Programmatically launch local DynamoDB via Docker

Right now, database_test will unconditionally try to launch a local
DynamoDB, so this commit can't land upstream until I add more controls.
However, this commit does successfully launch, configure, and tear down
a local DynamoDB instance as a proof of concept.

db/database_test.go

@@ -1,12 +1,145 @@
 package db
 
 import (
+	"context"
+	"errors"
+	"fmt"
+	"log"
+	"net"
+	"os"
+	"os/exec"
+	"strings"
 	"testing"
 
 	"github.com/aws/aws-sdk-go-v2/aws"
+	"github.com/aws/aws-sdk-go-v2/config"
 	"gotest.tools/assert"
 )
 
+var dbConfig *aws.Config
+
+func TestMain(m *testing.M) {
+	var teardown func() error
+	var err error
+
+	if dbConfig, teardown, err = setupDynamoDb(); err != nil {
+		log.Print(err.Error())
+		os.Exit(1)
+	}
+
+	retval := m.Run()
+	if err := teardown(); err != nil {
+		log.Print(err.Error())
+		retval = 1
+	}
+	os.Exit(retval)
+}
+
+func setupDynamoDb() (conf *aws.Config, teardown func() error, err error) {
+	// TODO: Add logic to decide whether to test against local or remote
+	// DynamoDB.
+	//
+	// Also create a single random table for all tests. Local teardown will stop
+	// the Docker container. Remote teardown will drop the test table.
+	return setupLocalDynamoDb()
+}
+
+// See also:
+// - https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/DynamoDBLocal.DownloadingAndRunning.html
+// - https://github.com/aws-samples/aws-sam-java-rest
+// - https://hub.docker.com/r/amazon/dynamodb-local
+// - https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/DynamoDBLocal.UsageNotes.html
+func setupLocalDynamoDb() (conf *aws.Config, teardown func() error, err error) {
+	var endpoint string
+	var containerId string
+
+	if err = checkDockerIsRunning(); err != nil {
+		return
+	} else if endpoint, err = pickUnusedEndpoint(); err != nil {
+		return
+	} else if containerId, err = launchLocalDb(endpoint); err != nil {
+		return
+	} else if conf, err = localDbConfig(endpoint); err != nil {
+		return
+	}
+	teardown = func() error { return cleanupLocalDb(containerId) }
+	return
+}
+
+func checkDockerIsRunning() (err error) {
+	cmd := exec.Command("docker", "info")
+	if err = cmd.Run(); err != nil {
+		err = errors.New("Please start Docker before running this test.")
+	}
+	return
+}
+
+func pickUnusedEndpoint() (string, error) {
+	if listener, err := net.Listen("tcp", ":0"); err != nil {
+		return "", errors.New("failed to pick unused endpoint: " + err.Error())
+	} else {
+		listener.Close()
+		return listener.Addr().String(), nil
+	}
+}
+
+func launchLocalDb(localEndpoint string) (string, error) {
+	portMap := localEndpoint + ":8000"
+	cmd := exec.Command(
+		"docker", "run", "-d", "-p", portMap, "amazon/dynamodb-local",
+	)
+
+	if output, err := cmd.CombinedOutput(); err != nil {
+		const errFmt = "failed to start local DynamoDB at %s: %s:\n%s"
+		return "", fmt.Errorf(errFmt, localEndpoint, err, output)
+	} else {
+		const logFmt = "local DynamoDB running at %s with container ID: %s"
+		containerId := string(output)
+		log.Printf(logFmt, localEndpoint, containerId)
+		return strings.TrimSpace(containerId), nil
+	}
+}
+
+func cleanupLocalDb(containerId string) (errResult error) {
+	stopCmd := exec.Command("docker", "stop", "-t", "0", containerId)
+	rmCmd := exec.Command("docker", "rm", containerId)
+
+	raiseErr := func(action string, err error, output []byte) error {
+		const errFmt = "failed to %s local DynamoDB container %s: %s:\n%s"
+		return fmt.Errorf(errFmt, action, containerId, err, output)
+	}
+
+	log.Printf("stopping local DynamoDB with container ID: " + containerId)
+
+	if output, err := stopCmd.CombinedOutput(); err != nil {
+		errResult = raiseErr("stop", err, output)
+	} else if output, err = rmCmd.CombinedOutput(); err != nil {
+		errResult = raiseErr("remove", err, output)
+	}
+	return
+}
+
+// Inspired by:
+// - https://davidagood.com/dynamodb-local-go/
+// - https://github.com/aws/aws-sdk-go-v2/blob/main/config/example_test.go
+func localDbConfig(localEndpoint string) (*aws.Config, error) {
+	localResolver := aws.EndpointResolverWithOptionsFunc(
+		func(
+			service, region string, options ...interface{},
+		) (aws.Endpoint, error) {
+			return aws.Endpoint{URL: "http://" + localEndpoint}, nil
+		},
+	)
+	dbConfig, err := config.LoadDefaultConfig(
+		context.TODO(), config.WithEndpointResolverWithOptions(localResolver),
+	)
+	if err != nil {
+		const errFmt = "failed to configure local DynamoDB at: %s: %s"
+		return nil, fmt.Errorf(errFmt, localEndpoint, err)
+	}
+	return &dbConfig, nil
+}
+
 func newTestDatabase() *DynamoDb {
 	cfg := aws.Config{}
 	return NewDynamoDb(cfg, "TestTable")