new protocol method: blockchain.transaction.id_from_pos (#524)

docs/protocol-changes.rst

@@ -133,6 +133,11 @@ Changes
   * Optional *cp_height* argument added to
     :func:`blockchain.block.header` and :func:`blockchain.block.headers`
 
+New methods
+-----------
+
+  * :func:`blockchain.transaction.id_from_pos`
+
 Removed methods
 ---------------
 

docs/protocol-methods.rst

@@ -671,6 +671,70 @@ and height.
     "pos": 710
   }
 
+blockchain.transaction.id_from_pos
+==================================
+
+Return a transaction hash and optionally a merkle proof,
+given a block height and a position in the block.
+
+**Signature**
+
+  .. function:: blockchain.transaction.id_from_pos(height, tx_pos, merkle=False)
+  .. versionadded:: 1.4
+
+  *height*
+
+    A block height in the main chain, an integer.
+
+  *tx_pos*
+
+    A zero-based index of the transaction in the given block, an integer.
+
+  *merkle*
+
+    Whether a merkle proof should also be returned, a boolean.
+
+**Result**
+
+  If *merkle* is :const:`False`, the transaction hash as a hexadecimal string.
+  If :const:`True`, a dictionary with the following keys:
+
+  * *tx_hash*
+
+    The transaction hash as a hexadecimal string.
+
+  * *merkle*
+
+    A list of transaction hashes the current hash is paired with,
+    recursively, in order to trace up to obtain merkle root of the
+    block, deepest pairing first.
+
+**Example Results**
+
+When *merkle* is :const:`False`::
+
+  "fc12dfcb4723715a456c6984e298e00c479706067da81be969e8085544b0ba08"
+
+When *merkle* is :const:`True`::
+
+  {
+    "tx_hash": "fc12dfcb4723715a456c6984e298e00c479706067da81be969e8085544b0ba08",
+    "merkle":
+    [
+      "928c4275dfd6270349e76aa5a49b355eefeb9e31ffbe95dd75fed81d219a23f8",
+      "5f35bfb3d5ef2ba19e105dcd976928e675945b9b82d98a93d71cbad0e714d04e",
+      "f136bcffeeed8844d54f90fc3ce79ce827cd8f019cf1d18470f72e4680f99207",
+      "6539b8ab33cedf98c31d4e5addfe40995ff96c4ea5257620dfbf86b34ce005ab",
+      "7ecc598708186b0b5bd10404f5aeb8a1a35fd91d1febbb2aac2d018954885b1e",
+      "a263aae6c470b9cde03b90675998ff6116f3132163911fafbeeb7843095d3b41",
+      "c203983baffe527edb4da836bc46e3607b9a36fa2c6cb60c1027f0964d971b29",
+      "306d89790df94c4632d652d142207f53746729a7809caa1c294b895a76ce34a9",
+      "c0b4eff21eea5e7974fe93c62b5aab51ed8f8d3adad4583c7a84a98f9e428f04",
+      "f0bd9d2d4c4cf00a1dd7ab3b48bbbb4218477313591284dcc2d7ca0aaa444e8d",
+      "503d3349648b985c1b571f59059e4da55a57b0163b08cc50379d73be80c4c8f3"
+    ]
+  }
+
 mempool.get_fee_histogram
 =========================
 

electrumx/server/session.py

@@ -1068,6 +1068,17 @@ class ElectrumX(SessionBase):
         block = await self.daemon_request('deserialised_block', block_hash)
         return block_hash, block['tx']
 
+    def _get_merkle_branch(self, tx_hashes, tx_pos):
+        '''Return a merkle branch to a transaction.
+
+        tx_hashes: ordered list of hex strings of tx hashes in a block
+        tx_pos: index of transaction in tx_hashes to create branch for
+        '''
+        hashes = [hex_str_to_hash(hash) for hash in tx_hashes]
+        branch, root = self.bp.merkle.branch_and_root(hashes, tx_pos)
+        branch = [hash_to_hex_str(hash) for hash in branch]
+        return branch
+
     async def transaction_merkle(self, tx_hash, height):
         '''Return the markle tree to a confirmed transaction given its hash
         and height.
@@ -1082,13 +1093,30 @@ class ElectrumX(SessionBase):
         except ValueError:
             raise RPCError(BAD_REQUEST, f'tx hash {tx_hash} not in '
                            f'block {block_hash} at height {height:,d}')
-
-        hashes = [hex_str_to_hash(hash) for hash in tx_hashes]
-        branch, root = self.bp.merkle.branch_and_root(hashes, pos)
-        branch = [hash_to_hex_str(hash) for hash in branch]
-
+        branch = self._get_merkle_branch(tx_hashes, pos)
         return {"block_height": height, "merkle": branch, "pos": pos}
 
+    async def transaction_id_from_pos(self, height, tx_pos, merkle=False):
+        '''Return the txid and optionally a merkle proof, given
+        a block height and position in the block.
+        '''
+        tx_pos = non_negative_integer(tx_pos)
+        if merkle not in (True, False):
+            raise RPCError(BAD_REQUEST, f'"merkle" must be a boolean')
+
+        block_hash, tx_hashes = await self.block_hash_and_tx_hashes(height)
+        try:
+            tx_hash = tx_hashes[tx_pos]
+        except IndexError:
+            raise RPCError(BAD_REQUEST, f'no tx at position {tx_pos:,d} in '
+                           f'block {block_hash} at height {height:,d}')
+        branch = self._get_merkle_branch(tx_hashes, tx_pos)
+
+        if merkle:
+            return {"tx_hash": tx_hash, "merkle": branch}
+        else:
+            return tx_hash
+
     def set_protocol_handlers(self, ptuple):
         self.protocol_tuple = ptuple
 
@@ -1126,6 +1154,8 @@ class ElectrumX(SessionBase):
                 'blockchain.block.header': self.block_header,
                 'blockchain.block.headers': self.block_headers,
                 'blockchain.headers.subscribe': self.headers_subscribe,
+                'blockchain.transaction.id_from_pos':
+                    self.transaction_id_from_pos,
             })
         elif ptuple >= (1, 3):
             handlers.update({