orphan check needs to handle fast sync "edge case" (#3418)

chain/src/chain.rs

@@ -342,8 +342,12 @@ impl Chain {
 
 	// Check if the provided block is an orphan.
 	// If block is an orphan add it to our orphan block pool for deferred processing.
+	// If this is the "next" block immediately following current head then not an orphan.
+	// Or if we have the previous full block then not an orphan.
 	fn check_orphan(&self, block: &Block, opts: Options) -> Result<(), Error> {
-		if self.block_exists(block.header.prev_hash)? {
+		let head = self.head()?;
+		let is_next = block.header.prev_hash == head.last_block_h;
+		if is_next || self.block_exists(block.header.prev_hash)? {
 			return Ok(());
 		}
 

chain/src/pipe.rs

@@ -103,9 +103,8 @@ pub fn process_block(
 	// want to do this now and not later during header validation.
 	validate_pow_only(&b.header, ctx)?;
 
-	// Get previous header from the db and check we have the corresponding full block.
+	// Get previous header from the db.
 	let prev = prev_header_store(&b.header, &mut ctx.batch)?;
-	ctx.batch.block_exists(&prev.hash())?;
 
 	// Process the header for the block.
 	// Note: We still want to process the full block if we have seen this header before