magi-code 0.77.1

Repository-aware CLI coding agent for terminal work
Documentation
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use super::{
    capture::{CHECKPOINT_SCHEMA_VERSION, LedgerRead, SnapshotBlobRef, SnapshotLedgerRecord},
    sanitized_diagnostic,
};
use crate::{
    config::McPaths,
    persistence::{CrossProcessFileLock, atomic_write, sync_parent_dir},
};
use sha2::{Digest, Sha256};
use std::{
    error::Error,
    fmt, fs,
    io::{ErrorKind, Read, Seek, SeekFrom, Write},
    path::{Path, PathBuf},
};

#[derive(Debug)]
pub(crate) enum CheckpointAppendFailure {
    OutcomeUncertain {
        path: PathBuf,
        source: anyhow::Error,
    },
    CommittedButUndurable {
        path: PathBuf,
        source: anyhow::Error,
    },
}

impl CheckpointAppendFailure {
    fn uncertain(path: &Path, source: anyhow::Error) -> anyhow::Error {
        anyhow::Error::new(Self::OutcomeUncertain {
            path: path.to_path_buf(),
            source,
        })
    }

    fn committed_but_undurable(path: &Path, source: anyhow::Error) -> anyhow::Error {
        anyhow::Error::new(Self::CommittedButUndurable {
            path: path.to_path_buf(),
            source,
        })
    }
}

impl fmt::Display for CheckpointAppendFailure {
    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
        let (path, outcome) = match self {
            Self::OutcomeUncertain { path, .. } => {
                (path, "append outcome is uncertain; do not retry blindly")
            }
            Self::CommittedButUndurable { path, .. } => (
                path,
                "record is visible but final synchronization failed; do not retry blindly",
            ),
        };
        write!(
            formatter,
            "checkpoint ledger append failed for {}: {outcome}",
            path.display()
        )
    }
}

impl Error for CheckpointAppendFailure {
    fn source(&self) -> Option<&(dyn Error + 'static)> {
        let source = match self {
            Self::OutcomeUncertain { source, .. } | Self::CommittedButUndurable { source, .. } => {
                source
            }
        };
        Some(source.as_ref())
    }
}

#[derive(Debug, Default)]
struct LedgerIoMetrics {
    read_bytes: u64,
    write_bytes: u64,
    serialized_record_bytes: u64,
}

#[derive(Debug, Clone, PartialEq, Eq)]
pub(crate) struct CheckpointStore {
    pub(super) root: PathBuf,
}

impl CheckpointStore {
    pub(crate) fn new(root: PathBuf) -> Self {
        Self { root }
    }

    pub(crate) fn from_paths(paths: &McPaths) -> Self {
        Self::new(paths.checkpoints.clone())
    }

    pub(crate) fn copy_before_turn(
        &self,
        source: &str,
        target: &str,
        turn: u64,
    ) -> anyhow::Result<()> {
        crate::sessions::validate_session_id(source.to_owned())?;
        crate::sessions::validate_session_id(target.to_owned())?;
        let read = self.read_records(source);
        anyhow::ensure!(
            read.diagnostics.is_empty(),
            "cannot copy unreadable file checkpoints"
        );
        for mut record in read
            .records
            .into_iter()
            .filter(|record| record.event.user_turn < turn)
        {
            record.event.session_id = target.to_owned();
            self.append_record(&record)?;
        }
        Ok(())
    }

    pub(super) fn blobs_dir(&self) -> PathBuf {
        self.root.join("blobs")
    }

    pub(super) fn ledgers_dir(&self) -> PathBuf {
        self.root.join("ledgers")
    }

    pub(super) fn ledger_path(&self, session_id: &str) -> PathBuf {
        self.ledgers_dir().join(format!("{session_id}.jsonl"))
    }

    pub(super) fn blob_path(&self, sha256: &str) -> PathBuf {
        self.blobs_dir().join(sha256)
    }

    fn mutation_lock_target(&self) -> PathBuf {
        self.root.with_file_name("checkpoint-store")
    }

    pub(super) fn lock_mutations(&self) -> anyhow::Result<CrossProcessFileLock> {
        CrossProcessFileLock::acquire(&self.mutation_lock_target())
    }

