Support savepoints on AWS S3

Gopkg.toml

@@ -40,3 +40,7 @@
 [[constraint]]
   branch = "master"
   name = "github.com/hashicorp/go-retryablehttp"
+
+[[constraint]]
+  name = "github.com/aws/aws-sdk-go"
+  version = "1.15.87"

cmd/cli/operations/retrieve_savepoint.go

@@ -2,12 +2,78 @@ package operations
 
 import (
 	"errors"
+	"net/url"
+	"os"
 	"strings"
+	"time"
 
+	"github.com/aws/aws-sdk-go/aws"
+	"github.com/aws/aws-sdk-go/aws/credentials"
+	"github.com/aws/aws-sdk-go/aws/session"
+	"github.com/aws/aws-sdk-go/service/s3"
 	"github.com/spf13/afero"
 )
 
+var s3Schemes = map[string]bool{
+	"s3":  true,
+	"s3a": true,
+	"s3p": true,
+}
+
 func (o RealOperator) retrieveLatestSavepoint(dir string) (string, error) {
+	u, err := url.Parse(dir)
+
+	if err == nil && s3Schemes[u.Scheme] {
+		return o.retrieveLatestSavepointS3(u)
+	}
+
+	return o.retrieveLatestSavepointLocal(dir)
+}
+
+func (o RealOperator) retrieveLatestSavepointS3(dir *url.URL) (string, error) {
+	region := os.Getenv("AWS_REGION")
+	if region == "" {
+		return "", errors.New("AWS_REGION env var must be specified for S3 savepoint directories")
+	}
+
+	config := &aws.Config{
+		Region:      aws.String(os.Getenv("AWS_REGION")),
+		Credentials: credentials.NewEnvCredentials(),
+	}
+
+	sess, err := session.NewSession()
+	if err != nil {
+		return "", errors.New("creating S3 session: " + err.Error())
+	}
+
+	client := s3.New(sess, config)
+
+	input := &s3.ListObjectsV2Input{
+		Bucket: aws.String(dir.Host),
+	}
+	if dir.Path != "" {
+		input.Prefix = aws.String(strings.TrimLeft(dir.Path, "/"))
+	}
+
+	var newestFile url.URL
+	var newestTime time.Time
+	err = client.ListObjectsV2Pages(input, func(output *s3.ListObjectsV2Output, lastPage bool) bool {
+		for _, object := range output.Contents {
+			if strings.HasSuffix(*object.Key, "_metadata") && object.LastModified.After(newestTime) {
+				newestTime = *object.LastModified
+				newestFile = url.URL{Scheme: dir.Scheme, Host: dir.Host, Path: *object.Key}
+			}
+		}
+		return true
+	})
+	if err != nil {
+		return "", errors.New("listing S3 objects: " + err.Error())
+	}
+
+	return newestFile.String(), nil
+}
+
+func (o RealOperator) retrieveLatestSavepointLocal(dir string) (string, error) {
 	if strings.HasSuffix(dir, "/") {
 		dir = strings.TrimSuffix(dir, "/")
 	}