feat(firestore): add GetCommitTime TransactionOption

firestore/transaction.go

@@ -17,6 +17,7 @@ package firestore
 import (
 	"context"
 	"errors"
+	"time"
 
 	"cloud.google.com/go/internal/trace"
 	gax "github.com/googleapis/gax-go/v2"
@@ -40,6 +41,7 @@ type Transaction struct {
 // A TransactionOption is an option passed to Client.Transaction.
 type TransactionOption interface {
 	config(t *Transaction)
+	handleCommitResponse(r *pb.CommitResponse)
 }
 
 // MaxAttempts is a TransactionOption that configures the maximum number of times to
@@ -48,7 +50,8 @@ func MaxAttempts(n int) maxAttempts { return maxAttempts(n) }
 
 type maxAttempts int
 
-func (m maxAttempts) config(t *Transaction) { t.maxAttempts = int(m) }
+func (m maxAttempts) config(t *Transaction)                     { t.maxAttempts = int(m) }
+func (m maxAttempts) handleCommitResponse(r *pb.CommitResponse) {}
 
 // DefaultTransactionMaxAttempts is the default number of times to attempt a transaction.
 const DefaultTransactionMaxAttempts = 5
@@ -59,7 +62,23 @@ var ReadOnly = ro{}
 
 type ro struct{}
 
-func (ro) config(t *Transaction) { t.readOnly = true }
+func (ro) config(t *Transaction)                     { t.readOnly = true }
+func (ro) handleCommitResponse(r *pb.CommitResponse) {}
+
+// GetCommitTime is a TransactionOption that allows the caller to indicate where the commit
+// time for the transaction should be stored, upon successful commit.
+func GetCommitTime(t *time.Time) commitTime {
+	return commitTime{Time: t}
+}
+
+type commitTime struct {
+	*time.Time
+}
+
+func (c commitTime) config(t *Transaction) {}
+func (c commitTime) handleCommitResponse(r *pb.CommitResponse) {
+	*c.Time = r.CommitTime.AsTime()
+}
 
 var (
 	// Defined here for testing.
@@ -114,6 +133,7 @@ func (c *Client) RunTransaction(ctx context.Context, f func(context.Context, *Tr
 		}
 	}
 	var backoff gax.Backoff
+	var commitResponse *pb.CommitResponse
 	// TODO(jba): use other than the standard backoff parameters?
 	// TODO(jba): get backoff time from gRPC trailer metadata? See
 	// extractRetryDelay in https://code.googlesource.com/gocloud/+/master/spanner/retry.go.
@@ -141,13 +161,20 @@ func (c *Client) RunTransaction(ctx context.Context, f func(context.Context, *Tr
 			return err
 		}
 		t.ctx = trace.StartSpan(t.ctx, "cloud.google.com/go/firestore.Client.Commit")
-		_, err = t.c.c.Commit(t.ctx, &pb.CommitRequest{
+		commitResponse, err = t.c.c.Commit(t.ctx, &pb.CommitRequest{
 			Database:    t.c.path(),
 			Writes:      t.writes,
 			Transaction: t.id,
 		})
 		trace.EndSpan(t.ctx, err)
 
+		// on success, handle the commit response
+		if err == nil {
+			for _, opt := range opts {
+				opt.handleCommitResponse(commitResponse)
+			}
+		}
+
 		// If a read-write transaction returns Aborted, retry.
 		// On success or other failures, return here.
 		if t.readOnly || status.Code(err) != codes.Aborted {

firestore/transaction_test.go

@@ -84,6 +84,7 @@ func TestRunTransaction(t *testing.T) {
 		},
 		&pb.CommitResponse{CommitTime: aTimestamp3},
 	)
+	var commitTime time.Time
 	err = c.RunTransaction(ctx, func(_ context.Context, tx *Transaction) error {
 		docref := c.Collection("C").Doc("a")
 		doc, err := tx.Get(docref)
@@ -95,11 +96,16 @@ func TestRunTransaction(t *testing.T) {
 			return err
 		}
 		return tx.Update(docref, []Update{{Path: "count", Value: count.(int64) + 1}})
-	})
+	}, GetCommitTime(&commitTime))
 	if err != nil {
 		t.Fatal(err)
 	}
 
+	// validate commit time
+	if commitTime != aTimestamp3.AsTime() {
+		t.Fatalf("commit time %v should equal %v", commitTime, aTimestamp3)
+	}
+
 	// Query
 	srv.reset()
 	srv.addRPC(beginReq, beginRes)