Fail fast when no AWS region is resolved and use it for S3 object URLs

The switch to resolving AWS credentials/region from the SDK default chain
left the region passed to the S3 service empty. Virtual-host style object
URLs embed the region in the hostname, so stored archive URLs came out as
https://<bucket>.s3..amazonaws.com/... which doesn't resolve. Resolve the
region from the SDK chain, error if it's empty, and pass it to the S3
service.

Author: rowanseymour

archives/s3.go

@@ -12,6 +12,7 @@ import (
 	"strings"
 
 	"github.com/aws/aws-sdk-go-v2/aws"
+	"github.com/aws/aws-sdk-go-v2/config"
 	"github.com/aws/aws-sdk-go-v2/feature/s3/manager"
 	"github.com/aws/aws-sdk-go-v2/service/s3"
 	"github.com/aws/aws-sdk-go-v2/service/s3/types"
@@ -30,7 +31,20 @@ const chunkSizeBytes = 1e9 // 1GB
 
 // NewS3Client creates a new s3 service from the passed in config, testing it as necessary
 func NewS3Client(cfg *runtime.Config, test bool) (*s3x.Service, error) {
-	svc, err := s3x.NewService(context.TODO(), "", cfg.S3Endpoint, cfg.S3PathStyle)
+	ctx := context.TODO()
+
+	// resolve the AWS region from the standard SDK default chain (AWS_REGION / AWS_DEFAULT_REGION env
+	// vars, shared config, etc.). The SDK doesn't error when no region can be resolved, it just leaves
+	// it empty, so fail fast here rather than let an empty region break virtual-host style S3 URLs.
+	awsCfg, err := config.LoadDefaultConfig(ctx)
+	if err != nil {
+		return nil, fmt.Errorf("error resolving AWS config: %w", err)
+	}
+	if awsCfg.Region == "" {
+		return nil, fmt.Errorf("no AWS region resolved - set AWS_REGION or AWS_DEFAULT_REGION")
+	}
+
+	svc, err := s3x.NewService(ctx, awsCfg.Region, cfg.S3Endpoint, cfg.S3PathStyle)
 	if err != nil {
 		return nil, err
 	}