env: add new cfg option "DAEMON_HAS_TXINDEX", and support "False"

docs/HOWTO.rst

@@ -29,17 +29,20 @@ functions. For example, `x11_hash`_ is required for DASH. Scrypt coins
 require a Python interpreter compiled and/or linked with OpenSSL 1.1.0
 or higher.
 
-You **must** be running a non-pruning bitcoin daemon with::
+You **must** be running a non-pruning bitcoin daemon.
+It is also recommended to have::
 
   txindex=1
 
-set in its configuration file.  If you have an existing installation
-of bitcoind and have not previously set this you will need to reindex
-the blockchain with::
+set in its configuration file, for better performance when serving many sessions.
+If you have an existing installation of bitcoind and have not previously set this
+but you do now, you will need to reindex the blockchain with::
 
   bitcoind -reindex
 
 which can take some time.
+If you intend to use a bitcoind without txindex enabled, you need to configure the
+`DAEMON_HAS_TXINDEX` :ref:`environment variable <environment>`.
 
 While not a requirement for running ElectrumX, it is intended to be
 run with supervisor software such as Daniel Bernstein's

docs/environment.rst

@@ -281,13 +281,14 @@ These environment variables are optional:
   version string. For example to drop versions from 1.0 to 1.2 use
   the regex ``1\.[0-2]\.\d+``.
 
-.. envvar:: DROP_CLIENT_UNKNOWN
+.. envvar:: DAEMON_HAS_TXINDEX
 
-  Set to anything non-empty to deny serving clients which do not
-  identify themselves first by issuing the server.version method
-  call with a non-empty client identifier. The connection is dropped 
-  on first actual method call. This might help to filter out simple 
-  robots. This behavior is off by default.
+  Set this environment variable to empty if the connected bitcoind
+  does not have txindex enabled. Defaults to True (has txindex).
+  We rely on bitcoind to lookup arbitrary txs, regardless of this setting.
+  Without txindex, tx lookup works as in `bitcoind PR #10275`_.
+  Note that performance when serving many sessions is better with txindex
+  (but initial sync should be unaffected).
 
 
 Resource Usage Limits
@@ -506,3 +507,4 @@ your available physical RAM:
 
 .. _lib/coins.py: https://github.com/spesmilo/electrumx/blob/master/electrumx/lib/coins.py
 .. _uvloop: https://pypi.python.org/pypi/uvloop
+.. _bitcoind PR #10275: https://github.com/bitcoin/bitcoin/pull/10275

electrumx/server/daemon.py

@@ -277,11 +277,16 @@ async def relayfee(self):
         network_info = await self.getnetworkinfo()
         return network_info['relayfee']
 
-    async def getrawtransaction(self, hex_hash, verbose=False):
+    async def getrawtransaction(self, hex_hash, verbose=False, blockhash=None):
         '''Return the serialized raw transaction with the given hash.'''
         # Cast to int because some coin daemons are old and require it
-        return await self._send_single('getrawtransaction',
-                                       (hex_hash, int(verbose)))
+        verbose = int(verbose)
+        if blockhash is None:
+            return await self._send_single('getrawtransaction', (hex_hash, verbose))
+        else:
+            # given a blockhash, modern bitcoind can lookup the tx even without txindex:
+            # https://github.com/bitcoin/bitcoin/pull/10275
+            return await self._send_single('getrawtransaction', (hex_hash, verbose, blockhash))
 
     async def getrawtransactions(self, hex_hashes, replace_errs=True):
         '''Return the serialized raw transactions with the given hashes.

electrumx/server/env.py

@@ -76,6 +76,7 @@ def __init__(self, coin=None):
         self.blacklist_url = self.default('BLACKLIST_URL', self.coin.BLACKLIST_URL)
         self.cache_MB = self.integer('CACHE_MB', 1200)
         self.reorg_limit = self.integer('REORG_LIMIT', self.coin.REORG_LIMIT)
+        self.daemon_has_txindex = self.boolean('DAEMON_HAS_TXINDEX', True)
 
         # Server limits to help prevent DoS
 

electrumx/server/session.py

@@ -1436,12 +1436,22 @@ async def transaction_get(self, tx_hash, verbose=False):
         tx_hash: the transaction hash as a hexadecimal string
         verbose: passed on to the daemon
         '''
-        assert_tx_hash(tx_hash)
+        tx_hash_bytes = assert_tx_hash(tx_hash)
+        tx_hash_hex = tx_hash
+        del tx_hash
         if verbose not in (True, False):
             raise RPCError(BAD_REQUEST, '"verbose" must be a boolean')
 
         self.bump_cost(1.0)
-        return await self.daemon_request('getrawtransaction', tx_hash, verbose)
+
+        blockhash = None
+        if not self.env.daemon_has_txindex:
+            height, tx_pos = await self.db.get_blockheight_and_txpos_for_txhash(tx_hash_bytes)
+            if height is not None:
+                block_header = self.db.raw_header(height)
+                blockhash = self.coin.header_hash(block_header).hex()
+
+        return await self.daemon_request('getrawtransaction', tx_hash_hex, verbose, blockhash)
 
     async def transaction_merkle(self, tx_hash, height=None):
         '''Return the merkle branch to a confirmed transaction given its hash