foo.go

package main

import (
	"context"
	"errors"

	log "github.com/sirupsen/logrus"
)

type someContextKey string

var (
	keyA = someContextKey("a")
	keyB = someContextKey("b")
)

func main() {
	ctx := context.Background()
	ctx = context.WithValue(ctx, keyA, "foo")
	ctx = context.WithValue(ctx, keyB, "bar")

	log2(ctx, nil).Info("did nothing")
	err := errors.New("an error")
	log2(ctx, err).Fatal("unrecoverable error")
}

func log2(ctx context.Context, err error) *log.Entry {
	entry := log.WithField("component", "main")
	if err != nil {
		entry = entry.WithField("error", err)
	}
	if a := ctx.Value(keyA); a != nil {
		entry = entry.WithField("a", a)
	}
	if b := ctx.Value(keyB); b != nil {
		entry = entry.WithField("b", b)
	}
	return entry
}