    pub(super) fn write_blob(&self, bytes: &[u8]) -> anyhow::Result<SnapshotBlobRef> {
        let sha256 = sha256_hex(bytes);
        let path = self.blob_path(&sha256);
        if path.exists() {
            return Ok(SnapshotBlobRef {
                sha256,
                bytes: bytes.len() as u64,
            });
        }
        atomic_write(&path, bytes)?;
        Ok(SnapshotBlobRef {
            sha256,
            bytes: bytes.len() as u64,
        })
    }

    pub(super) fn read_blob(&self, blob: &SnapshotBlobRef) -> anyhow::Result<Vec<u8>> {
        let bytes = fs::read(self.blob_path(&blob.sha256))?;
        if sha256_hex(&bytes) != blob.sha256 {
            anyhow::bail!("checkpoint blob hash mismatch");
        }
        Ok(bytes)
    }

    pub(super) fn append_record(&self, record: &SnapshotLedgerRecord) -> anyhow::Result<()> {
        self.append_record_with(
            record,
            None,
            |file| file.sync_all().map_err(anyhow::Error::from),
            sync_parent_dir,
        )
    }

    fn append_record_with(
        &self,
        record: &SnapshotLedgerRecord,
        mut metrics: Option<&mut LedgerIoMetrics>,
        sync_file: impl FnOnce(&fs::File) -> anyhow::Result<()>,
        sync_parent: impl FnOnce(&Path) -> anyhow::Result<()>,
    ) -> anyhow::Result<()> {
        crate::sessions::validate_session_id(record.event.session_id.clone())?;
        let ledger = self.ledger_path(&record.event.session_id);
        if let Some(parent) = ledger.parent() {
            fs::create_dir_all(parent)?;
        }
        let mut line = serde_json::to_vec(record)?;
        line.push(b'\n');
        if let Some(metrics) = metrics.as_deref_mut() {
            metrics.serialized_record_bytes += line.len() as u64;
        }

        let _lock = CrossProcessFileLock::acquire(&ledger)?;
        let (mut file, created) = open_ledger_for_append(&ledger)?;
        let pre_append_len = file.metadata()?.len();
        let needs_separator = if pre_append_len == 0 {
            false
        } else {
            file.seek(SeekFrom::End(-1))?;
            let mut tail = [0_u8; 1];
            file.read_exact(&mut tail)?;
            if let Some(metrics) = metrics.as_deref_mut() {
                metrics.read_bytes += 1;
            }
            tail[0] != b'\n'
        };
        if needs_separator {
            line.insert(0, b'\n');
        }
        if let Some(metrics) = metrics {
            metrics.write_bytes += line.len() as u64;
        }

        if let Err(write_error) = file.write_all(&line) {
            let rollback = file.set_len(pre_append_len).and_then(|()| file.sync_all());
            return match rollback {
                Ok(()) => Err(anyhow::Error::new(write_error).context(
                    "checkpoint ledger append failed; partial data was durably rolled back",
                )),
                Err(rollback_error) => Err(CheckpointAppendFailure::uncertain(
                    &ledger,
                    anyhow::anyhow!(
                        "append write failed ({write_error}); rollback failed ({rollback_error})"
                    ),
                )),
            };
        }
        file.flush().map_err(|error| {
            CheckpointAppendFailure::committed_but_undurable(&ledger, error.into())
        })?;
        sync_file(&file)
            .map_err(|error| CheckpointAppendFailure::committed_but_undurable(&ledger, error))?;
        if created {
            let parent = ledger
                .parent()
                .ok_or_else(|| anyhow::anyhow!("checkpoint ledger has no parent"))?;
            sync_parent(parent).map_err(|error| {
                CheckpointAppendFailure::committed_but_undurable(&ledger, error)
            })?;
        }
        Ok(())
    }

