store: fsck improvements, fix index rebuild and batch offset bug

- Add fsck_full(): compares current index with rebuilt, reports zombies/missing
- Add repair_index(): rebuilds index from capnp log
- Index rebuild now uses timestamp (not version) for "latest" detection
  Fixes tombstones shadowing restored nodes when version numbers reset
- Add read_node_at_offset_for_key() to handle batch writes correctly
  When multiple nodes share an offset, filter by key to get the right one
- Add find_latest_by_key() and find_last_live_version() for restore support

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/hippocampus/store/capnp.rs

@@ -212,7 +212,9 @@ impl Relation {
 
 /// Read a single node at the given offset in the capnp log.
 /// The offset must point to a valid message containing the node.
-pub fn read_node_at_offset(offset: u64) -> Result<Node> {
+/// Read a node at a given offset. If `target_key` is provided, find that specific
+/// node in the message (handles batch writes where multiple nodes share an offset).
+pub fn read_node_at_offset_for_key(offset: u64, target_key: Option<&str>) -> Result<Node> {
     let path = nodes_path();
     let mut file = fs::File::open(&path)
         .with_context(|| format!("open {}", path.display()))?;
@@ -229,13 +231,22 @@ pub fn read_node_at_offset(offset: u64) -> Result<Node> {
     let nodes = log.get_nodes()
         .with_context(|| "get nodes")?;
 
-    // A message at this offset should have exactly one node (from upsert),
-    // or we take the last one if there are multiple (from batch operations like rename)
     if nodes.is_empty() {
         anyhow::bail!("no nodes in message at offset {}", offset);
     }
 
-    // Return the first non-deleted node, or the first one if all are deleted
+    // If target_key specified, find that specific node
+    if let Some(key) = target_key {
+        for node_reader in nodes.iter() {
+            let node = Node::from_capnp_migrate(node_reader)?;
+            if node.key == key {
+                return Ok(node);
+            }
+        }
+        anyhow::bail!("node '{}' not found in message at offset {}", key, offset);
+    }
+
+    // No target key - return first non-deleted, or first if all deleted
     for node_reader in nodes.iter() {
         let node = Node::from_capnp_migrate(node_reader)?;
         if !node.deleted {
@@ -243,10 +254,14 @@ pub fn read_node_at_offset(offset: u64) -> Result<Node> {
         }
     }
 
-    // All nodes in this message are deleted - shouldn't happen if index is correct
     Node::from_capnp_migrate(nodes.get(0))
 }
 
+/// Read a node at offset (legacy, no key filtering)
+pub fn read_node_at_offset(offset: u64) -> Result<Node> {
+    read_node_at_offset_for_key(offset, None)
+}
+
 // ---------------------------------------------------------------------------
 // Store persistence methods
 // ---------------------------------------------------------------------------
@@ -414,6 +429,88 @@ impl Store {
         Ok(by_key)
     }
 
