Generic header length

If header lengths change in the future, only one function needs
to change in coins.py.

lib/coins.py

@@ -34,7 +34,6 @@ class Coin(object):
 
     REORG_LIMIT=200
     # Not sure if these are coin-specific
-    HEADER_LEN = 80
     RPC_URL_REGEX = re.compile('.+@[^:]+(:[0-9]+)?')
     VALUE_PER_COIN = 100000000
     CHUNK_SIZE=2016
@@ -191,9 +190,24 @@ class Coin(object):
         return header[4:36]
 
     @classmethod
-    def read_block(cls, block):
-        '''Return a tuple (header, tx_hashes, txs) given a raw block.'''
-        header, rest = block[:cls.HEADER_LEN], block[cls.HEADER_LEN:]
+    def header_offset(cls, height):
+        '''Given a header height return its offset in the headers file.
+
+        If header sizes change at some point, this is the only code
+        that needs updating.'''
+        return height * 80
+
+    @classmethod
+    def header_len(cls, height):
+        '''Given a header height return its length.'''
+        return cls.header_offset(height + 1) - cls.header_offset(height)
+
+    @classmethod
+    def read_block(cls, block, height):
+        '''Return a tuple (header, tx_hashes, txs) given a raw block at
+        the given height.'''
+        hlen = cls.header_len(height)
+        header, rest = block[:hlen], block[hlen:]
         return (header, ) + Deserializer(rest).read_block()
 
     @classmethod

server/block_processor.py

@@ -128,7 +128,7 @@ class Prefetcher(LoggedClass):
 
                 # Strip the unspendable genesis coinbase
                 if first == 0:
-                    blocks[0] = blocks[0][:self.coin.HEADER_LEN] + bytes(1)
+                    blocks[0] = blocks[0][:self.coin.header_len(0)] + bytes(1)
 
                 # Update our recent average block size estimate
                 size = sum(len(block) for block in blocks)
@@ -503,7 +503,7 @@ class BlockProcessor(server.db.DB):
         self.tx_counts.append(prior_tx_count + len(txs))
 
     def advance_block(self, block, touched):
-        header, tx_hashes, txs = self.coin.read_block(block)
+        header, tx_hashes, txs = self.coin.read_block(block, self.height + 1)
         if self.tip != self.coin.header_prevhash(header):
             raise ChainReorg
 
@@ -562,13 +562,13 @@ class BlockProcessor(server.db.DB):
     def backup_blocks(self, blocks, touched):
         '''Backup the blocks and flush.
 
-        The blocks should be in order of decreasing height.
-        A flush is performed once the blocks are backed up.
+        The blocks should be in order of decreasing height, starting at.
+        self.height.  A flush is performed once the blocks are backed up.
         '''
         self.assert_flushed()
 
         for block in blocks:
-            header, tx_hashes, txs = self.coin.read_block(block)
+            header, tx_hashes, txs = self.coin.read_block(block, self.height)
             header_hash = self.coin.header_hash(header)
             if header_hash != self.tip:
                 raise ChainError('backup block {} is not tip {} at height {:,d}'

server/db.py

@@ -258,7 +258,7 @@ class DB(util.LoggedClass):
         assert len(hashes) // 32 == txs_done
 
         # Write the headers, tx counts, and tx hashes
-        offset = (fs_height + 1) * self.coin.HEADER_LEN
+        offset = self.coin.header_offset(fs_height + 1)
         self.headers_file.write(offset, b''.join(headers))
         offset = (fs_height + 1) * self.tx_counts.itemsize
         self.tx_counts_file.write(offset,
@@ -274,9 +274,9 @@ class DB(util.LoggedClass):
             raise self.DBError('{:,d} headers starting at {:,d} not on disk'
                                .format(count, start))
         if disk_count:
-            header_len = self.coin.HEADER_LEN
-            offset = start * header_len
-            return self.headers_file.read(offset, disk_count * header_len)
+            offset = self.coin.header_offset(start)
+            size = self.coin.header_offset(start + disk_count) - offset
+            return self.headers_file.read(offset, size)
         return b''
 
     def fs_tx_hash(self, tx_num):
@@ -288,14 +288,18 @@ class DB(util.LoggedClass):
             tx_hash = None
         else:
             tx_hash = self.hashes_file.read(tx_num * 32, 32)
-        return tx_hash, height
+        return tx_hash, tx_height
 
     def fs_block_hashes(self, height, count):
-        headers = self.read_headers(height, count)
-        # FIXME: move to coins.py
-        hlen = self.coin.HEADER_LEN
-        return [self.coin.header_hash(header)
-                for header in util.chunks(headers, hlen)]
+        headers_concat = self.read_headers(height, count)
+        offset = 0
+        headers = []
+        for n in range(count):
+            hlen = self.coin.header_len(height + n)
+            headers.append(headers_concat[offset:offset + hlen])
+            offset += hlen
+
+        return [self.coin.header_hash(header) for header in headers]
 
     @staticmethod
     def _resolve_limit(limit):