Value sources

With several drivers it is not always obvious where the final value came from. Sources returns which driver has set every field in the last Parse call.

if err := config.Parse(c); err != nil {
    panic(err)
}
for _, source := range config.Sources(c) {
    fmt.Printf("%s is set by %s driver from %s\n", source.Field, source.Driver, source.Source)
}

HTTP.Host is set by env driver from HTTP_HOST
HTTP.Port is set by yaml driver from http.port
Password is set by env driver from PASSWORD

FieldSource

Field	Description
Field	struct path of the field in the config, HTTP.Port for example
Driver	name of the driver that has set the final value
Source	source of the value inside the driver

Source depends on the driver:

Driver	Source
tag	name of the tag, initial
yaml, json, toml, ini, properties	path by the tags, http.port
env	name of the variable, with the prefix; HOSTS_* for the value from indexed keys
flag	name of the flag, -http-port
consul	the key, app/http/port
vault	path by the tags, database.password
etcd	the key, app/http/port

Rules

• Sources are returned in the order of the config fields.
• Only the last driver that has set the value is reported, the overridden ones are not.
• Fields without values from drivers are not included: they have the value that was in the config before Parse.
• Sources are stored per config type, pass the same pointer type that was used in Parse. For a sub config the paths are relative to it.

It is handy to log the sources once at the start of the application: the answer to "why is this value here" is in the logs. Values themselves are not included, so secrets are not leaked.