Bound a VS subsegment by its own size, and follow its chunk chain across holes

src/heap.rs

@@ -159,6 +159,9 @@ pub struct HeapWalkReport {
     pub diagnostic_count: usize,
     pub unreadable_gaps: usize,
     pub refused_headers: u64,
+    /// Committed VS bytes the walk declined to decode because it could not place a chunk
+    /// boundary in them; see [`crate::pool::PoolSnapshot::unplaced_bytes`].
+    pub unplaced_bytes: u64,
     pub stalls: WalkStalls,
 }
 
@@ -669,6 +672,7 @@ fn from_pool_snapshot(
             .filter(|allocation| allocation.state == HeapState::Unreadable)
             .count(),
         refused_headers: snapshot.refused_chunks,
+        unplaced_bytes: snapshot.unplaced_bytes,
         stalls: snapshot.stalls,
     };
     let diagnostics = HeapDiagnosticReport {

src/pool/index.rs

@@ -39,6 +39,8 @@ pub(crate) struct PoolIndex {
     pub stalls: WalkStalls,
     /// Carried through from [`PoolSnapshot::refused_chunks`].
     pub refused_chunks: u64,
+    /// Carried through from [`PoolSnapshot::unplaced_bytes`].
+    pub unplaced_bytes: u64,
     row_postings: HashMap<RowIdentity, Vec<usize>>,
     span_rows: Vec<RowIdentity>,
 }
@@ -91,6 +93,7 @@ impl PoolIndex {
             budget_expired: snapshot.budget_expired,
             stalls: snapshot.stalls,
             refused_chunks: snapshot.refused_chunks,
+            unplaced_bytes: snapshot.unplaced_bytes,
             row_postings,
             span_rows,
         }

src/pool/query.rs

@@ -241,6 +241,9 @@ pub struct PoolSnapshotReport {
     /// Chunk headers the walk refused and resynchronised past; see
     /// [`crate::pool::PoolSnapshot::refused_chunks`] for why this is not read off a diagnostic.
     pub refused_chunks: u64,
+    /// Committed VS bytes the walk declined to decode because it could not place a chunk
+    /// boundary in them; see [`crate::pool::PoolSnapshot::unplaced_bytes`].
+    pub unplaced_bytes: u64,
 }
 
 /// How much of the pool a walk covered.
@@ -319,6 +322,7 @@ fn report_of(index: &PoolIndex) -> PoolSnapshotReport {
         diagnostics: index.diagnostics.clone(),
         stalls: index.stalls,
         refused_chunks: index.refused_chunks,
+        unplaced_bytes: index.unplaced_bytes,
     }
 }
 

src/pool/render.rs

@@ -414,6 +414,7 @@ mod tests {
             budget_expired: false,
             stalls: Default::default(),
             refused_chunks: 0,
+            unplaced_bytes: 0,
             diagnostics: PoolDiagnostics::from_iter([
                 "per-session paged heaps are not included".to_string()
             ]),
@@ -508,6 +509,7 @@ mod tests {
             budget_expired: false,
             stalls: Default::default(),
             refused_chunks: 0,
+            unplaced_bytes: 0,
             diagnostics: PoolDiagnostics::default(),
         });
         let dml_chunks = render_pool_map(