From 5cb0aac55ec0a326966f568ca9ac6d0c10231429 Mon Sep 17 00:00:00 2001
From: Antioch Peverell <apeverell@protonmail.com>
Date: Wed, 26 Feb 2020 19:15:13 +0000
Subject: [PATCH] only apply full rewind to bitmap, not per block (#3246)

---
 chain/src/txhashset/txhashset.rs | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/chain/src/txhashset/txhashset.rs b/chain/src/txhashset/txhashset.rs
index 3d3332cb5..3cc5facfb 100644
--- a/chain/src/txhashset/txhashset.rs
+++ b/chain/src/txhashset/txhashset.rs
@@ -1119,11 +1119,15 @@ impl<'a> Extension<'a> {
 			self.rewind_mmrs_to_pos(header.output_mmr_size, header.kernel_mmr_size, &vec![])?;
 			self.apply_to_bitmap_accumulator(&[header.output_mmr_size])?;
 		} else {
+			let mut affected_pos = vec![];
 			let mut current = head_header;
 			while header.height < current.height {
-				self.rewind_single_block(&current, batch)?;
+				let mut affected_pos_single_block = self.rewind_single_block(&current, batch)?;
+				affected_pos.append(&mut affected_pos_single_block);
 				current = batch.get_previous_header(&current)?;
 			}
+			// Now apply a single aggregate "affected_pos" to our bitmap accumulator.
+			self.apply_to_bitmap_accumulator(&affected_pos)?;
 		}
 
 		// Update our head to reflect the header we rewound to.
@@ -1132,12 +1136,14 @@ impl<'a> Extension<'a> {
 		Ok(())
 	}
 
-	// Rewind the MMRs, the bitmap accumulator and the output_pos index.
+	// Rewind the MMRs and the output_pos index.
+	// Returns a vec of "affected_pos" so we can apply the necessary updates to the bitmap
+	// accumulator in a single pass for all rewound blocks.
 	fn rewind_single_block(
 		&mut self,
 		header: &BlockHeader,
 		batch: &Batch<'_>,
-	) -> Result<(), Error> {
+	) -> Result<Vec<u64>, Error> {
 		// The spent index allows us to conveniently "unspend" everything in a block.
 		let spent = batch.get_spent_index(&header.hash());
 
@@ -1165,7 +1171,6 @@ impl<'a> Extension<'a> {
 		// Treat last_pos as an affected output to ensure we rebuild far enough back.
 		let mut affected_pos = spent_pos.clone();
 		affected_pos.push(self.output_pmmr.last_pos);
-		self.apply_to_bitmap_accumulator(&affected_pos)?;
 
 		// Remove any entries from the output_pos created by the block being rewound.
 		let block = batch.get_block(&header.hash())?;
@@ -1183,7 +1188,7 @@ impl<'a> Extension<'a> {
 			}
 		}
 
-		Ok(())
+		Ok(affected_pos)
 	}
 
 	/// Rewinds the MMRs to the provided positions, given the output and