# Task 06: `tidalctl recover --verify-only`

## Delivers

A `recover` subcommand for the `tidalctl` CLI that performs a dry-run WAL replay against a tidalDB data directory. Reports: total event count, last WAL sequence number, checkpoint sequence number, number of inconsistencies (corrupted batches, deserialization failures), estimated recovery time, and segment file inventory. Writes no state -- purely diagnostic.

Operators use this to answer "what happened?" after a crash, and "how long will recovery take?" before restarting the database.

## Complexity: S

## Dependencies

- Task 05 (recovery benchmark -- provides the recovery time estimation model)

## Technical Design

### 1. CLI argument parsing

The `tidalctl` binary already exists (from m0p2). Add a `recover` subcommand:

```rust
// In the tidalctl clap definition:

#[derive(clap::Subcommand)]
enum Commands {
    // ... existing subcommands ...

    /// Inspect WAL state and verify crash recovery without modifying data.
    Recover {
        /// Path to the tidalDB data directory.
        #[arg(long)]
        path: std::path::PathBuf,

        /// Dry-run: report WAL state without writing any changes.
        /// This is the only supported mode initially.
        #[arg(long, default_value_t = true)]
        verify_only: bool,
    },
}
```

### 2. Recovery report struct

```rust
// tidal/src/wal/diagnostics.rs (new file)

use std::path::Path;

use super::checkpoint::CheckpointManager;
use super::error::WalError;
use super::format::{self, BatchHeader, EventRecord, HEADER_SIZE, MAGIC};
use super::segment;

/// Diagnostic report from a dry-run WAL inspection.
#[derive(Debug)]
pub struct WalDiagnosticReport {
    /// Total number of valid signal events found in WAL segments.
    pub total_events: u64,
    /// Number of events that would be replayed (after checkpoint filtering).
    pub replay_events: u64,
    /// Last WAL sequence number seen in any segment.
    pub last_wal_seq: u64,
    /// Checkpoint sequence number (from checkpoint.meta), or 0 if none.
    pub checkpoint_seq: u64,
    /// Checkpoint timestamp (nanos), or 0 if none.
    pub checkpoint_ts: u64,
    /// Number of WAL segment files.
    pub segment_count: usize,
    /// Total bytes across all segment files.
    pub total_segment_bytes: u64,
    /// Number of corrupted or truncated batches encountered.
    pub inconsistency_count: u64,
    /// Per-segment summary.
    pub segments: Vec<SegmentSummary>,
    /// Estimated recovery time in seconds (based on event count heuristic).
    pub estimated_recovery_secs: f64,
}

/// Summary of a single WAL segment file.
#[derive(Debug)]
pub struct SegmentSummary {
    /// First sequence number in this segment.
    pub first_seq: u64,
    /// File size in bytes.
    pub file_size: u64,
    /// Number of valid batches.
    pub batch_count: u64,
    /// Number of valid events.
    pub event_count: u64,
    /// Number of corrupted batches.
    pub corrupt_batches: u64,
}
```

