fluent/fluentd · error · Fluent::ConfigError

Incomplete bracket. Invalid syntax: #{orig_param}

Error message

Incomplete bracket. Invalid syntax: #{orig_param}

What it means

Fluent::ConfigError from Fluent::RecordAccessor's parse_bracket_notation. When a quoted key is opened inside brackets, the parser looks for the closing quote-plus-bracket sequence ('] or "]); if neither appears in the remaining string, the bracket is unterminated and the whole path is rejected as incomplete.

Source

Thrown at lib/fluent/plugin_helper/record_accessor.rb:177

          result
        end

        def self.parse_bracket_notation(param)
          orig_param = param
          result = []
          param = param[1..-1]
          in_bracket = false

          until param.empty?
            if in_bracket
              if param[0] == "'" || param[0] == '"'
                if i = param.index("']") || param.index('"]')
                  raise Fluent::ConfigError, "Mismatched quotes. Invalid syntax: #{orig_param}" unless param[0] == param[i]
                  result << param[1..i - 1]
                  param = param[i + 2..-1]
                  in_bracket = false
                else
                  raise Fluent::ConfigError, "Incomplete bracket. Invalid syntax: #{orig_param}"
                end
              else
                if i = param.index(']')
                  index_value = param[0..i - 1]
                  raise Fluent::ConfigError, "missing array index in '[]'. Invalid syntax: #{param}" if index_value == ']'
                  result << Integer(index_value)
                  param = param[i + 1..-1]
                  in_bracket = false
                else
                  raise Fluent::ConfigError, "'[' found but ']' not found. Invalid syntax: #{orig_param}"
                end
              end
            else
              if i = param.index('[')
                param = param[i + 1..-1]
                in_bracket = true
              else
                raise Fluent::ConfigError, "found more characters after ']'. Invalid syntax: #{orig_param}"

View on GitHub (pinned to dd45c6e18d)

Solutions

  1. Terminate the key: $['key'] or $["key"]
  2. Validate generated paths with a regex such as /\A\$\[(['"]).*\1\]\z/ before passing them on
  3. Keep accessor params on one line in config files

Example fix

# before
accessor = record_accessor_create("$['key")

# after
accessor = record_accessor_create("$['key']")
Defensive patterns

Strategy: try-catch

Validate before calling

# reject unterminated quoted keys early
raise ArgumentError, 'accessor path not terminated' unless param.match?(%r{\]\z})
accessor = record_accessor_create(param)

Try / catch

begin
  Fluent::RecordAccessor::Accessor.new(param)
rescue Fluent::ConfigError => e
  log.warn "dropping malformed path #{param.inspect}"
  nil
end

Prevention

When it happens

Trigger: record_accessor_create with a bracket-notation path that never closes a quoted key: $['key, $["nested.field - no closing '] at the end.

Common situations: Paths truncated by shell heredocs, YAML or ERB templating that strips the tail; hand-written multiline configs breaking the path; forgetting the final ']' after the closing quote.

Related errors

AI-assisted analysis of fluent/fluentd@dd45c6e18d (2026-08-21). Data as JSON: /api/errors/72756f382257bd6e. Report an issue: GitHub.