    pub(crate) fn read_records(&self, session_id: &str) -> LedgerRead {
        let ledger = self.ledger_path(session_id);
        let mut read = LedgerRead::default();
        let bytes = match fs::read(&ledger) {
            Ok(bytes) => bytes,
            Err(error) if error.kind() == ErrorKind::NotFound => return read,
            Err(error) => {
                read.diagnostics.push(sanitized_diagnostic(format!(
                    "checkpoint ledger read failed: {error}"
                )));
                return read;
            }
        };
        let final_segment = bytes.split(|byte| *byte == b'\n').count().saturating_sub(1);
        let has_terminated_tail = bytes.ends_with(b"\n");
        for (index, line) in bytes.split(|byte| *byte == b'\n').enumerate() {
            if line.iter().all(u8::is_ascii_whitespace) {
                continue;
            }
            match serde_json::from_slice::<SnapshotLedgerRecord>(line) {
                Ok(record) if record.schema_version == CHECKPOINT_SCHEMA_VERSION => {
                    read.records.push(record);
                }
                Ok(_) => read.diagnostics.push(format!(
                    "ignored checkpoint ledger line {}: unsupported_schema_version",
                    index + 1
                )),
                Err(_) if index == final_segment && !has_terminated_tail => {
                    read.diagnostics.push(format!(
                        "ignored checkpoint ledger line {}: incomplete_tail",
                        index + 1
                    ))
                }
                Err(_) => read.diagnostics.push(format!(
                    "ignored checkpoint ledger line {}: malformed_json",
                    index + 1
                )),
            }
        }
        read
    }

    pub(crate) fn prune_session(&self, session_id: &str) -> anyhow::Result<()> {
        if !self.validate_deletion_directories()? {
            return Ok(());
        }
        let _mutation_lock = self.lock_mutations()?;
        if !self.validate_deletion_directories()? {
            return Ok(());
        }
        let ledger = self.ledger_path(session_id);
        if ledger.exists() {
            fs::remove_file(&ledger)?;
        }
        self.prune_unreferenced_blobs_locked()
    }

    pub(crate) fn prune_unreferenced_blobs(&self) -> anyhow::Result<()> {
        if !self.validate_deletion_directories()? {
            return Ok(());
        }
        let _mutation_lock = self.lock_mutations()?;
        if !self.validate_deletion_directories()? {
            return Ok(());
        }
        self.prune_unreferenced_blobs_locked()
    }

    fn validate_deletion_directories(&self) -> anyhow::Result<bool> {
        if !validate_owned_directory(&self.root)? {
            return Ok(false);
        }
        for path in [self.ledgers_dir(), self.blobs_dir()] {
            match fs::symlink_metadata(&path) {
                Ok(_) => {
                    if !validate_owned_directory(&path)? {
                        anyhow::bail!("unsafe checkpoint directory: {}", path.display());
                    }
                }
                Err(error) if error.kind() == ErrorKind::NotFound => {}
                Err(error) => return Err(error.into()),
            }
        }
        Ok(true)
    }

    fn prune_unreferenced_blobs_locked(&self) -> anyhow::Result<()> {
        let mut referenced = std::collections::BTreeSet::new();
        let ledgers = self.ledgers_dir();
        if ledgers.exists() {
            for entry in fs::read_dir(&ledgers)? {
                let entry = entry?;
                let file_type = entry.file_type()?;
                if file_type.is_symlink() {
                    anyhow::bail!("unsafe checkpoint ledger entry");
                }
                if !file_type.is_file() {
                    continue;
                }
                let file_name = entry
                    .file_name()
                    .into_string()
                    .map_err(|_| anyhow::anyhow!("checkpoint ledger name is not UTF-8"))?;
                let Some(session_id) = file_name.strip_suffix(".jsonl") else {
                    continue;
                };
                crate::sessions::validate_session_id(session_id.to_string())?;
                let read = self.read_records(session_id);
                if !read.diagnostics.is_empty() {
                    anyhow::bail!("checkpoint ledger could not be read completely");
                }
                for record in read.records {
                    if let Some(blob) = record.event.pre {
                        referenced.insert(blob.sha256);
                    }
                    if let Some(blob) = record.event.post {
                        referenced.insert(blob.sha256);
                    }
                }
            }
        }
        let blobs = self.blobs_dir();
        if !blobs.exists() {
            return Ok(());
        }
        for entry in fs::read_dir(&blobs)? {
            let entry = entry?;
            if !entry.file_type()?.is_file() {
                continue;
            }
            let name = entry.file_name().to_string_lossy().into_owned();
            if !referenced.contains(&name) {
                fs::remove_file(entry.path())?;
            }
        }
        Ok(())
    }
}

