see https://github.com/winebarrel/rdslog

downloadCompleteLogFile.go

package main

import (
	"context"
	"fmt"
	"io"
	"log"
	"net/http"
	"os"
	"time"

	"github.com/aws/aws-sdk-go-v2/aws"
	v4 "github.com/aws/aws-sdk-go-v2/aws/signer/v4"
	"github.com/aws/aws-sdk-go-v2/config"
	"github.com/aws/aws-sdk-go-v2/service/rds"
)

const EmptyStringSHA256 = "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855"

func downloadCompleteLogFile(ctx context.Context, dbInstanceIdentifier string, logFileName string, dst io.Writer) error {
	cfg, err := config.LoadDefaultConfig(ctx)

	if err != nil {
		return err
	}

	endpointResolver := rds.NewDefaultEndpointResolverV2()

	endpoint, err := endpointResolver.ResolveEndpoint(ctx, rds.EndpointParameters{
		Region: aws.String(cfg.Region),
	})

	if err != nil {
		return err
	}

	url := endpoint.URI
	url.Path = fmt.Sprintf("/v15/downloadCompleteLogFile/%s/%s", dbInstanceIdentifier, logFileName)
	req, err := http.NewRequest(http.MethodGet, url.String(), nil)

	if err != nil {
		return err
	}

	creds, err := cfg.Credentials.Retrieve(ctx)

	if err != nil {
		return err
	}

	signer := v4.NewSigner()
	err = signer.SignHTTP(ctx, creds, req, EmptyStringSHA256, "rds", cfg.Region, time.Now())

	if err != nil {
		return err
	}

	res, err := http.DefaultClient.Do(req)

	if err != nil {
		return err
	}

	defer res.Body.Close()

	_, err = io.Copy(dst, res.Body)

	return err
}

func main() {
	dbInstanceIdentifier := "..."
	logFileName := "error/postgresql.log.yyyy-mm-dd-hhmi"

	ctx := context.Background()
	err := downloadCompleteLogFile(ctx, dbInstanceIdentifier, logFileName, os.Stdout)

	if err != nil {
		log.Fatal(err)
	}
}