add comments

crates/primitives/src/stage/id.rs

@@ -3,6 +3,7 @@
 /// For custom stages, use [`StageId::Other`]
 #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
 pub enum StageId {
+    /// Snapshot stage in the process.
     Snapshot,
     /// Header stage in the process.
     Headers,

crates/stages/src/error.rs

@@ -106,6 +106,8 @@ pub enum StageError {
         /// Expected static file block number.
         static_file: BlockNumber,
     },
+    #[error("unwind not supported for this stage")]
+    UnwindNotSupported,
     /// Internal error
     #[error(transparent)]
     Internal(#[from] RethError),

crates/stages/src/stages/snapshot.rs

@@ -4,7 +4,10 @@ use reth_primitives::stage::{StageCheckpoint, StageId};
 use reth_provider::{BlockNumReader, DatabaseProviderRW};
 use reth_snapshot::Snapshotter;
 
-/// The snapshot stage.
+/// The snapshot stage _copies_ all data from database to static files using [Snapshotter]. The
+/// block range for copying is determined by the current highest blocks contained in static files
+/// and [BlockNumReader::best_block_number],
+/// i.e. the range is `highest_snapshotted_block_number..=best_block_number`.
 #[derive(Debug)]
 pub struct SnapshotStage<DB: Database> {
     snapshotter: Snapshotter<DB>,
@@ -29,15 +32,14 @@ impl<DB: Database> Stage<DB> for SnapshotStage<DB> {
     ) -> Result<ExecOutput, StageError> {
         let targets = self.snapshotter.get_snapshot_targets(provider.best_block_number()?)?;
         self.snapshotter.run(targets)?;
-        Ok(ExecOutput { checkpoint: StageCheckpoint::new(input.target()), done: true })
+        Ok(ExecOutput::done(input.checkpoint()))
     }
 
     fn unwind(
         &mut self,
         _provider: &DatabaseProviderRW<DB>,
         _input: UnwindInput,
     ) -> Result<UnwindOutput, StageError> {
-        // TODO(alexey): return proper error
-        Err(StageError::ChannelClosed)
+        Err(StageError::UnwindNotSupported)
     }
 }