feat: write out parsed configs (#707)

## Which problem is this PR solving?

We have often been asked for the ability to see what refinery thinks its
configurations are. We support this with a debug command, but it
requires a running refinery configured to allow debug queries, and a
configuration that allows connecting to it.

This PR adds command-line switches to read the configs, apply defaults
and environment variables, and write the data back out to a file. If you
use this feature, refinery doesn't start, it just writes the file and
exits.

## Short description of the changes

- Add function to write YAML out
- Add cmdline parameters to control it

Dependent on #706

config/cmdenv.go

@@ -45,6 +45,8 @@ type CmdEnv struct {
 	InterfaceNames        bool   `long:"interface-names" description:"Print system's network interface names and exit."`
 	Validate              bool   `short:"V" long:"validate" description:"Validate the configuration files, writing results to stdout, and exit with 0 if valid, 1 if invalid."`
 	NoValidate            bool   `long:"no-validate" description:"Do not attempt to validate the configuration files. Makes --validate meaningless."`
+	WriteConfig           string `long:"write-config" description:"After applying defaults, environment variables, and command line values, write the loaded configuration to the specified file as YAML and exit."`
+	WriteRules            string `long:"write-rules" description:"After applying defaults, write the loaded rules to the specified file as YAML and exit."`
 }
 
 func NewCmdEnvOptions(args []string) (*CmdEnv, error) {

config/file_config.go

@@ -2,10 +2,14 @@ package config
 
 import (
 	"errors"
+	"fmt"
 	"net"
+	"os"
 	"strings"
 	"sync"
 	"time"
+
+	"gopkg.in/yaml.v3"
 )
 
 // In order to be able to unmarshal "15s" etc. into time.Duration, we need to
@@ -295,9 +299,22 @@ func newFileConfig(opts *CmdEnv) (*fileConfig, error) {
 	return cfg, nil
 }
 
+// writeYAMLToFile renders the given data item to a YAML file
+func writeYAMLToFile(data any, filename string) error {
+	f, err := os.Create(filename)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+	encoder := yaml.NewEncoder(f)
+	encoder.SetIndent(2)
+	return encoder.Encode(data)
+}
+
 // NewConfig creates a new Config object from the given arguments; if args is
-// nil, it uses the command line arguments. Because errors may be caused by
-// validation problems, it returns the config even in the case of errors.
+// nil, it uses the command line arguments.
+// It also dumps the config and rules to the given files, if specified, which
+// will cause the program to exit.
 func NewConfig(opts *CmdEnv, errorCallback func(error)) (Config, error) {
 	cfg, err := newFileConfig(opts)
 	// only exit if we have no config at all; if it fails validation, we'll
@@ -306,6 +323,22 @@ func NewConfig(opts *CmdEnv, errorCallback func(error)) (Config, error) {
 		return nil, err
 	}
 
+	if opts.WriteConfig != "" {
+		if err := writeYAMLToFile(cfg.mainConfig, opts.WriteConfig); err != nil {
+			fmt.Printf("Error writing config: %s\n", err)
+			os.Exit(1)
+		}
+	}
+	if opts.WriteRules != "" {
+		if err := writeYAMLToFile(cfg.rulesConfig, opts.WriteRules); err != nil {
+			fmt.Printf("Error writing rules: %s\n", err)
+			os.Exit(1)
+		}
+	}
+	if opts.WriteConfig != "" || opts.WriteRules != "" {
+		os.Exit(0)
+	}
+
 	cfg.callbacks = make([]func(), 0)
 	cfg.errorCallback = errorCallback
 

config_complete.yaml

@@ -470,7 +470,7 @@ LegacyMetrics:
     ## metrics.
     ##
     ## Not eligible for live reload.
-    APIKey: abcd1234
+    APIKey: SetThisToAHoneycombKey
 
     ## Dataset is the Honeycomb dataset to which metrics will be sent.
     ##