loopdb: add Secret type for reading password from file

[kaldun-tech]

Summary

• Add Secret type that implements go-flags.Unmarshaler to support reading sensitive values from files using @/path/to/file syntax
• Change PostgresConfig.Password field from string to Secret
• This allows the postgres password to be stored in a file rather than passed directly on the command line, avoiding exposure in process listings and shell history

Usage

# Direct password (existing behavior)
loopd --postgres.password=mypassword

# Password from file (new behavior)
echo "mypassword" > /path/to/secret.txt
chmod 600 /path/to/secret.txt
loopd --postgres.password=@/path/to/secret.txt

Trailing whitespace (spaces, tabs, newlines) is automatically stripped from file contents.

Design notes

• No fmt.Stringer redaction: PostgresConfig.DSN(hidePassword bool) already handles password hiding explicitly. Automatic redaction could mask debugging issues.
• No MarshalFlag: The resolved secret should not be written back to config files. Users should store the @/path syntax, not the resolved value.

Test plan

• Unit tests for Secret type covering direct values, file references, edge cases
• go-flags integration test confirming parser compatibility
• Manual test: loopd --postgres.password=@/path/to/secret.txt parses successfully

Fixes #1088

🤖 Generated with Claude Code

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request improves the security posture of the application by allowing sensitive database credentials to be read from external files rather than being passed as plain-text command-line arguments. By implementing a custom unmarshaler, the configuration system now supports a file-reference syntax, ensuring that secrets remain protected from accidental exposure in system logs or process monitoring tools.

Highlights

• New Secret Type: Introduced a new Secret type that implements go-flags.Unmarshaler to support reading sensitive configuration values from files.
• Enhanced Security: Updated PostgresConfig.Password to use the Secret type, allowing passwords to be loaded via @/path/to/file to prevent exposure in process lists and shell history.
• Robust Testing: Added comprehensive unit and integration tests to verify file reading, newline handling, and go-flags compatibility.

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[gemini-code-assist]

Code Review

This pull request introduces a new Secret type that allows database passwords to be read from files when prefixed with '@', enhancing security by preventing secrets from appearing in process listings. The feedback suggests restoring a gosec linter suppression to avoid CI failures, implementing the fmt.Stringer interface to prevent accidental logging of sensitive data, and broadening the whitespace trimming logic to include spaces and tabs when reading from secret files.

[gemini-code-assist]

The //nolint:gosec directive was removed in this change. Since the field name contains "Password", it is likely to be flagged by security linters (like gosec G101) as a potential hardcoded credential. It's recommended to keep the suppression to avoid CI failures.

Suggested change

	Password Secret `long:"password" description:"Database user's password. Use @/path/to/file to read from a file."`
	Password Secret `long:"password" description:"Database user's password. Use @/path/to/file to read from a file."` //nolint:gosec

[kaldun-tech]

I think this can be ignored: The gosec G101 rule looks for hardcoded credentials in string literals. Since Secret is a type (not a hardcoded value), it shouldn't trigger.
Running the linter showed no warnings in this file via golangci-lint run -v

[gemini-code-assist]

It is a best practice for sensitive types like Secret to implement the fmt.Stringer interface. This prevents the actual secret value from being accidentally exposed in logs if the configuration struct or the Secret value itself is printed using %v or %+v.

type Secret string

// String implements the fmt.Stringer interface to prevent the secret from
// being leaked in logs.
func (s Secret) String() string {
	if s == "" {
		return ""
	}
	return "xxxxxxxx"
}

[kaldun-tech]

This is a deliberate design decision made because:

• DSN() already handles password hiding explicitly via the hidePassword parameter
• Automatic redaction could mask debugging issues
• This keeps the Secret type simple and predictable