### 3. Diagnostic scan implementation

```rust
/// Perform a dry-run WAL inspection and produce a diagnostic report.
///
/// This function reads WAL segments and the checkpoint file but writes
/// nothing. It is safe to run against a live database directory (reads
/// are independent of the writer thread's append-only operations).
///
/// # Errors
///
/// Returns `WalError::Io` on filesystem failure.
pub fn diagnose_wal(data_dir: &Path) -> Result<WalDiagnosticReport, WalError> {
    let wal_dir = data_dir.join("wal");

    // Read checkpoint.
    let checkpoint = CheckpointManager::read(&wal_dir)?;
    let (checkpoint_seq, checkpoint_ts) = checkpoint.unwrap_or((0, 0));

    // List and scan segments.
    let segment_list = segment::list_segments(&wal_dir)?;
    let segment_count = segment_list.len();
    let mut total_events = 0u64;
    let mut replay_events = 0u64;
    let mut last_wal_seq = 0u64;
    let mut inconsistency_count = 0u64;
    let mut total_segment_bytes = 0u64;
    let mut segments = Vec::with_capacity(segment_count);

    for (seg_first_seq, seg_path) in &segment_list {
        let file_size = std::fs::metadata(seg_path)
            .map(|m| m.len())
            .unwrap_or(0);
        total_segment_bytes += file_size;

        let data = std::fs::read(seg_path)?;
        let mut offset = 0usize;
        let mut batch_count = 0u64;
        let mut event_count = 0u64;
        let mut corrupt_batches = 0u64;

        while offset < data.len() {
            if data.len() - offset < HEADER_SIZE {
                corrupt_batches += 1;
                inconsistency_count += 1;
                break;
            }
            if data[offset..offset + 4] != MAGIC {
                corrupt_batches += 1;
                inconsistency_count += 1;
                break;
            }

            let payload_len = u32::from_le_bytes(
                data[offset + 24..offset + 28]
                    .try_into()
                    .unwrap_or([0; 4]),
            ) as usize;

            let batch_end = offset + HEADER_SIZE + payload_len;
            if batch_end > data.len() {
                corrupt_batches += 1;
                inconsistency_count += 1;
                break;
            }

            match format::decode_batch(&data[offset..batch_end]) {
                Ok((header, events)) => {
                    batch_count += 1;
                    let n = events.len() as u64;
                    event_count += n;
                    total_events += n;

                    // Count replay events (after checkpoint).
                    for (i, _) in events.iter().enumerate() {
                        let event_seq = header.first_seq + i as u64;
                        if event_seq >= checkpoint_seq {
                            replay_events += 1;
                        }
                        let candidate = event_seq + 1;
                        if candidate > last_wal_seq {
                            last_wal_seq = candidate;
                        }
                    }

                    offset = batch_end;
                }
                Err(_) => {
                    corrupt_batches += 1;
                    inconsistency_count += 1;
                    break;
                }
            }
        }

        segments.push(SegmentSummary {
            first_seq: *seg_first_seq,
            file_size,
            batch_count,
            event_count,
            corrupt_batches,
        });
    }

    // Estimate recovery time.
    // Heuristic: ~100K events/sec replay throughput based on benchmarks.
    // Checkpoint restore adds ~10ms per 1000 entries.
    // Total = replay_events / 100_000 + checkpoint_restore_estimate.
    let replay_secs = replay_events as f64 / 100_000.0;
    // We don't know the checkpoint size from the WAL alone, so estimate
    // conservatively as 5 seconds for checkpoint restore.
    let estimated_recovery_secs = replay_secs + 5.0;

    Ok(WalDiagnosticReport {
        total_events,
        replay_events,
        last_wal_seq,
        checkpoint_seq,
        checkpoint_ts,
        segment_count,
        total_segment_bytes,
        inconsistency_count,
        segments,
        estimated_recovery_secs,
    })
}
```

### 4. CLI output formatting

```rust
// In tidalctl, in the recover subcommand handler:

fn handle_recover(path: &std::path::Path, verify_only: bool) -> anyhow::Result<()> {
    use tidaldb::wal::diagnostics::diagnose_wal;

    if !verify_only {
        anyhow::bail!("only --verify-only mode is supported");
    }

    let report = diagnose_wal(path)
        .map_err(|e| anyhow::anyhow!("WAL diagnosis failed: {e}"))?;

    println!("WAL Diagnostic Report");
    println!("=====================");
    println!();
    println!("Checkpoint:");
    if report.checkpoint_seq > 0 {
        println!("  Sequence:  {}", report.checkpoint_seq);
        println!("  Timestamp: {} ns", report.checkpoint_ts);
    } else {
        println!("  (no checkpoint found)");
    }
    println!();
    println!("WAL Segments: {}", report.segment_count);
    println!("Total Segment Bytes: {} ({:.1} MB)",
        report.total_segment_bytes,
        report.total_segment_bytes as f64 / 1_048_576.0,
    );
    println!();
    println!("Events:");
    println!("  Total in WAL:    {}", report.total_events);
    println!("  To replay:       {}", report.replay_events);
    println!("  Last WAL seq:    {}", report.last_wal_seq);
    println!("  Inconsistencies: {}", report.inconsistency_count);
    println!();
    println!("Estimated Recovery Time: {:.1}s", report.estimated_recovery_secs);
    println!();

    if !report.segments.is_empty() {
        println!("Segment Inventory:");
        println!("  {:>10}  {:>10}  {:>8}  {:>8}  {:>8}",
            "first_seq", "file_size", "batches", "events", "corrupt");
        for seg in &report.segments {
            println!("  {:>10}  {:>10}  {:>8}  {:>8}  {:>8}",
                seg.first_seq, seg.file_size, seg.batch_count,
                seg.event_count, seg.corrupt_batches);
        }
    }

    if report.inconsistency_count > 0 {
        println!();
        println!("WARNING: {} inconsistencies found. Recovery will truncate corrupted tails.",
            report.inconsistency_count);
    }

    Ok(())
}
```

### 5. Module registration

Add `pub mod diagnostics;` to `tidal/src/wal/mod.rs`.