fn open_ledger_for_append(path: &Path) -> anyhow::Result<(fs::File, bool)> {
    match fs::symlink_metadata(path) {
        Ok(metadata) if metadata.file_type().is_symlink() || !metadata.is_file() => {
            anyhow::bail!("unsafe checkpoint ledger entry")
        }
        Ok(_) => {
            let file = fs::OpenOptions::new().read(true).append(true).open(path)?;
            ensure_open_ledger_matches_path(&file, path)?;
            Ok((file, false))
        }
        Err(error) if error.kind() == ErrorKind::NotFound => {
            let file = fs::OpenOptions::new()
                .read(true)
                .append(true)
                .create_new(true)
                .open(path)?;
            ensure_open_ledger_matches_path(&file, path)?;
            Ok((file, true))
        }
        Err(error) => Err(error.into()),
    }
}

fn ensure_open_ledger_matches_path(file: &fs::File, path: &Path) -> anyhow::Result<()> {
    let path_metadata = fs::symlink_metadata(path)?;
    let file_metadata = file.metadata()?;
    if path_metadata.file_type().is_symlink()
        || !path_metadata.is_file()
        || !file_metadata.is_file()
    {
        anyhow::bail!("unsafe checkpoint ledger entry");
    }
    #[cfg(unix)]
    {
        use std::os::unix::fs::MetadataExt;
        if path_metadata.dev() != file_metadata.dev()
            || path_metadata.ino() != file_metadata.ino()
            || file_metadata.uid() != unsafe { libc::geteuid() }
        {
            anyhow::bail!("unsafe checkpoint ledger entry");
        }
    }
    Ok(())
}

fn validate_owned_directory(path: &std::path::Path) -> anyhow::Result<bool> {
    let metadata = match fs::symlink_metadata(path) {
        Ok(metadata) => metadata,
        Err(error) if error.kind() == ErrorKind::NotFound => return Ok(false),
        Err(error) => return Err(error.into()),
    };
    if metadata.file_type().is_symlink() || !metadata.file_type().is_dir() {
        anyhow::bail!("unsafe checkpoint directory: {}", path.display());
    }
    #[cfg(unix)]
    {
        use std::os::unix::fs::MetadataExt;
        if metadata.uid() != unsafe { libc::geteuid() } {
            anyhow::bail!("checkpoint directory is not owned by the current user");
        }
    }
    Ok(true)
}

pub(crate) fn sha256_hex(bytes: &[u8]) -> String {
    let digest = Sha256::digest(bytes);
    crate::hex::lower_hex(digest)
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::checkpoints::{
        SnapshotContext, SnapshotExclusionReason, SnapshotTool,
        capture::capture_file_snapshot_with_hook,
        capture::{FileSnapshotEvent, SnapshotCaptureStatus, SnapshotLedgerRecord},
    };
    use std::{sync::mpsc, thread, time::Duration};
    use tempfile::TempDir;

    fn store(temp: &TempDir) -> CheckpointStore {
        CheckpointStore::new(temp.path().join("mc/checkpoints"))
    }

    fn context(temp: &TempDir) -> SnapshotContext {
        let paths = McPaths::from_root(temp.path().join("mc"));
        SnapshotContext {
            store: CheckpointStore::from_paths(&paths),
            paths,
            session_id: "session".to_string(),
            user_turn: 1,
        }
    }

    fn test_record(session_id: &str, cwd: &Path, turn: u64) -> SnapshotLedgerRecord {
        SnapshotLedgerRecord::new(FileSnapshotEvent {
            session_id: session_id.to_string(),
            user_turn: turn,
            tool: SnapshotTool::Edit,
            cwd: cwd.to_path_buf(),
            relative_path: PathBuf::from("file.txt"),
            pre: None,
            post: None,
            status: SnapshotCaptureStatus::Excluded {
                reason: SnapshotExclusionReason::NotFile,
            },
        })
    }

    #[test]
    fn blob_store_round_trips_by_hash() {
        let temp = TempDir::new().unwrap();
        let store = store(&temp);
        let blob = store.write_blob(b"hello").unwrap();
        assert_eq!(store.read_blob(&blob).unwrap(), b"hello");
        assert_eq!(blob.sha256, sha256_hex(b"hello"));
    }

