init: Error if ignored bitcoin.conf file is found

Show an error on startup if a bitcoin datadir that is being used contains a
`bitcoin.conf` file that is ignored. There are two cases where this could
happen:

- One case reported in
  https://github.com/bitcoin/bitcoin/issues/27246#issuecomment-1470006043
  happens when a bitcoin.conf file in the default datadir (e.g.
  $HOME/.bitcoin/bitcoin.conf) has a "datadir=/path" line that sets different
  datadir containing a second bitcoin.conf file. Currently the second
  bitcoin.conf file is ignored with no warning.

- Another way this could happen is if a -conf= command line argument points
  to a configuration file with a "datadir=/path" line and that specified path
  contains a bitcoin.conf file, which is currently ignored.

This change only adds an error message and doesn't change anything about way
settings are applied. It also doesn't trigger errors if there are redundant
-datadir or -conf settings pointing at the same configuration file, only if
they are pointing at different files and one file is being ignored.

1 files changed, 1 insertions, 0 deletions

diff --git a/src/init.cpp b/src/init.cpp
index 525648b812..78db4a7a82 100644
--- a/src/init.cpp
+++ b/src/init.cpp
@@ -441,6 +441,7 @@ void SetupServerArgs(ArgsManager& argsman)
     argsman.AddArg("-dbbatchsize", strprintf("Maximum database write batch size in bytes (default: %u)", nDefaultDbBatchSize), ArgsManager::ALLOW_ANY | ArgsManager::DEBUG_ONLY, OptionsCategory::OPTIONS);
     argsman.AddArg("-dbcache=<n>", strprintf("Maximum database cache size <n> MiB (%d to %d, default: %d). In addition, unused mempool memory is shared for this cache (see -maxmempool).", nMinDbCache, nMaxDbCache, nDefaultDbCache), ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
     argsman.AddArg("-includeconf=<file>", "Specify additional configuration file, relative to the -datadir path (only useable from configuration file, not command line)", ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
+    argsman.AddArg("-allowignoredconf", strprintf("For backwards compatibility, treat an unused %s file in the datadir as a warning, not an error.", BITCOIN_CONF_FILENAME), ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
     argsman.AddArg("-loadblock=<file>", "Imports blocks from external file on startup", ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
     argsman.AddArg("-maxmempool=<n>", strprintf("Keep the transaction memory pool below <n> megabytes (default: %u)", DEFAULT_MAX_MEMPOOL_SIZE_MB), ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
     argsman.AddArg("-maxorphantx=<n>", strprintf("Keep at most <n> unconnectable transactions in memory (default: %u)", DEFAULT_MAX_ORPHAN_TRANSACTIONS), ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);