rollout: extract reverse JSONL scanner

codex-rs/rollout/src/lib.rs

@@ -11,6 +11,7 @@ pub(crate) mod metadata;
 mod persistence_metrics;
 pub(crate) mod policy;
 pub(crate) mod recorder;
+mod reverse_jsonl_scanner;
 pub(crate) mod search;
 pub(crate) mod session_index;
 mod sqlite_metrics;

codex-rs/rollout/src/reverse_jsonl_scanner.rs

@@ -0,0 +1,102 @@
+use std::io;
+use std::io::Read;
+use std::io::Seek;
+use std::io::SeekFrom;
+
+use serde::de::DeserializeOwned;
+
+const READ_CHUNK_SIZE: usize = 8 * 1024;
+
+#[derive(Debug)]
+pub(crate) enum ScanOutcome<T> {
+    Parsed(T),
+    Rejected(serde_json::Error),
+}
+
+/// Read-only scanner for newline-delimited JSON records, starting from the end.
+pub(crate) struct ReverseJsonlScanner<R> {
+    reader: R,
+    next_chunk_end: u64,
+    chunk_position: usize,
+    chunk: Vec<u8>,
+    record_reversed: Vec<u8>,
+}
+
+impl<R> ReverseJsonlScanner<R>
+where
+    R: Read + Seek,
+{
+    pub(crate) fn new(mut reader: R) -> io::Result<Self> {
+        let next_chunk_end = reader.seek(SeekFrom::End(0))?;
+        Ok(Self {
+            reader,
+            next_chunk_end,
+            chunk_position: 0,
+            chunk: vec![0; READ_CHUNK_SIZE],
+            record_reversed: Vec::new(),
+        })
+    }
+
+    /// Scans the next nonblank record.
+    ///
+    /// I/O failures are returned as [`Err`]. Invalid JSON records are returned as
+    /// [`ScanOutcome::Rejected`], and the scanner remains usable.
+    pub(crate) fn scan_next<T>(&mut self) -> io::Result<Option<ScanOutcome<T>>>
+    where
+        T: DeserializeOwned,
+    {
+        loop {
+            let Some(byte) = self.read_previous_byte()? else {
+                return Ok(self.finish_record());
+            };
+
+            if byte != b'\n' {
+                self.record_reversed.push(byte);
+                continue;
+            }
+
+            if let Some(outcome) = self.finish_record() {
+                return Ok(Some(outcome));
+            }
+        }
+    }
+
+    fn read_previous_byte(&mut self) -> io::Result<Option<u8>> {
+        if self.chunk_position == 0 {
+            if self.next_chunk_end == 0 {
+                return Ok(None);
+            }
+
+            let read_size = usize::try_from(self.next_chunk_end.min(READ_CHUNK_SIZE as u64))
+                .map_err(io::Error::other)?;
+            self.next_chunk_end -= read_size as u64;
+            self.reader.seek(SeekFrom::Start(self.next_chunk_end))?;
+            self.reader.read_exact(&mut self.chunk[..read_size])?;
+            self.chunk_position = read_size;
+        }
+
+        self.chunk_position -= 1;
+        Ok(Some(self.chunk[self.chunk_position]))
+    }
+
+    fn finish_record<T>(&mut self) -> Option<ScanOutcome<T>>
+    where
+        T: DeserializeOwned,
+    {
+        self.record_reversed.reverse();
+        let outcome = if self.record_reversed.iter().all(u8::is_ascii_whitespace) {
+            None
+        } else {
+            Some(match serde_json::from_slice::<T>(&self.record_reversed) {
+                Ok(value) => ScanOutcome::Parsed(value),
+                Err(error) => ScanOutcome::Rejected(error),
+            })
+        };
+        self.record_reversed.clear();
+        outcome
+    }
+}
+
+#[cfg(test)]
+#[path = "reverse_jsonl_scanner_tests.rs"]
+mod tests;

