WIP: Log with slog to stdout in JSON form

cmd/root.go

@@ -5,6 +5,7 @@ import (
 	"os"
 	"strings"
 
+	"github.com/jetstack/preflight/pkg/logs"
 	"github.com/spf13/cobra"
 	"github.com/spf13/pflag"
 )
@@ -17,6 +18,9 @@ var rootCmd = &cobra.Command{
 configuration checks using Open Policy Agent (OPA).
 
 Preflight checks are bundled into Packages`,
+	PersistentPreRun: func(cmd *cobra.Command, args []string) {
+		logs.Initialize()
+	},
 }
 
 func init() {
@@ -28,6 +32,8 @@ func init() {
 // Execute adds all child commands to the root command and sets flags appropriately.
 // This is called by main.main(). It only needs to happen once to the rootCmd.
 func Execute() {
+	logs.AddFlags(rootCmd.PersistentFlags())
+
 	if err := rootCmd.Execute(); err != nil {
 		fmt.Println(err)
 		os.Exit(1)

pkg/logs/logs.go

@@ -1,8 +1,54 @@
 package logs
 
 import (
+	"io"
 	"log"
+	"log/slog"
 	"os"
+
+	"github.com/spf13/pflag"
 )
 
-var Log = log.New(os.Stderr, "", log.LstdFlags)
+// Deprecated: Log is a `log` logger, which is being phased out.
+var Log = log.Default()
+
+// Write logs to stdout by default
+var logOutput io.Writer = os.Stdout
+
+// SetOutput changes the global log output writer
+func SetOutput(w io.Writer) {
+	logOutput = w
+}
+
+var logLevel int
+
+// AddFlags adds log related flags to the supplied flag set
+func AddFlags(fs *pflag.FlagSet) {
+	fs.IntVarP(&logLevel,
+		"log-level", "v", int(slog.LevelInfo),
+		"Set the logging verbosity. 8: >= ERROR, 4: >= WARN, 0: >=INFO, -4: >= DEBUG")
+}
+
+// Initialize configures the global log and slog loggers to write JSON to stdout.
+//
+// Errors, warnings, info and debug messages are all logged to stdout.
+// By default, log messages with level >= INFO will be written.
+// If verbose logging is enabled, log messages with level >= DEBUG will be written.
+//
+// The log module doesn't support levels. All its messages will be logged in
+// JSON format, at INFO level.
+func Initialize() {
+	// This is a work around to remove the `vcert: ` prefix which is added by the vcert module.
+	// See https://github.com/Venafi/vcert/pull/512
+	log.SetPrefix("")
+	slog.SetLogLoggerLevel(slog.LevelInfo)
+
+	level := slog.Level(logLevel)
+	logger := slog.New(
+		slog.NewJSONHandler(logOutput, &slog.HandlerOptions{
+			Level: level,
+		}),
+	)
+	// This sets both the global slog logger and the global legacy log logger.
+	slog.SetDefault(logger)
+}