Dynamic header support

Block headers can have a dynamic size that is being indexed on a
new meta file "headers_offsets".

The offsets are 64 bits in order to accommodate coins with big
headers that will accumulate GBs of header data after some years.

Closes #128

docs/AUTHORS

@@ -1,2 +1,3 @@
 Neil Booth: creator and maintainer
-Johann Bauer: backend DB abstraction
+Johann Bauer: backend DB abstraction
+John Jegutanis: alt-chain integrations

server/db.py

@@ -50,7 +50,8 @@ class DB(util.LoggedClass):
             self.header_offset = self.coin.static_header_offset
             self.header_len = self.coin.static_header_len
         else:
-            raise Exception("Non static headers are not supported")
+            self.header_offset = self.dynamic_header_offset
+            self.header_len = self.dynamic_header_len
 
         self.logger.info('switching current directory to {}'
                          .format(env.db_dir))
@@ -69,6 +70,12 @@ class DB(util.LoggedClass):
         self.headers_file = util.LogicalFile('meta/headers', 2, 16000000)
         self.tx_counts_file = util.LogicalFile('meta/txcounts', 2, 2000000)
         self.hashes_file = util.LogicalFile('meta/hashes', 4, 16000000)
+        if not self.coin.STATIC_BLOCK_HEADERS:
+            self.headers_offsets_file = util.LogicalFile(
+                'meta/headers_offsets', 2, 16000000)
+            # Write the offset of the genesis block
+            if self.headers_offsets_file.read(0, 8) != b'\x00' * 8:
+                self.headers_offsets_file.write(0, b'\x00' * 8)
 
         # tx_counts[N] has the cumulative number of txs at the end of
         # height N.  So tx_counts[0] is 1 - the genesis coinbase
@@ -192,6 +199,28 @@ class DB(util.LoggedClass):
             self.clear_excess_history(self.utxo_flush_count)
         self.clear_excess_undo_info()
 
+    def fs_update_header_offsets(self, offset_start, height_start, headers):
+        if self.coin.STATIC_BLOCK_HEADERS:
+            return
+        offset = offset_start
+        offsets = []
+        for h in headers:
+            offset += len(h)
+            offsets.append(pack("<Q", offset))
+        # For each header we get the offset of the next header, hence we
+        # start writing from the next height
+        pos = (height_start + 1) * 8
+        self.headers_offsets_file.write(pos, b''.join(offsets))
+
+    def dynamic_header_offset(self, height):
+        assert not self.coin.STATIC_BLOCK_HEADERS
+        offset, = unpack('<Q', self.headers_offsets_file.read(height * 8, 8))
+        return offset
+
+    def dynamic_header_len(self, height):
+        return self.dynamic_header_offset(height + 1)\
+               - self.dynamic_header_offset(height)
+
     def fs_update(self, fs_height, headers, block_tx_hashes):
         '''Write headers, the tx_count array and block tx hashes to disk.
 
@@ -215,6 +244,7 @@ class DB(util.LoggedClass):
         # Write the headers, tx counts, and tx hashes
         offset = self.header_offset(height_start)
         self.headers_file.write(offset, b''.join(headers))
+        self.fs_update_header_offsets(offset, height_start, headers)
         offset = height_start * self.tx_counts.itemsize
         self.tx_counts_file.write(offset,
                                   self.tx_counts[height_start:].tobytes())