bugfix: Fix incorrect debug.log config file path

Currently debug.log will show the wrong bitcoin.conf config file path when
bitcoind is invoked without -conf or -datadir arguments, and there's a default
bitcoin.conf file which specifies another datadir= location. When this happens,
the debug.log will include an incorrect "Config file:" line referring to a
bitcoin.conf file in the other datadir, instead of the referring to the actual
configuration file in the default datadir which was parsed.

The bad log print was reported and originally fixed in
https://github.com/bitcoin/bitcoin/pull/27303 by
Matthew Zipkin <pinheadmz@gmail.com>

This PR takes a slightly different approach to fixing the bug, trying to avoid
future bugs by not allowing the GetConfigFilePath function to be called before
the the configuration is parsed, and deleting GetConfigFile function which
could be confused with GetConfigFilePath. It also includes a test for the bug
which the original fix did not have.

Co-authored-by: Matthew Zipkin <pinheadmz@gmail.com>

src/common/args.cpp

@@ -714,7 +714,8 @@ bool CheckDataDirOption(const ArgsManager& args)
 
 fs::path ArgsManager::GetConfigFilePath() const
 {
-    return GetConfigFile(*this, GetPathArg("-conf", BITCOIN_CONF_FILENAME));
+    LOCK(cs_args);
+    return *Assert(m_config_path);
 }
 
 std::string ArgsManager::GetChainName() const

src/common/args.h

@@ -26,7 +26,6 @@ extern const char * const BITCOIN_SETTINGS_FILENAME;
 
 // Return true if -datadir option points to a valid directory or is not specified.
 bool CheckDataDirOption(const ArgsManager& args);
-fs::path GetConfigFile(const ArgsManager& args, const fs::path& configuration_file_path);
 
 /**
  * Most paths passed as configuration arguments are treated as relative to
@@ -136,6 +135,7 @@ protected:
     std::map<OptionsCategory, std::map<std::string, Arg>> m_available_args GUARDED_BY(cs_args);
     bool m_accept_any_command GUARDED_BY(cs_args){true};
     std::list<SectionInfo> m_config_sections GUARDED_BY(cs_args);
+    std::optional<fs::path> m_config_path GUARDED_BY(cs_args);
     mutable fs::path m_cached_blocks_path GUARDED_BY(cs_args);
     mutable fs::path m_cached_datadir_path GUARDED_BY(cs_args);
     mutable fs::path m_cached_network_datadir_path GUARDED_BY(cs_args);

src/common/config.cpp

@@ -26,11 +26,6 @@
 #include <utility>
 #include <vector>
 
-fs::path GetConfigFile(const ArgsManager& args, const fs::path& configuration_file_path)
-{
-    return AbsPathForConfigVal(args, configuration_file_path, /*net_specific=*/false);
-}
-
 static bool GetConfigOptions(std::istream& stream, const std::string& filepath, std::string& error, std::vector<std::pair<std::string, std::string>>& options, std::list<SectionInfo>& sections)
 {
     std::string str, prefix;
@@ -125,6 +120,7 @@ bool ArgsManager::ReadConfigFiles(std::string& error, bool ignore_invalid_keys)
         LOCK(cs_args);
         m_settings.ro_config.clear();
         m_config_sections.clear();
+        m_config_path = AbsPathForConfigVal(*this, GetPathArg("-conf", BITCOIN_CONF_FILENAME), /*net_specific=*/false);
     }
 
     const auto conf_path{GetConfigFilePath()};
@@ -175,7 +171,7 @@ bool ArgsManager::ReadConfigFiles(std::string& error, bool ignore_invalid_keys)
             const size_t default_includes = add_includes({});
 
             for (const std::string& conf_file_name : conf_file_names) {
-                std::ifstream conf_file_stream{GetConfigFile(*this, fs::PathFromString(conf_file_name))};
+                std::ifstream conf_file_stream{AbsPathForConfigVal(*this, fs::PathFromString(conf_file_name), /*net_specific=*/false)};
                 if (conf_file_stream.good()) {
                     if (!ReadConfigStream(conf_file_stream, conf_file_name, error, ignore_invalid_keys)) {
                         return false;

src/common/init.cpp

@@ -32,7 +32,7 @@ std::optional<ConfigError> InitConfig(ArgsManager& args, SettingsAbortFn setting
         // 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();
+        const fs::path orig_config_path{AbsPathForConfigVal(args, args.GetPathArg("-conf", BITCOIN_CONF_FILENAME), /*net_specific=*/false)};
 
         std::string error;
         if (!args.ReadConfigFiles(error, true)) {