Merge bitcoin/bitcoin#22622: util: Check if specified config file cannot be opened

127b460 test: Check if specified config file cannot be opened (nthumann)
6bb5470 util: Check if specified config file cannot be opened (nthumann)

Pull request description:

  Fixes bitcoin/bitcoin#22612.
  When running e.g. `./src/bitcoind -datadir=/tmp/bitcoin -regtest -conf=/tmp/bitcoin/regtest/bitcoin.conf` and the specified config cannot be opened (doesn't exist, permission denied, ...), the initialization silently uses the default config.

  As voidburn already noted:
  > I can't think of a situation in which a config file is specified explicitly (in the startup options, as per service unit linked above), but inaccessible, where the fail condition should be to keep booting using defaults instead.

  With this patch applied, the initialization will fail immediately, if the specified config file cannot be opened. If no config file is explicitly specified, the behavior is unchanged. This not only affects `bitcoind`, but also `bitcoin-cli` and `bitcoin-qt`.

  In the example below the datadir is accessible, but the config file is not due to insufficient permissions:
  ```
  $ ./src/bitcoind -datadir=/tmp/bitcoin -regtest --debug=1 -conf=/tmp/bitcoin/regtest/bitcoin.conf
  Error: Error reading configuration file: specified config file "/tmp/bitcoin/regtest/bitcoin.conf" could not be opened.
  ```

ACKs for top commit:
  0xB10C:
    ACK 127b460
  Zero-1729:
    tACK 127b460
  theStack:
    Tested ACK 127b460

Tree-SHA512: 4fe487921485426f1d1da8d256c388af517b984b639d776aec7b159b3e23b669824093d3bdd31139d9415ed5f5de405b3e6a51b110c8ab471f12b9c99ac67cc1

Author: MarcoFalke

src/util/system.cpp

@@ -904,6 +904,11 @@ bool ArgsManager::ReadConfigFiles(std::string& error, bool ignore_invalid_keys)
     const std::string confPath = GetArg("-conf", BITCOIN_CONF_FILENAME);
     fsbridge::ifstream stream(GetConfigFile(confPath));
 
+    // not ok to have a config file specified that cannot be opened
+    if (IsArgSet("-conf") && !stream.good()) {
+        error = strprintf("specified config file \"%s\" could not be opened.", confPath);
+        return false;
+    }
     // ok to not have a config file
     if (stream.good()) {
         if (!ReadConfigStream(stream, confPath, error, ignore_invalid_keys)) {

test/functional/feature_config_args.py

@@ -248,6 +248,10 @@ def run_test(self):
 
         self.nodes[0].assert_start_raises_init_error([f'-conf={conf_file}'], f'Error: Error reading configuration file: specified data directory "{new_data_dir}" does not exist.')
 
+        # Check that an explicitly specified config file that cannot be opened fails
+        none_existent_conf_file = os.path.join(default_data_dir, "none_existent_bitcoin.conf")
+        self.nodes[0].assert_start_raises_init_error(['-conf=' + none_existent_conf_file], 'Error: Error reading configuration file: specified config file "' + none_existent_conf_file + '" could not be opened.')
+
         # Create the directory and ensure the config file now works
         os.mkdir(new_data_dir)
         self.start_node(0, [f'-conf={conf_file}'])