Do not use os package in filestate backend

Since the filestate backend is now written using the go-cloud blob
abstraction, places where we were using functions from the `os`
package were very unlikely to be correct.

This change removes their uses in favor of APIs provided by
go-cloud (which sometimes requires more work than before).

pkg/backend/filestate/backend.go

@@ -32,6 +32,7 @@ import (
 	_ "gocloud.dev/blob/fileblob"  // driver for file://
 	_ "gocloud.dev/blob/gcsblob"   // driver for gs://
 	_ "gocloud.dev/blob/s3blob"    // driver for s3://
+	"gocloud.dev/gcerrors"
 
 	"github.com/pulumi/pulumi/pkg/apitype"
 	"github.com/pulumi/pulumi/pkg/backend"
@@ -252,7 +253,7 @@ func (b *localBackend) GetStack(ctx context.Context, stackRef backend.StackRefer
 	stackName := stackRef.Name()
 	snapshot, path, err := b.getStack(stackName)
 	switch {
-	case os.IsNotExist(errors.Cause(err)):
+	case gcerrors.Code(errors.Cause(err)) == gcerrors.NotFound:
 		return nil, nil
 	case err != nil:
 		return nil, err
@@ -305,12 +306,13 @@ func (b *localBackend) RenameStack(ctx context.Context, stackRef backend.StackRe
 	}
 
 	// Ensure the destination stack does not already exist.
-	_, err = os.Stat(b.stackPath(newName))
-	if err == nil {
-		return errors.Errorf("a stack named %s already exists", newName)
-	} else if !os.IsNotExist(err) {
+	hasExisting, err := b.bucket.Exists(ctx, b.stackPath(newName))
+	if err != nil {
 		return err
 	}
+	if hasExisting {
+		return errors.Errorf("a stack named %s already exists", newName)
+	}
 
 	// If we have a snapshot, we need to rename the URNs inside it to use the new stack name.
 	if snap != nil {
@@ -328,16 +330,8 @@ func (b *localBackend) RenameStack(ctx context.Context, stackRef backend.StackRe
 	file := b.stackPath(stackName)
 	backupTarget(b.bucket, file)
 
-	// And move the history over as well, if it exists.
-	oldHistoryDir := b.historyDirectory(stackName)
-	if _, err := os.Stat(oldHistoryDir); err == nil {
-		newHistoryDir := b.historyDirectory(newName)
-		if err := os.Rename(oldHistoryDir, newHistoryDir); err != nil {
-			return errors.Wrap(err, "renaming history")
-		}
-	}
-
-	return nil
+	// And rename the histoy folder as well.
+	return b.renameHistory(stackName, newName)
 }
 
 func (b *localBackend) GetLatestConfiguration(ctx context.Context,

pkg/backend/filestate/bucket.go

@@ -20,6 +20,7 @@ type Bucket interface {
 	SignedURL(ctx context.Context, key string, opts *blob.SignedURLOptions) (string, error)
 	ReadAll(ctx context.Context, key string) (_ []byte, err error)
 	WriteAll(ctx context.Context, key string, p []byte, opts *blob.WriterOptions) (err error)
+	Exists(ctx context.Context, key string) (bool, error)
 }
 
 // wrappedBucket encapsulates a true gocloud blob.Bucket, but ensures that all paths we send to it
@@ -54,6 +55,10 @@ func (b *wrappedBucket) WriteAll(ctx context.Context, key string, p []byte, opts
 	return b.bucket.WriteAll(ctx, filepath.ToSlash(key), p, opts)
 }
 
+func (b *wrappedBucket) Exists(ctx context.Context, key string) (bool, error) {
+	return b.bucket.Exists(ctx, filepath.ToSlash(key))
+}
+
 // listBucket returns a list of all files in the bucket within a given directory. go-cloud sorts the results by key
 func listBucket(bucket Bucket, dir string) ([]*blob.ListObject, error) {
 	bucketIter := bucket.List(&blob.ListOptions{

pkg/backend/filestate/state.go

@@ -25,6 +25,7 @@ import (
 	"time"
 
 	"github.com/pkg/errors"
+	"gocloud.dev/gcerrors"
 
 	"github.com/pulumi/pulumi/pkg/apitype"
 	"github.com/pulumi/pulumi/pkg/backend"
@@ -267,7 +268,7 @@ func (b *localBackend) getHistory(name tokens.QName) ([]backend.UpdateInfo, erro
 	allFiles, err := listBucket(b.bucket, dir)
 	if err != nil {
 		// History doesn't exist until a stack has been updated.
-		if os.IsNotExist(err) {
+		if gcerrors.Code(errors.Cause(err)) == gcerrors.NotFound {
 			return nil, nil
 		}
 		return nil, err
@@ -302,6 +303,42 @@ func (b *localBackend) getHistory(name tokens.QName) ([]backend.UpdateInfo, erro
 	return updates, nil
 }
 
+func (b *localBackend) renameHistory(oldName tokens.QName, newName tokens.QName) error {
+	contract.Require(oldName != "", "oldName")
+	contract.Require(newName != "", "newName")
+
+	oldHistory := b.historyDirectory(oldName)
+	newHistory := b.historyDirectory(newName)
+
+	allFiles, err := listBucket(b.bucket, oldHistory)
+	if err != nil {
+		// if there's nothing there, we don't really need to do a rename.
+		if gcerrors.Code(errors.Cause(err)) == gcerrors.NotFound {
+			return nil
+		}
+		return err
+	}
+
+	for _, file := range allFiles {
+		fileName := objectName(file)
+		oldBlob := path.Join(oldHistory, fileName)
+
+		// The filename format is <stack-name>-<timestamp>.[checkpoint|history].json, we need to change
+		// the stack name part but retain the other parts.
+		newFileName := string(newName) + fileName[strings.LastIndex(fileName, "-"):]
+		newBlob := path.Join(newHistory, newFileName)
+
+		if err := b.bucket.Copy(context.TODO(), newBlob, oldBlob, nil); err != nil {
+			return errors.Wrap(err, "copying history file")
+		}
+		if err := b.bucket.Delete(context.TODO(), oldBlob); err != nil {
+			return errors.Wrap(err, "deleting existing history file")
+		}
+	}
+
+	return nil
+}
+
 // addToHistory saves the UpdateInfo and makes a copy of the current Checkpoint file.
 func (b *localBackend) addToHistory(name tokens.QName, update backend.UpdateInfo) error {
 	contract.Require(name != "", "name")