session: rework transaction.get_merkle: avoid calling bitcoind (#789)

use ElectrumX's db instead

electrumx/server/db.py

@@ -402,7 +402,7 @@ class DB(object):
         return await run_in_thread(read_headers)
 
     def fs_tx_hash(self, tx_num):
-        '''Return a par (tx_hash, tx_height) for the given tx number.
+        '''Return a pair (tx_hash, tx_height) for the given tx number.
 
         If the tx_height is not on disk, returns (None, tx_height).'''
         tx_height = bisect_right(self.tx_counts, tx_num)
@@ -412,6 +412,25 @@ class DB(object):
             tx_hash = self.hashes_file.read(tx_num * 32, 32)
         return tx_hash, tx_height
 
+    def fs_tx_hashes_at_blockheight(self, block_height):
+        '''Return a list of tx_hashes at given block height,
+        in the same order as in the block.
+        '''
+        if block_height > self.db_height:
+            raise self.DBError(f'block {block_height:,d} not on disk (>{self.db_height:,d})')
+        assert block_height >= 0
+        if block_height > 0:
+            first_tx_num = self.tx_counts[block_height - 1]
+        else:
+            first_tx_num = 0
+        num_txs_in_block = self.tx_counts[block_height] - first_tx_num
+        tx_hashes = self.hashes_file.read(first_tx_num * 32, num_txs_in_block * 32)
+        assert num_txs_in_block == len(tx_hashes) // 32
+        return [tx_hashes[idx * 32: (idx+1) * 32] for idx in range(num_txs_in_block)]
+
+    async def tx_hashes_at_blockheight(self, block_height):
+        return await run_in_thread(self.fs_tx_hashes_at_blockheight, block_height)
+
     async def fs_block_hashes(self, height, count):
         headers_concat, headers_count = await self.read_headers(height, count)
         if headers_count != count:

electrumx/server/session.py

@@ -1194,10 +1194,12 @@ class ElectrumX(SessionBase):
         ordered list of hexadecimal strings.
         '''
         height = non_negative_integer(height)
-        hex_hashes = await self.daemon_request('block_hex_hashes', height, 1)
-        block_hash = hex_hashes[0]
-        block = await self.daemon_request('deserialised_block', block_hash)
-        tx_hashes = block['tx']
+        try:
+            block_hash = hash_to_hex_str((await self.db.fs_block_hashes(height, 1))[0])
+            tx_hashes = await self.db.tx_hashes_at_blockheight(height)
+        except self.db.DBError as e:
+            raise RPCError(BAD_REQUEST, f'db error: {e!r}')
+        tx_hashes = [hash_to_hex_str(hash) for hash in tx_hashes]
         self.bump_cost(1.0 + len(tx_hashes) / 1000)
         return block_hash, tx_hashes