit@a39730c5cd (2026-09-08). Data as JSON: /api/errors/5dd4ce598bed35bf. Report an issue: GitHub.

Appendix: source

Thrown at quickwit/quickwit-common/src/sorted_iter.rs:77

                    let left = self
                        .left
                        .next()
                        .expect("The left iterator should not be empty.");
                    Some(Diff::Removed(left))
                }
                Ordering::Equal => {
                    let left = self
                        .left
                        .next()
                        .expect("The left iterator should not be empty.");
                    self.right.next();
                    Some(Diff::Unchanged(left))
                }
                Ordering::Greater => {
                    let right = self
                        .right
                        .next()
                        .expect("The right iterator should not be empty.");
                    Some(Diff::Added(right))
                }
            },
            (Some(_), None) => {
                let left = self
                    .left
                    .next()
                    .expect("The left iterator should not be empty.");
                Some(Diff::Removed(left))
            }
            (None, Some(_)) => {
                let right = self
                    .right
                    .next()
                    .expect("The right iterator should not be empty.");
                Some(Diff::Added(right))
            }
            (None, None) => None,

View on GitHub (pinned to a39730c5cd)