rpc: add nBits to getmininginfo

Also expands nBits test coverage.

test/functional/mining_basic.py

@@ -16,6 +16,8 @@ from test_framework.blocktools import (
     get_witness_script,
     NORMAL_GBT_REQUEST_PARAMS,
     TIME_GENESIS_BLOCK,
+    REGTEST_N_BITS,
+    nbits_str,
 )
 from test_framework.messages import (
     BLOCK_HEADER_SIZE,
@@ -206,6 +208,7 @@ class MiningTest(BitcoinTestFramework):
         assert_equal(mining_info['chain'], self.chain)
         assert 'currentblocktx' not in mining_info
         assert 'currentblockweight' not in mining_info
+        assert_equal(mining_info['bits'], nbits_str(REGTEST_N_BITS))
         assert_equal(mining_info['difficulty'], Decimal('4.656542373906925E-10'))
         assert_equal(mining_info['networkhashps'], Decimal('0.003333333333333334'))
         assert_equal(mining_info['pooledtx'], 0)

test/functional/rpc_blockchain.py

@@ -1,5 +1,5 @@
 #!/usr/bin/env python3
-# Copyright (c) 2014-2022 The Bitcoin Core developers
+# Copyright (c) 2014-present The Bitcoin Core developers
 # Distributed under the MIT software license, see the accompanying
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.
 """Test RPCs related to blockchainstate.
@@ -30,9 +30,11 @@ import textwrap
 from test_framework.blocktools import (
     MAX_FUTURE_BLOCK_TIME,
     TIME_GENESIS_BLOCK,
+    REGTEST_N_BITS,
     create_block,
     create_coinbase,
     create_tx_with_script,
+    nbits_str,
 )
 from test_framework.messages import (
     CBlockHeader,
@@ -412,7 +414,7 @@ class BlockchainTest(BitcoinTestFramework):
         assert_is_hash_string(header['hash'])
         assert_is_hash_string(header['previousblockhash'])
         assert_is_hash_string(header['merkleroot'])
-        assert_is_hash_string(header['bits'], length=None)
+        assert_equal(header['bits'], nbits_str(REGTEST_N_BITS))
         assert isinstance(header['time'], int)
         assert_equal(header['mediantime'], TIME_RANGE_MTP)
         assert isinstance(header['nonce'], int)

test/functional/test_framework/blocktools.py

@@ -65,6 +65,10 @@ NORMAL_GBT_REQUEST_PARAMS = {"rules": ["segwit"]}
 VERSIONBITS_LAST_OLD_BLOCK_VERSION = 4
 MIN_BLOCKS_TO_KEEP = 288
 
+REGTEST_N_BITS = 0x207fffff  # difficulty retargeting is disabled in REGTEST chainparams"
+
+def nbits_str(nbits):
+    return f"{nbits:08x}"
 
 def create_block(hashprev=None, coinbase=None, ntime=None, *, version=None, tmpl=None, txlist=None):
     """Create a block (with regtest difficulty)."""
@@ -77,7 +81,7 @@ def create_block(hashprev=None, coinbase=None, ntime=None, *, version=None, tmpl
     if tmpl and tmpl.get('bits') is not None:
         block.nBits = struct.unpack('>I', bytes.fromhex(tmpl['bits']))[0]
     else:
-        block.nBits = 0x207fffff  # difficulty retargeting is disabled in REGTEST chainparams
+        block.nBits = REGTEST_N_BITS
     if coinbase is None:
         coinbase = create_coinbase(height=tmpl['height'])
     block.vtx.append(coinbase)