codex-rs/rollout/src/reverse_jsonl_scanner_tests.rs

@@ -0,0 +1,127 @@
+use std::io::Cursor;
+use std::io::Read;
+use std::io::Seek;
+
+use pretty_assertions::assert_eq;
+use serde::Deserialize;
+use serde::Serialize;
+
+use super::ReverseJsonlScanner;
+use super::ScanOutcome;
+
+#[derive(Debug, Deserialize, Serialize, PartialEq)]
+struct TestRecord {
+    value: String,
+}
+
+fn record(value: &str) -> TestRecord {
+    TestRecord {
+        value: value.to_string(),
+    }
+}
+
+fn parsed<T>(outcome: Option<ScanOutcome<T>>) -> T {
+    let Some(ScanOutcome::Parsed(record)) = outcome else {
+        panic!("expected parsed record");
+    };
+    record
+}
+
+fn assert_records<R>(scanner: &mut ReverseJsonlScanner<R>, expected: &[&str]) -> std::io::Result<()>
+where
+    R: Read + Seek,
+{
+    for value in expected {
+        assert_eq!(parsed(scanner.scan_next::<TestRecord>()?), record(value));
+    }
+    assert!(scanner.scan_next::<TestRecord>()?.is_none());
+    Ok(())
+}
+
+#[test]
+fn scans_jsonl_records_from_end() -> std::io::Result<()> {
+    let input = br#"{"value":"first"}
+{"value":"second"}
+{"value":"third"}
+"#;
+
+    assert_records(
+        &mut ReverseJsonlScanner::new(Cursor::new(input))?,
+        &["third", "second", "first"],
+    )
+}
+
+#[test]
+fn rejects_invalid_json_and_continues_scanning() -> std::io::Result<()> {
+    let input = br#"{"value":"first"}
+not-json
+{"value":"third"}
+"#;
+    let mut scanner = ReverseJsonlScanner::new(Cursor::new(input))?;
+
+    assert_eq!(parsed(scanner.scan_next::<TestRecord>()?), record("third"));
+    let Some(ScanOutcome::Rejected(error)) = scanner.scan_next::<TestRecord>()? else {
+        panic!("expected rejected record");
+    };
+    assert!(error.is_syntax());
+    assert_eq!(parsed(scanner.scan_next::<TestRecord>()?), record("first"));
+    Ok(())
+}
+
+#[test]
+fn accepts_valid_json_at_eof() -> std::io::Result<()> {
+    let input = b"{\"value\":\"first\"}\n{\"value\":\"second\"}";
+
+    assert_records(
+        &mut ReverseJsonlScanner::new(Cursor::new(input))?,
+        &["second", "first"],
+    )
+}
+
+#[test]
+fn rejects_invalid_json_at_eof_and_continues_scanning() -> std::io::Result<()> {
+    let input = b"{\"value\":\"first\"}\n{\"value\":";
+    let mut scanner = ReverseJsonlScanner::new(Cursor::new(input))?;
+
+    let Some(ScanOutcome::Rejected(error)) = scanner.scan_next::<TestRecord>()? else {
+        panic!("expected rejected record");
+    };
+    assert!(error.is_eof());
+    assert_eq!(parsed(scanner.scan_next::<TestRecord>()?), record("first"));
+    Ok(())
+}
+
+#[test]
+fn skips_blank_lines_with_or_without_termination() -> std::io::Result<()> {
+    let input = b"{\"value\":\"first\"}\r\n\n \t\r";
+
+    assert_records(
+        &mut ReverseJsonlScanner::new(Cursor::new(input))?,
+        &["first"],
+    )
+}
+
+#[test]
+fn scans_across_read_chunk_boundaries() -> std::io::Result<()> {
+    let empty_record_len = serde_json::to_string(&record(""))?.len();
+    for distance_from_eof in [
+        super::READ_CHUNK_SIZE - 1,
+        super::READ_CHUNK_SIZE,
+        super::READ_CHUNK_SIZE + 1,
+    ] {
+        let large_value = "x".repeat(distance_from_eof - empty_record_len - 2);
+        let input = format!(
+            "{}\n{}\n",
+            serde_json::to_string(&record("first"))?,
+            serde_json::to_string(&record(&large_value))?
+        );
+        let mut scanner = ReverseJsonlScanner::new(Cursor::new(input.into_bytes()))?;
+
+        assert_eq!(
+            parsed(scanner.scan_next::<TestRecord>()?),
+            record(&large_value)
+        );
+        assert_eq!(parsed(scanner.scan_next::<TestRecord>()?), record("first"));
+    }
+    Ok(())
+}

codex-rs/rollout/src/session_index.rs

@@ -2,15 +2,14 @@ use std::collections::HashMap;
 use std::collections::HashSet;
 use std::fs::File;
 use std::io::ErrorKind;
-use std::io::Read;
-use std::io::Seek;
-use std::io::SeekFrom;
 use std::io::Write;
 use std::path::Path;
 use std::path::PathBuf;
 use std::sync::LazyLock;
 use std::sync::Mutex;
 
