Fix BigTable upload early return (#29378)

* Change Err when slot range is empty to Ok and log; add method docs

* Update var and log to be more correct

* Promote log level to warn

ledger-tool/src/bigtable.rs

@@ -60,7 +60,7 @@ async fn upload(
             ending_slot,
             starting_slot.saturating_add(config.max_num_slots_to_check as u64 * 2),
         );
-        let last_slot_uploaded = solana_ledger::bigtable_upload::upload_confirmed_blocks(
+        let last_slot_checked = solana_ledger::bigtable_upload::upload_confirmed_blocks(
             blockstore.clone(),
             bigtable.clone(),
             starting_slot,
@@ -69,8 +69,8 @@ async fn upload(
             Arc::new(AtomicBool::new(false)),
         )
         .await?;
-        info!("last slot uploaded: {}", last_slot_uploaded);
-        starting_slot = last_slot_uploaded.saturating_add(1);
+        info!("last slot checked: {}", last_slot_checked);
+        starting_slot = last_slot_checked.saturating_add(1);
     }
     info!("No more blocks to upload.");
     Ok(())

ledger/src/bigtable_upload.rs

@@ -41,6 +41,9 @@ struct BlockstoreLoadStats {
     pub elapsed: Duration,
 }
 
+/// Uploads a range of blocks from a Blockstore to bigtable LedgerStorage
+/// Returns the Slot of the last block checked. If no blocks in the range `[staring_slot,
+/// ending_slot]` are found in Blockstore, this value is equal to `ending_slot`.
 pub async fn upload_confirmed_blocks(
     blockstore: Arc<Blockstore>,
     bigtable: solana_storage_bigtable::LedgerStorage,
@@ -61,7 +64,8 @@ pub async fn upload_confirmed_blocks(
         .collect();
 
     if blockstore_slots.is_empty() {
-        return Err(format!("Ledger has no slots from {starting_slot} to {ending_slot:?}").into());
+        warn!("Ledger has no slots from {starting_slot} to {ending_slot:?}");
+        return Ok(ending_slot);
     }
 
     let first_blockstore_slot = blockstore_slots.first().unwrap();