+    /// Find the most recent version of a node by key (including deleted).
+    /// Scans the entire log. Used for version continuity when recreating deleted nodes.
+    pub fn find_latest_by_key(&self, target_key: &str) -> Result<Option<Node>> {
+        let path = nodes_path();
+        if !path.exists() { return Ok(None); }
+
+        let file = fs::File::open(&path)
+            .with_context(|| format!("open {}", path.display()))?;
+        let mut reader = BufReader::new(file);
+
+        let mut latest: Option<Node> = None;
+
+        while let Ok(msg) = serialize::read_message(&mut reader, message::ReaderOptions::new()) {
+            let log = match msg.get_root::<memory_capnp::node_log::Reader>() {
+                Ok(l) => l,
+                Err(_) => continue,
+            };
+            let nodes = match log.get_nodes() {
+                Ok(n) => n,
+                Err(_) => continue,
+            };
+            for node_reader in nodes {
+                let node = match Node::from_capnp_migrate(node_reader) {
+                    Ok(n) => n,
+                    Err(_) => continue,
+                };
+                if node.key != target_key { continue; }
+                // Keep if newer timestamp (handles version resets)
+                let dominated = latest.as_ref()
+                    .map(|l| node.timestamp >= l.timestamp)
+                    .unwrap_or(true);
+                if dominated {
+                    latest = Some(node);
+                }
+            }
+        }
+
+        Ok(latest)
+    }
+
+    /// Find the last non-deleted version of a node by key.
+    /// Scans the entire log. Used for restore operations.
+    pub fn find_last_live_version(&self, target_key: &str) -> Result<Option<Node>> {
+        let path = nodes_path();
+        if !path.exists() { return Ok(None); }
+
+        let file = fs::File::open(&path)
+            .with_context(|| format!("open {}", path.display()))?;
+        let mut reader = BufReader::new(file);
+
+        let mut last_live: Option<Node> = None;
+
+        while let Ok(msg) = serialize::read_message(&mut reader, message::ReaderOptions::new()) {
+            let log = match msg.get_root::<memory_capnp::node_log::Reader>() {
+                Ok(l) => l,
+                Err(_) => continue,
+            };
+            let nodes = match log.get_nodes() {
+                Ok(n) => n,
+                Err(_) => continue,
+            };
+            for node_reader in nodes {
+                let node = match Node::from_capnp_migrate(node_reader) {
+                    Ok(n) => n,
+                    Err(_) => continue,
+                };
+                if node.key != target_key { continue; }
+                if !node.deleted {
+                    // Keep the most recent non-deleted version by timestamp
+                    let dominated = last_live.as_ref()
+                        .map(|l| node.timestamp >= l.timestamp)
+                        .unwrap_or(true);
+                    if dominated {
+                        last_live = Some(node);
+                    }
+                }
+            }
+        }
+
+        Ok(last_live)
+    }
+
     /// Append nodes to the log file. Returns the offset where the message was written.
     pub fn append_nodes(&self, nodes: &[Node]) -> Result<u64> {
         use std::sync::atomic::Ordering;
@@ -646,9 +743,9 @@ fn rebuild_index(db_path: &Path, capnp_path: &Path) -> Result<redb::Database> {
                 }
             }
 
-            // Keep if newer version
+            // Keep if newer timestamp (not version - version can reset after delete/recreate)
             let dominated = latest.get(&key)
-                .map(|(_, _, v, _, _, _, _)| version >= *v)
+                .map(|(_, _, _, _, _, ts, _)| timestamp >= *ts)
                 .unwrap_or(true);
             if dominated {
                 latest.insert(key, (offset, uuid, version, deleted, node_type, timestamp, provenance));
@@ -690,3 +787,100 @@ fn rebuild_index(db_path: &Path, capnp_path: &Path) -> Result<redb::Database> {
 