    #[test]
    fn blob_write_is_idempotent() {
        let temp = TempDir::new().unwrap();
        let store = store(&temp);
        let first = store.write_blob(b"same").unwrap();
        let second = store.write_blob(b"same").unwrap();
        assert_eq!(first, second);
    }

    #[test]
    fn checkpoint_gc_waits_for_blob_publication() {
        let temp = TempDir::new().unwrap();
        let context = context(&temp);
        let old_record = SnapshotLedgerRecord::new(FileSnapshotEvent {
            session_id: "old-session".to_string(),
            user_turn: 1,
            tool: SnapshotTool::Edit,
            cwd: temp.path().to_path_buf(),
            relative_path: PathBuf::from("old.txt"),
            pre: None,
            post: None,
            status: SnapshotCaptureStatus::Excluded {
                reason: SnapshotExclusionReason::NotFile,
            },
        });
        context.store.append_record(&old_record).unwrap();
        let target = temp.path().join("new.txt");
        fs::write(&target, "new").unwrap();
        let (published_tx, published_rx) = mpsc::sync_channel(0);
        let (release_tx, release_rx) = mpsc::sync_channel(0);
        let capture_context = context.clone();
        let capture_root = temp.path().to_path_buf();
        let capture_target = target.clone();
        let capture = thread::spawn(move || {
            capture_file_snapshot_with_hook(
                &capture_context,
                &capture_root,
                SnapshotTool::WriteFile,
                &capture_target,
                None,
                Some(b"new"),
                || {
                    published_tx.send(()).unwrap();
                    release_rx.recv().unwrap();
                },
            )
        });
        published_rx.recv().unwrap();
        let prune_store = context.store.clone();
        let (started_tx, started_rx) = mpsc::sync_channel(0);
        let (pruned_tx, pruned_rx) = mpsc::sync_channel(0);
        let prune = thread::spawn(move || {
            started_tx.send(()).unwrap();
            pruned_tx
                .send(prune_store.prune_session("old-session"))
                .unwrap();
        });
        started_rx.recv().unwrap();
        assert!(matches!(
            pruned_rx.recv_timeout(Duration::from_millis(100)),
            Err(mpsc::RecvTimeoutError::Timeout)
        ));

        release_tx.send(()).unwrap();
        capture.join().unwrap().unwrap();
        pruned_rx.recv().unwrap().unwrap();
        prune.join().unwrap();

        let record = context.store.read_records("session").records.pop().unwrap();
        let post = record.event.post.unwrap();
        assert_eq!(context.store.read_blob(&post).unwrap(), b"new");

        let orphan = context.store.blobs_dir().join("orphan-after-failed-gc");
        fs::write(&orphan, "orphan").unwrap();
        context.store.prune_unreferenced_blobs().unwrap();
        assert!(!orphan.exists());
    }

    #[cfg(unix)]
    #[test]
    fn checkpoint_gc_rejects_symlinked_blob_directory() {
        use std::os::unix::fs::symlink;

        let temp = TempDir::new().unwrap();
        let store = store(&temp);
        fs::create_dir_all(&store.root).unwrap();
        let outside = temp.path().join("outside");
        fs::create_dir_all(&outside).unwrap();
        let retained = outside.join("retained");
        fs::write(&retained, "keep").unwrap();
        symlink(&outside, store.blobs_dir()).unwrap();

        assert!(store.prune_unreferenced_blobs().is_err());
        assert_eq!(fs::read_to_string(retained).unwrap(), "keep");
    }

    #[test]
    fn retention_safety_preserves_blobs_for_malformed_retained_ledger() {
        let temp = TempDir::new().unwrap();
        let store = store(&temp);
        fs::create_dir_all(store.blobs_dir()).unwrap();
        fs::create_dir_all(store.ledgers_dir()).unwrap();
        let retained = store.blobs_dir().join("retained");
        fs::write(&retained, "checkpoint").unwrap();
        fs::write(store.ledger_path("live-session"), "{malformed\n").unwrap();

        assert!(store.prune_unreferenced_blobs().is_err());
        assert_eq!(fs::read_to_string(retained).unwrap(), "checkpoint");
    }