## Acceptance Criteria

- [ ] `tidalctl recover --path <dir> --verify-only` runs without modifying any files
- [ ] Reports: total event count, replay event count, last WAL sequence, checkpoint sequence
- [ ] Reports: inconsistency count (corrupted/truncated batches)
- [ ] Reports: estimated recovery time in seconds
- [ ] Reports: per-segment inventory (first_seq, file_size, batch_count, event_count, corrupt_batches)
- [ ] `WalDiagnosticReport` struct in `tidal/src/wal/diagnostics.rs`
- [ ] `diagnose_wal(data_dir)` function scans WAL without writing state
- [ ] Handles missing WAL directory gracefully (reports 0 segments)
- [ ] Handles missing checkpoint file gracefully (reports checkpoint_seq=0)
- [ ] Unit tests: `diagnose_empty_dir`, `diagnose_single_segment`, `diagnose_with_checkpoint`, `diagnose_with_corruption`

## Test Strategy

```rust
#[cfg(test)]
#[allow(clippy::unwrap_used)]
mod tests {
    use super::*;
    use crate::wal::format::encode_batch;
    use crate::wal::format::EventRecord;
    use crate::wal::segment::segment_filename;

    fn sample_event(id: u64) -> EventRecord {
        EventRecord {
            entity_id: id,
            signal_type: 1,
            weight: 1.0,
            timestamp_nanos: id * 1_000_000_000,
        }
    }

    #[test]
    fn diagnose_empty_dir() {
        let dir = tempfile::tempdir().unwrap();
        // Create the wal subdirectory (diagnose_wal expects data_dir, not wal_dir).
        std::fs::create_dir_all(dir.path().join("wal")).unwrap();

        let report = diagnose_wal(dir.path()).unwrap();
        assert_eq!(report.total_events, 0);
        assert_eq!(report.segment_count, 0);
        assert_eq!(report.checkpoint_seq, 0);
        assert_eq!(report.inconsistency_count, 0);
    }

    #[test]
    fn diagnose_single_segment() {
        let dir = tempfile::tempdir().unwrap();
        let wal_dir = dir.path().join("wal");
        std::fs::create_dir_all(&wal_dir).unwrap();

        let events = vec![sample_event(1), sample_event(2), sample_event(3)];
        let batch = encode_batch(&events, 1, 1000).unwrap();
        let seg_name = segment_filename(1);
        std::fs::write(wal_dir.join(seg_name), &batch).unwrap();

        let report = diagnose_wal(dir.path()).unwrap();
        assert_eq!(report.total_events, 3);
        assert_eq!(report.replay_events, 3); // no checkpoint
        assert_eq!(report.segment_count, 1);
        assert_eq!(report.inconsistency_count, 0);
        assert_eq!(report.segments[0].event_count, 3);
    }

    #[test]
    fn diagnose_with_checkpoint() {
        let dir = tempfile::tempdir().unwrap();
        let wal_dir = dir.path().join("wal");
        std::fs::create_dir_all(&wal_dir).unwrap();

        // Write checkpoint at seq=2.
        CheckpointManager::write(&wal_dir, 2, 2000).unwrap();

        // Write events at seq 1, 2, 3.
        let events = vec![sample_event(1), sample_event(2), sample_event(3)];
        let batch = encode_batch(&events, 1, 1000).unwrap();
        std::fs::write(wal_dir.join(segment_filename(1)), &batch).unwrap();

        let report = diagnose_wal(dir.path()).unwrap();
        assert_eq!(report.total_events, 3);
        assert_eq!(report.replay_events, 2); // events at seq 2, 3
        assert_eq!(report.checkpoint_seq, 2);
    }

    #[test]
    fn diagnose_with_corruption() {
        let dir = tempfile::tempdir().unwrap();
        let wal_dir = dir.path().join("wal");
        std::fs::create_dir_all(&wal_dir).unwrap();

        let events = vec![sample_event(1)];
        let mut batch = encode_batch(&events, 1, 1000).unwrap();
        // Append garbage to simulate a torn write.
        batch.extend_from_slice(&[0xDE, 0xAD, 0xBE, 0xEF]);

        std::fs::write(wal_dir.join(segment_filename(1)), &batch).unwrap();

        let report = diagnose_wal(dir.path()).unwrap();
        assert_eq!(report.total_events, 1); // one valid event
        assert_eq!(report.inconsistency_count, 1); // one corrupt tail
    }
}
```
