main.go

package main

import (
	"fmt"
	"log"
	"os"
	"time"

	"github.com/broady/sdlog"
)

func init() {
	fmt.Printf("sdlog: %v\n", sdlog.Default("cbro-scratch", "sdlog-example"))
}

func main() {
	defer func() {
		fmt.Printf("sdlog.Flush: %v\n", sdlog.Flush())
	}()

	log.Print("hello there")
	log.Print("hello there2")
	fmt.Fprintln(os.Stderr, "this is to stderr")

	time.Sleep(time.Second)
}

sdlog.go

package sdlog

import (
	"bufio"
	"context"
	"errors"
	"fmt"
	"os"
	"syscall"

	"cloud.google.com/go/logging"
)

var logger *logging.Logger

func Init(c *logging.Client, logID string, opts ...logging.LoggerOption) error {
	if logger != nil {
		return errors.New("sdlog: already initialized")
	}
	return redirect(c, logID)
}

func Default(parent string, logID string, opts ...logging.LoggerOption) error {
	if logger != nil {
		return errors.New("sdlog: already initialized")
	}

	ctx := context.Background()

	c, err := logging.NewClient(ctx, parent)
	if err != nil {
		return err
	}

	return redirect(c, logID)
}

func Flush() error {
	if logger == nil {
		return errors.New("sdlog: not initialized; not flushing")
	}

	return logger.Flush()
}

func redirect(client *logging.Client, logID string, opts ...logging.LoggerOption) error {
	r, w, err := os.Pipe()
	if err != nil {
		return err
	}
	br := bufio.NewReader(r)

	logger = client.Logger(logID, opts...)

	if err := syscall.Dup2(int(w.Fd()), 2); err != nil {
		return fmt.Errorf("dup2: %v", err)
	}

	go func() {
		for {
			line, err := br.ReadString('\n')
			if err != nil {
				fmt.Fprintf(os.Stdout, "sdlog: pipe fail: %v", err)
				continue
			}
			fmt.Fprintf(os.Stdout, "sdlog: writing %q\n", line)
			logger.Log(logging.Entry{Payload: line})
		}
	}()

	return nil
}