    #[test]
    fn ledger_skips_malformed_lines() {
        let temp = TempDir::new().unwrap();
        let store = store(&temp);
        let ledger = store.ledger_path("session");
        fs::create_dir_all(ledger.parent().unwrap()).unwrap();
        fs::write(&ledger, "not json\n").unwrap();
        let read = store.read_records("session");
        assert!(read.records.is_empty());
        assert_eq!(read.diagnostics.len(), 1);
    }

    #[test]
    fn ledger_append_recovers_after_truncated_line() {
        let temp = TempDir::new().unwrap();
        let store = store(&temp);
        let ledger = store.ledger_path("session");
        fs::create_dir_all(ledger.parent().unwrap()).unwrap();
        fs::write(&ledger, "truncated").unwrap();
        let event = FileSnapshotEvent {
            session_id: "session".to_string(),
            user_turn: 1,
            tool: SnapshotTool::WriteFile,
            cwd: temp.path().to_path_buf(),
            relative_path: PathBuf::from("file.txt"),
            pre: None,
            post: None,
            status: SnapshotCaptureStatus::Excluded {
                reason: SnapshotExclusionReason::SecretPath,
            },
        };
        store
            .append_record(&SnapshotLedgerRecord::new(event))
            .unwrap();
        let read = store.read_records("session");
        assert_eq!(read.records.len(), 1);
        assert_eq!(read.diagnostics.len(), 1);
    }

    #[test]
    fn ledger_reader_preserves_valid_history_before_incomplete_tail() {
        let temp = TempDir::new().unwrap();
        let store = store(&temp);
        store
            .append_record(&test_record("session", temp.path(), 1))
            .unwrap();
        let ledger = store.ledger_path("session");
        let mut file = fs::OpenOptions::new().append(true).open(&ledger).unwrap();
        file.write_all(b"{\"schema_version\":1").unwrap();
        file.sync_all().unwrap();

        let read = store.read_records("session");
        assert_eq!(read.records.len(), 1);
        assert_eq!(
            read.diagnostics,
            vec!["ignored checkpoint ledger line 2: incomplete_tail"]
        );
    }

    #[test]
    fn ledger_reader_surfaces_malformed_committed_middle_record() {
        let temp = TempDir::new().unwrap();
        let store = store(&temp);
        let first = serde_json::to_vec(&test_record("session", temp.path(), 1)).unwrap();
        let second = serde_json::to_vec(&test_record("session", temp.path(), 2)).unwrap();
        let mut ledger_bytes = first;
        ledger_bytes.extend_from_slice(b"\n{malformed}\n");
        ledger_bytes.extend_from_slice(&second);
        ledger_bytes.push(b'\n');
        let ledger = store.ledger_path("session");
        fs::create_dir_all(ledger.parent().unwrap()).unwrap();
        fs::write(ledger, ledger_bytes).unwrap();

        let read = store.read_records("session");
        assert_eq!(read.records.len(), 2);
        assert_eq!(
            read.diagnostics,
            vec!["ignored checkpoint ledger line 2: malformed_json"]
        );
    }

    #[test]
    fn complete_unterminated_record_is_read_and_separated_from_next_append() {
        let temp = TempDir::new().unwrap();
        let store = store(&temp);
        let ledger = store.ledger_path("session");
        fs::create_dir_all(ledger.parent().unwrap()).unwrap();
        fs::write(
            &ledger,
            serde_json::to_vec(&test_record("session", temp.path(), 1)).unwrap(),
        )
        .unwrap();

        store
            .append_record(&test_record("session", temp.path(), 2))
            .unwrap();
        let read = store.read_records("session");
        assert_eq!(read.records.len(), 2);
        assert!(read.diagnostics.is_empty());
    }

    #[test]
    fn visible_record_with_failed_sync_has_distinguishable_outcome() {
        let temp = TempDir::new().unwrap();
        let store = store(&temp);
        let record = test_record("session", temp.path(), 1);
        let error = store
            .append_record_with(
                &record,
                None,
                |_| Err(anyhow::anyhow!("injected file sync failure")),
                |_| Ok(()),
            )
            .unwrap_err();
        let failure = error
            .downcast_ref::<CheckpointAppendFailure>()
            .expect("typed checkpoint append failure");
        assert!(matches!(
            failure,
            CheckpointAppendFailure::CommittedButUndurable { .. }
        ));
        assert_eq!(store.read_records("session").records, vec![record]);
    }

