args: Properly support -noconf and catch directories

peek() in order to catch directories being specified instead of configuration files. Previously passing a directory path as -conf would lead to an ifstream being opened for it, and would not trigger any errors.

-noconf would previously lead to an ifstream "successfully" being opened to the ".bitcoin"-directory (not a file)). With the change to AbsPathForConfigVal we will now try to open the marginally more correct absolute path "".

Other users of AbsPathForConfigVal() have been updated to handle negation in 2 prior commits.

src/common/config.cpp

@@ -129,9 +129,11 @@ bool ArgsManager::ReadConfigFiles(std::string& error, bool ignore_invalid_keys)
 
     const auto conf_path{GetConfigFilePath()};
     std::ifstream stream{conf_path};
+    // Calling peek() on ifstreams opened on directories flags them as !good().
+    (void)stream.peek();
 
     // not ok to have a config file specified that cannot be opened
-    if (IsArgSet("-conf") && !stream.good()) {
+    if ((IsArgSet("-conf") && !IsArgNegated("-conf")) && !stream.good()) {
         error = strprintf("specified config file \"%s\" could not be opened.", fs::PathToString(conf_path));
         return false;
     }
@@ -176,6 +178,7 @@ bool ArgsManager::ReadConfigFiles(std::string& error, bool ignore_invalid_keys)
 
             for (const std::string& conf_file_name : conf_file_names) {
                 std::ifstream conf_file_stream{AbsPathForConfigVal(*this, fs::PathFromString(conf_file_name), /*net_specific=*/false)};
+                (void)conf_file_stream.peek();
                 if (conf_file_stream.good()) {
                     if (!ReadConfigStream(conf_file_stream, conf_file_name, error, ignore_invalid_keys)) {
                         return false;
@@ -213,7 +216,7 @@ bool ArgsManager::ReadConfigFiles(std::string& error, bool ignore_invalid_keys)
 
 fs::path AbsPathForConfigVal(const ArgsManager& args, const fs::path& path, bool net_specific)
 {
-    if (path.is_absolute()) {
+    if (path.is_absolute() || path.empty()) {
         return path;
     }
     return fsbridge::AbsPathJoin(net_specific ? args.GetDataDirNet() : args.GetDataDirBase(), path);