bitcoind_: return support for older Bitcoin Core versions

Refactor to take Bitcoin Core minor version into consideration,
and use that to implement legacy version support.

plugins/currency/bitcoin/bitcoind_

@@ -63,6 +63,45 @@ import urllib.request
 DEBUG = os.getenv('MUNIN_DEBUG') == '1'
 
 
+def _get_version(info):
+    # v0.15.2 version is represented as 150200
+    return info['version'] // 10000
+
+
+def _rpc_get_initial_info(connection):
+    (info, connect_error) = connection.getnetworkinfo()
+    if connect_error:
+        if isinstance(connect_error, urllib.error.HTTPError) and connect_error.code == 404:
+            # getinfo RPC exists in version <= 0.15
+            (info, connect_error) = connection.getinfo()
+            if connect_error:
+                return (None, None, connect_error)
+        else:
+            return (None, None, connect_error)  # pass all other not-404 errors
+
+    return (info, _get_version(info), None)
+
+
+def _rpc_get_balance(info, minor_version, connection):
+    # see https://github.com/bitcoin/bitcoin/blob/239d199667888e5d60309f15a38eed4d3afe56c4/
+    # doc/release-notes/release-notes-0.19.0.1.md#new-rpcs
+    if minor_version >= 19:
+        # we use getbalance*s* (plural) as old getbalance is being deprecated,
+        # and we have to calculate total balance (owned and watch-only) manually now.
+        (result, error) = connection.getbalances()
+
+        total = sum(result[wallet_mode]['trusted']
+                    for wallet_mode in ('mine', 'watchonly')
+                    if wallet_mode in result)
+
+        info['balance'] = total
+        return info
+    else:
+        (result, error) = connection.getbalance()
+        info['balance'] = result
+        return info
+
+
 def main():
     # getinfo variable is read from command name - probably the sym-link name.
     request_var = sys.argv[0].split('_', 1)[1] or 'balance'
@@ -119,7 +158,7 @@ def main():
                                                  bitcoin_options.rpcport),
                                username=bitcoin_options.rpcuser,
                                password=bitcoin_options.rpcpassword)
-        (info, connect_error) = bitcoin.getnetworkinfo()
+        (info, minor_version, connect_error) = _rpc_get_initial_info(bitcoin)
         if connect_error:
             error = "Could not connect to Bitcoin server: {}".format(connect_error)
 
@@ -135,15 +174,7 @@ def main():
         return False
 
     if request_var == 'balance':
-        # we use getbalance*s* (plural) as old getbalance is being deprecated,
+        info = _rpc_get_balance(info, minor_version, bitcoin)
-        # and we have to calculate total balance (owned and watch-only) manually now.
-        (info, error) = bitcoin.getbalances()
-
-        total = sum(info[wallet_mode]['trusted']
-                    for wallet_mode in ('mine', 'watchonly')
-                    if wallet_mode in info)
-
-        info['balance'] = total
 
     if request_var in ('transactions', 'block_age'):
         (info, error) = bitcoin.getblockchaininfo()