    #[test]
    fn checkpoint_append_logical_io_scales_linearly() {
        fn measure(count: usize) -> (LedgerIoMetrics, u64, Duration) {
            let temp = TempDir::new().unwrap();
            let store = store(&temp);
            let record = test_record("session", temp.path(), 1);
            let mut metrics = LedgerIoMetrics::default();
            let started = std::time::Instant::now();
            for _ in 0..count {
                // Logical-byte instrumentation intentionally bypasses physical fsync timing. The
                // first append is the documented warm-up that creates the ledger.
                store
                    .append_record_with(&record, Some(&mut metrics), |_| Ok(()), |_| Ok(()))
                    .unwrap();
            }
            let final_bytes = fs::metadata(store.ledger_path("session")).unwrap().len();
            (metrics, final_bytes, started.elapsed())
        }

        let (at_1k, final_1k, elapsed_1k) = measure(1_000);
        let (at_2k, final_2k, elapsed_2k) = measure(2_000);
        let logical_1k = at_1k.read_bytes + at_1k.write_bytes;
        let logical_2k = at_2k.read_bytes + at_2k.write_bytes;
        let ratio = logical_2k as f64 / logical_1k as f64;

        assert_eq!(at_1k.serialized_record_bytes, final_1k);
        assert_eq!(at_2k.serialized_record_bytes, final_2k);
        assert!(ratio <= 2.5, "logical-byte ratio was {ratio}");
        eprintln!(
            "checkpoint ledger logical I/O: 1k read={} write={} final={} elapsed={elapsed_1k:?}; 2k read={} write={} final={} elapsed={elapsed_2k:?}; ratio={ratio:.3}",
            at_1k.read_bytes,
            at_1k.write_bytes,
            final_1k,
            at_2k.read_bytes,
            at_2k.write_bytes,
            final_2k,
        );
    }

    #[test]
    fn ledger_process_append_helper() {
        let Some(root) = std::env::var_os("MAGI_CHECKPOINT_APPEND_HELPER_ROOT") else {
            return;
        };
        let writer = std::env::var("MAGI_CHECKPOINT_APPEND_HELPER_WRITER")
            .unwrap()
            .parse::<u64>()
            .unwrap();
        let store = CheckpointStore::new(PathBuf::from(root));
        for index in 0..20 {
            store
                .append_record(&test_record(
                    "shared-session",
                    &store.root,
                    writer * 1_000 + index,
                ))
                .unwrap();
        }
    }

    #[test]
    fn cross_process_lock_preserves_all_serialized_appends() {
        let temp = TempDir::new().unwrap();
        let root = temp.path().join("checkpoints");
        let executable = std::env::current_exe().unwrap();
        let mut children = Vec::new();
        for writer in 0..4 {
            children.push(
                std::process::Command::new(&executable)
                    .arg("--exact")
                    .arg("checkpoints::store::tests::ledger_process_append_helper")
                    .env("MAGI_CHECKPOINT_APPEND_HELPER_ROOT", &root)
                    .env("MAGI_CHECKPOINT_APPEND_HELPER_WRITER", writer.to_string())
                    .stdout(std::process::Stdio::piped())
                    .stderr(std::process::Stdio::piped())
                    .spawn()
                    .unwrap(),
            );
        }
        for child in children {
            let output = child.wait_with_output().unwrap();
            assert!(
                output.status.success(),
                "child failed: stdout={} stderr={}",
                String::from_utf8_lossy(&output.stdout),
                String::from_utf8_lossy(&output.stderr)
            );
        }

        let read = CheckpointStore::new(root).read_records("shared-session");
        assert!(read.diagnostics.is_empty(), "{:?}", read.diagnostics);
        assert_eq!(read.records.len(), 80);
        let turns = read
            .records
            .into_iter()
            .map(|record| record.event.user_turn)
            .collect::<std::collections::BTreeSet<_>>();
        assert_eq!(turns.len(), 80);
    }
}