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, 40 insertions, 0 deletions

diff --git a/src/common/init.cpp b/src/common/init.cpp
index 6ffa44847a..60df14de9a 100644
--- a/src/common/init.cpp
+++ b/src/common/init.cpp
@@ -5,6 +5,7 @@
 #include <chainparams.h>
 #include <common/args.h>
 #include <common/init.h>
+#include <logging.h>
 #include <tinyformat.h>
 #include <util/fs.h>
 #include <util/translation.h>
@@ -20,6 +21,19 @@ std::optional<ConfigError> InitConfig(ArgsManager& args, SettingsAbortFn setting
         if (!CheckDataDirOption(args)) {
             return ConfigError{ConfigStatus::FAILED, strprintf(_("Specified data directory \"%s\" does not exist."), args.GetArg("-datadir", ""))};
         }
+
+        // Record original datadir and config paths before parsing the config
+        // file. It is possible for the config file to contain a datadir= line
+        // that changes the datadir path after it is parsed. This is useful for
+        // CLI tools to let them use a different data storage location without
+        // needing to pass it every time on the command line. (It is not
+        // possible for the config file to cause another configuration to be
+        // used, though. Specifying a conf= option in the config file causes a
+        // parse error, and specifying a datadir= location containing another
+        // bitcoin.conf file just ignores the other file.)
+        const fs::path orig_datadir_path{args.GetDataDirBase()};
+        const fs::path orig_config_path = args.GetConfigFilePath();
+
         std::string error;
         if (!args.ReadConfigFiles(error, true)) {
             return ConfigError{ConfigStatus::FAILED, strprintf(_("Error reading configuration file: %s"), error)};
@@ -48,6 +62,32 @@ std::optional<ConfigError> InitConfig(ArgsManager& args, SettingsAbortFn setting
             fs::create_directories(net_path / "wallets");
         }
 
+        // Show an error or warning if there is a bitcoin.conf file in the
+        // datadir that is being ignored.
+        const fs::path base_config_path = base_path / BITCOIN_CONF_FILENAME;
+        if (fs::exists(base_config_path) && !fs::equivalent(orig_config_path, base_config_path)) {
+            const std::string cli_config_path = args.GetArg("-conf", "");
+            const std::string config_source = cli_config_path.empty()
+                ? strprintf("data directory %s", fs::quoted(fs::PathToString(orig_datadir_path)))
+                : strprintf("command line argument %s", fs::quoted("-conf=" + cli_config_path));
+            const std::string error = strprintf(
+                "Data directory %1$s contains a %2$s file which is ignored, because a different configuration file "
+                "%3$s from %4$s is being used instead. Possible ways to address this would be to:\n"
+                "- Delete or rename the %2$s file in data directory %1$s.\n"
+                "- Change datadir= or conf= options to specify one configuration file, not two, and use "
+                "includeconf= to include any other configuration files.\n"
+                "- Set allowignoredconf=1 option to treat this condition as a warning, not an error.",
+                fs::quoted(fs::PathToString(base_path)),
+                fs::quoted(BITCOIN_CONF_FILENAME),
+                fs::quoted(fs::PathToString(orig_config_path)),
+                config_source);
+            if (args.GetBoolArg("-allowignoredconf", false)) {
+                LogPrintf("Warning: %s\n", error);
+            } else {
+                return ConfigError{ConfigStatus::FAILED, Untranslated(error)};
+            }
+        }
+
         // Create settings.json if -nosettings was not specified.
         if (args.GetSettingsPath()) {
             std::vector<std::string> details;