don't allow leading slashes in s3 remote state key

S3 accepts objects with a leading slash and strips them off. This works
fine except in our workspace hierarchy, which then can no longer find
suffixes matching the full key name.

backend/remote-state/s3/backend.go

@@ -2,6 +2,8 @@ package s3
 
 import (
 	"context"
+	"fmt"
+	"strings"
 
 	"github.com/aws/aws-sdk-go/service/dynamodb"
 	"github.com/aws/aws-sdk-go/service/s3"
@@ -25,6 +27,14 @@ func New() backend.Backend {
 				Type:        schema.TypeString,
 				Required:    true,
 				Description: "The path to the state file inside the bucket",
+				ValidateFunc: func(v interface{}, s string) ([]string, []error) {
+					// s3 will strip leading slashes from an object, so while this will
+					// technically be accepted by s3, it will break our workspace hierarchy.
+					if strings.HasPrefix(v.(string), "/") {
+						return nil, []error{fmt.Errorf("key must not start with '/'")}
+					}
+					return nil, nil
+				},
 			},
 
 			"region": {

backend/remote-state/s3/backend_test.go

@@ -11,6 +11,7 @@ import (
 	"github.com/aws/aws-sdk-go/service/dynamodb"
 	"github.com/aws/aws-sdk-go/service/s3"
 	"github.com/hashicorp/terraform/backend"
+	"github.com/hashicorp/terraform/config"
 	"github.com/hashicorp/terraform/state/remote"
 	"github.com/hashicorp/terraform/terraform"
 )
@@ -65,6 +66,28 @@ func TestBackendConfig(t *testing.T) {
 	}
 }
 
+func TestBackendConfig_invalidKey(t *testing.T) {
+	testACC(t)
+	cfg := map[string]interface{}{
+		"region":         "us-west-1",
+		"bucket":         "tf-test",
+		"key":            "/leading-slash",
+		"encrypt":        true,
+		"dynamodb_table": "dynamoTable",
+	}
+
+	rawCfg, err := config.NewRawConfig(cfg)
+	if err != nil {
+		t.Fatal(err)
+	}
+	resCfg := terraform.NewResourceConfig(rawCfg)
+
+	_, errs := New().Validate(resCfg)
+	if len(errs) != 1 {
+		t.Fatal("expected config validation error")
+	}
+}
+
 func TestBackend(t *testing.T) {
 	testACC(t)