     Ok(database)
 }
+
+/// Fsck report — discrepancies found between capnp logs and redb index.
+#[derive(Debug, Default)]
+pub struct FsckReport {
+    /// Keys in current index but not in rebuilt (zombie entries)
+    pub zombies: Vec<String>,
+    /// Keys in rebuilt but not in current index (missing from index)
+    pub missing: Vec<String>,
+    /// Was capnp log repaired?
+    pub capnp_repaired: bool,
+}
+
+impl FsckReport {
+    pub fn is_clean(&self) -> bool {
+        self.zombies.is_empty() && self.missing.is_empty() && !self.capnp_repaired
+    }
+}
+
+/// Full fsck: verify capnp logs, rebuild index to temp, compare with current.
+/// Returns a report of discrepancies found.
+pub fn fsck_full() -> Result<FsckReport> {
+    use redb::{ReadableDatabase, ReadableTable};
+    use tempfile::TempDir;
+
+    let mut report = FsckReport::default();
+
+    // Step 1: Run capnp log fsck (may truncate corrupt messages)
+    // We need to check if it did repairs — currently fsck() just prints to stderr
+    // For now, we'll re-check after by comparing file sizes
+    let nodes_size_before = nodes_path().metadata().map(|m| m.len()).unwrap_or(0);
+    fsck()?;
+    let nodes_size_after = nodes_path().metadata().map(|m| m.len()).unwrap_or(0);
+    report.capnp_repaired = nodes_size_after != nodes_size_before;
+
+    // Step 2: Rebuild index to temp file
+    let temp_dir = TempDir::new().context("create temp dir")?;
+    let temp_db_path = temp_dir.path().join("rebuilt.redb");
+    let rebuilt_db = rebuild_index(&temp_db_path, &nodes_path())?;
+
+    // Step 3: Copy current index to temp and open (avoids write lock contention)
+    let current_db_path = db_path();
+    if !current_db_path.exists() {
+        // No current index — all rebuilt keys are "missing"
+        let txn = rebuilt_db.begin_read()?;
+        let table = txn.open_table(index::NODES)?;
+        for entry in table.iter()? {
+            let (key, _) = entry?;
+            report.missing.push(key.value().to_string());
+        }
+        return Ok(report);
+    }
+
+    // Copy to temp to avoid lock contention with running daemon
+    let current_copy_path = temp_dir.path().join("current.redb");
+    fs::copy(&current_db_path, &current_copy_path)
+        .with_context(|| format!("copy {} to temp", current_db_path.display()))?;
+
+    let current_db = redb::Database::open(&current_copy_path)
+        .with_context(|| format!("open current db copy"))?;
+
+    // Step 4: Compare NODES tables
+    // Collect all keys from both
+    let rebuilt_keys: std::collections::HashSet<String> = {
+        let txn = rebuilt_db.begin_read()?;
+        let table = txn.open_table(index::NODES)?;
+        table.iter()?.map(|e| e.map(|(k, _)| k.value().to_string())).collect::<Result<_, _>>()?
+    };
+
+    let current_keys: std::collections::HashSet<String> = {
+        let txn = current_db.begin_read()?;
+        let table = txn.open_table(index::NODES)?;
+        table.iter()?.map(|e| e.map(|(k, _)| k.value().to_string())).collect::<Result<_, _>>()?
+    };
+
+    // Keys in current but not rebuilt = zombies (shouldn't exist)
+    for key in current_keys.difference(&rebuilt_keys) {
+        report.zombies.push(key.clone());
+    }
+    report.zombies.sort();
+
+    // Keys in rebuilt but not current = missing (should exist but don't)
+    for key in rebuilt_keys.difference(&current_keys) {
+        report.missing.push(key.clone());
+    }
+    report.missing.sort();
+
+    Ok(report)
+}
+
+/// Repair the index by rebuilding from capnp logs.
+/// Use after fsck_full() reports discrepancies.
+pub fn repair_index() -> Result<()> {
+    let db_path = db_path();
+    rebuild_index(&db_path, &nodes_path())?;
+    eprintln!("index rebuilt from capnp log");
+    Ok(())
+}

src/hippocampus/store/mod.rs

@@ -27,8 +27,7 @@ pub use types::{
     new_node, new_relation,
 };
 pub use view::StoreView;
-pub use capnp::fsck;
-pub use ops::current_provenance;
+pub use capnp::{fsck, fsck_full, repair_index, FsckReport};
 
 use crate::graph::{self, Graph};
 
@@ -81,7 +80,7 @@ impl Store {
             .ok_or_else(|| anyhow::anyhow!("store not loaded"))?;
 
         match index::get_offset(db, key)? {
-            Some(offset) => Ok(Some(capnp::read_node_at_offset(offset)?)),
+            Some(offset) => Ok(Some(capnp::read_node_at_offset_for_key(offset, Some(key))?)),
             None => Ok(None),
         }
     }