+use crate::reverse_jsonl_scanner::ReverseJsonlScanner;
+use crate::reverse_jsonl_scanner::ScanOutcome;
 use codex_protocol::ThreadId;
 use codex_protocol::protocol::SessionMetaLine;
 use serde::Deserialize;
@@ -18,7 +17,6 @@ use serde::Serialize;
 use tokio::io::AsyncBufReadExt;
 
 const SESSION_INDEX_FILE: &str = "session_index.jsonl";
-const READ_CHUNK_SIZE: usize = 8192;
 static SESSION_INDEX_LOCK: LazyLock<Mutex<()>> = LazyLock::new(|| Mutex::new(()));
 
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
@@ -245,64 +243,18 @@ fn scan_index_from_end_for_each<F>(
 where
     F: FnMut(&SessionIndexEntry) -> std::io::Result<Option<SessionIndexEntry>>,
 {
-    let mut file = File::open(path)?;
-    let mut remaining = file.metadata()?.len();
-    let mut line_rev: Vec<u8> = Vec::new();
-    let mut buf = vec![0u8; READ_CHUNK_SIZE];
-
-    while remaining > 0 {
-        let read_size = usize::try_from(remaining.min(READ_CHUNK_SIZE as u64))
-            .map_err(std::io::Error::other)?;
-        remaining -= read_size as u64;
-        file.seek(SeekFrom::Start(remaining))?;
-        file.read_exact(&mut buf[..read_size])?;
-
-        for &byte in buf[..read_size].iter().rev() {
-            if byte == b'\n' {
-                if let Some(entry) = parse_line_from_rev(&mut line_rev, &mut visit_entry)? {
-                    return Ok(Some(entry));
-                }
-                continue;
-            }
-            line_rev.push(byte);
+    let mut scanner = ReverseJsonlScanner::new(File::open(path)?)?;
+    while let Some(outcome) = scanner.scan_next::<SessionIndexEntry>()? {
+        let ScanOutcome::Parsed(entry) = outcome else {
+            continue;
+        };
+        if let Some(entry) = visit_entry(&entry)? {
+            return Ok(Some(entry));
         }
     }
-
-    if let Some(entry) = parse_line_from_rev(&mut line_rev, &mut visit_entry)? {
-        return Ok(Some(entry));
-    }
-
     Ok(None)
 }
 
-fn parse_line_from_rev<F>(
-    line_rev: &mut Vec<u8>,
-    visit_entry: &mut F,
-) -> std::io::Result<Option<SessionIndexEntry>>
-where
-    F: FnMut(&SessionIndexEntry) -> std::io::Result<Option<SessionIndexEntry>>,
-{
-    if line_rev.is_empty() {
-        return Ok(None);
-    }
-    line_rev.reverse();
-    let line = std::mem::take(line_rev);
-    let Ok(mut line) = String::from_utf8(line) else {
-        return Ok(None);
-    };
-    if line.ends_with('\r') {
-        line.pop();
-    }
-    let trimmed = line.trim();
-    if trimmed.is_empty() {
-        return Ok(None);
-    }
-    let Ok(entry) = serde_json::from_str::<SessionIndexEntry>(trimmed) else {
-        return Ok(None);
-    };
-    visit_entry(&entry)
-}
-
 #[cfg(test)]
 #[path = "session_index_tests.rs"]
 mod tests;

codex-rs/rollout/src/session_index_tests.rs

@@ -234,6 +234,40 @@ fn scan_index_returns_none_when_entry_missing() -> std::io::Result<()> {
     Ok(())
 }
 
+#[tokio::test]
+async fn reverse_lookup_accepts_valid_eof_json_and_skips_invalid() -> std::io::Result<()> {
+    let temp = TempDir::new()?;
+    let path = session_index_path(temp.path());
+    let expected = SessionIndexEntry {
+        id: ThreadId::new(),
+        thread_name: "expected".to_string(),
+        updated_at: "2024-01-01T00:00:00Z".to_string(),
+    };
+    let unterminated = SessionIndexEntry {
+        id: ThreadId::new(),
+        thread_name: "unterminated".to_string(),
+        updated_at: "2024-01-02T00:00:00Z".to_string(),
+    };
+    std::fs::write(
+        &path,
+        format!(
+            "{}\nnot-json\n{}",
+            serde_json::to_string(&expected)?,
+            serde_json::to_string(&unterminated)?
+        ),
+    )?;
+
+    assert_eq!(
+        find_thread_name_by_id(temp.path(), &unterminated.id).await?,
+        Some("unterminated".to_string())
+    );
+    assert_eq!(
+        find_thread_name_by_id(temp.path(), &expected.id).await?,
+        Some("expected".to_string())
+    );
+    Ok(())
+}
+
 #[tokio::test]
 async fn find_thread_names_by_ids_prefers_latest_entry() -> std::io::Result<()> {
     let temp = TempDir::new()?;