config.TLS: Inline PEM-encoded Certs, Keys and CAs

[oxzi]

Introduced support for raw encoded PEM data instead of filenames for TLS.Cert, TLS.Key and TLS.Ca. This should make it easier to get rid of the additional entrypoint program for docker-icingadb and allow direct use of Icinga DB with its environment variable support.

In addition to the implementation, new tests were written with the goal of complete coverage. A consumer of this type, database.Config, got another test case using PEMs both as environment variables and as YAML

[lippserd]

I would introduce a utility function that does both pem.Decode() and os.ReadFile() in order to reduce code complexity. The first part could then look like this:

certPem, err := loadPemOrFile(t.Cert)
if err != nil {
    return nil, errors.Wrap(err, "can't load certificate")
}

keyPem, err := loadPemOrFile(t.Key)
if err != nil {
    return nil, errors.Wrap(err, "can't load key")
}

crt, err := tls.X509KeyPair(certPem, KeyPem)
if err != nil {
    return nil, errors.Wrap(err, "can't load X.509 key pair")
}

tlsConfig.Certificates = []tls.Certificate{crt}

func loadPemOrFile(subject string) ([]byte, error) {
    if block, _ := pem.Decode([]byte(subject)); block != nil {
        return []byte(subject), nil
    }

    data, err := os.ReadFile(subject)
    if err != nil {
        return nil, err
    }

    return data, nil
}

[oxzi]

@lippserd: Valid point. I have moved the decision logic to a helper function like you suggested. Now the body of the actual MakeConfig method looks cleaner. While working on it, I have flattened the if and removed the nested if, making it easier to read IMO.